2008. "Electron-Driven Acid-Base Chemistry: Proton Transfer from Hydrogen Chloride to Ammonia." Science 319(5865):936-939. doi:10.1126/science.1151614 Abstract It is well established that NH3 and HCl form in isolation a hydrogen bonded complex NH3…HCl rather than an ionic salt, NH4+Cl-. This experimental and theoretical study utilized anion photoelectron spectroscopy and ab initio theory to investigate the effect of an excess electron on the hydrogen bonded complex NH3 …HCl. Our results indicate that one electron is sufficient to drive the hydrogen bonded complex to form the ionic salt. We propose a stepwise mechanism for this process involving an initial dipole-bound state, followed by the formation of a distorted Rydberg species, NH40.
2007. "Valence anions in complexes of adenine and 9-methyladenine with formic acid - stabilization by intermolecular proton transfer." Journal of the American Chemical Society 129(5):1216-1224. doi:10.1021/ja066229h Abstract The photoelectron spectra of the adenine-formic acid (AFA)- and 9-methyladenine-formic acid (MAFA)- anionic complexes have been recorded with 2.540 eV photons. These spectra reveal broad features with maxima at 1.5-1.4 eV that indicate formation of stable valence anions in the gas phase. The neutral and anionic complexes of adenine/9- methyladenine and formic acid were also studied computationally at the B3LYP, second order Møller-Plesset and coupled clusters levels of theory, with the 6-31++G** and aug-cc-pVDZ basis sets. The neutral complexes form cyclic hydrogen bonds and the most stable dimers are bound by 17.7 and 16.0 kcal/mol for AFA and MAFA, respectively. The theoretical results indicate that the excess electron in both (AFA)- and (MAFA)- occupies a p* orbital localized on adenine/9-methyladenine and the adiabatic stability of the most stable anions amounts to 0.67 and 0.54 eV for AFA- and MAFA-, respectively. The excess electron attachment to the complexes induces a barrierfree proton transfer (BFPT) from the carboxylic group of formic acid to a N atom of adenine or 9-mathyladenine. As a result, the most stable structures of the anionic complexes can be characterized as neutral radicals of hydrogenated adenine(9-methyladenine) solvated by a deprotonated formic acid. The BFPT to the N atoms of adenine may be biologically relevant because some of these sites are not involved in the Watson-Crick pairing scheme and are easily accessible in the cellular environment. We suggest that valence anions of purines might be as important as those of pyrimidines in the process of DNA damage by low energy electrons. The calculations were performed at the Academic Computer Center in Gdańsk (TASK) and at the Molecular Science Computing Facility (MSCF) in the William R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the U.S. Department of Energy's Office of Biological and Environmental Research and located at the Pacific Northwest National Laboratory, which is operated by Battelle for the US Department of Energy. The MSCF resources were available through a Computational Grand Challenge Application grant.
2007. "Can an Excess Electron Localise on a Purine Moiety in the Adenine-thymine Watson-Crick Base Pair? A Computational Study." International Journal of Quantum Chemistry 107(12):2224-2232. doi:10.1002/qua.21359 Abstract The electron affinity and the propensity to electron-induced proton transfer (PT) of hydrogen-bonded complexes between the Watson–Crick adenine–thymine pair (AT) and simple organic acid (HX), attached to adenine in the Hoogsteen-type configuration, were studied at the B3LYP/6-31+G** level. Although the carboxyl group is deprotonated at physiological pH, its neutral form, COOH, resembles the peptide bond or the amide fragment in the side chain of asparagine (Asn) or glutamine (Gln). Thus, these complexes mimic the interaction between the DNA environment (e.g., proteins) and nucleobase pairs incorporated in the biopolymer. Electron attachment is thermodynamically feasible and adiabatic electron affinities range from 0.41 to 1.28 eV, while the vertical detachment energies of the resulting anions span the range of 0.39 –2.88 eV. Low-energy activation barriers separate the anionic minima: aHX(AT) from the more stable single-PT anionic geometry, aHX(AT)-SPT, and aHX(AT)-SPT from the double-PT anionic geometry, aHX(AT)-DPT. Interaction between the adenine of the Watson–Crick AT base pair with an acidic proton donor probably counterbalances the larger EA of isolated thymine, as SOMO is almost evenly delocalized over both types of nucleic bases in the aHX(AT) anions. Moreover, as a result of PT the excess electron localizes entirely on adenine. Thus, in DNA interacting with its physiological environment, damage induced by low-energy electrons could begin, contrary to the current view, with the formation of purine anions, which are not formed in isolated DNA because of the greater stability of anionic pyrimidines.
2007. "Photoelectron Spectroscopy of Adiabatically Bound Valence Anions of Rare Tautomers of the Nucleic Acid Bases." Journal of Chemical Physics 127(17):Art. 174309. doi:10.1063/1.2795719 Abstract The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Anionic states of nucleic acid bases (NABs) are involved in DNA damage by low-energy electrons and in charge transfer through DNA. Previous gas phase studies of free, unsolvated NAB parent anions probed mostly dipole-bound states, which are not present in condensed phase environments. Recently, we demonstrated that very rare tautomers of uracil (U), cytosine (C), adenine (A), and guanine (G), which are obtained from canonical tautomers through N-to-C proton transfers, support valence anionic states. Here we report the photoelectron spectrum of the final member of the NABs series: the valence state of the thymine (T) anion. Additionally, we summarized the work of all five NABs. All of the newfound anionic tautomers of the NABs may be formed via dissociative electron attachment followed by hydrogen atom reattachment to a carbon atom. Furthermore, these unusual tautomers may affect the structure and properties of DNA and RNA exposed to low-energy electrons. The new valence states observed here, unlike dipole bound states, could exist in condensed phases and may be relevant to radiobiological damage.
2007. "Differences in Electrostatic Potential Around DNA Fragments Containing Guanine and 8-oxo-Guanine." Theoretical Chemistry Accounts 117(2):291-296. doi:10.1007/s00214-006-0133-1 Abstract hanges of electrostatic potential (EP) around the DNA molecule resulting from chemical modifications of nucleotides may play a role in enzymatic recognition of damaged sites. Effects of chemical modifications of nucleotides on the structure of DNA have been characterized through large scale density functional theory computations. Quantum mechanical structural optimizations of DNA fragments with three pairs of nucleotoides and accompanying counteractions were performed with a B3LYP exchange-correlation functional and 6-31G** basis sets. The “intact” DNA fragment contained guanine in the middle layer, while the “damaged” fragment had the guanine replaced with 8-oxo-guanine. The electrostatic potential around these DNA fragments was projected on a surface around the double helix. The 2D maps of EP of intact and damaged DNA fragments were analyzed to identify these modifications of EP that result from the occurrence of 8-oxo-guanine. It was found that distortions of the phosphate groups and displacements of the accompanying countercations are clearly reflected in the EP maps.
2007. "Differences in Electrostatic Potential Around DNA Fragments Containing Adenine and 8-oxo-Adenine. An Analysis Based on Regular Cylindrical Projection ." Journal of Molecular Graphics and Modelling 26(1):282-289. Abstract Changes of electrostatic potential (EP) around the DNA molecule resulting from chemical modifications of nucleotides may play a role in enzymatic recognition of damaged sites. Effects of chemical modifications of nucleotides on the structure of DNA have been characterized through large scale density functional theory computations. Quantum mechanical structural optimizations of DNA fragments with three pairs of nucleotides and accompanying counteractions were performed with a B3LYP exchange-correlation functional and 6-31G** basis sets. The “intact” DNA fragment contained adenine in the middle layer, while the “damaged” fragment had the adenine replaced with 8-oxo-adenine. The electrostatic potential around these DNA fragments was projected on a cylindrical surface around the double helix. The two-dimensional maps of EP of the intact and damaged DNA fragments were analyzed to identify these modifications of EP that result from the occurrence of 8-oxo-adenine (8oA). It was found that distortions of a phosphate group neighboring 8oA and displacements of the accompanying countercation are clearly reflected in the EP maps. Helpful discussions Michel Dupuis are gratefully acknowledged. Authors wish to thank Marcel Swart for directing us to a compilation of van der Waals radii. This work was supported by the: (i) US DOE Office of Biological and Environmental Research, Low Dose Radiation Research Program (M.G. and M.H.), (ii) the Office of Science (BER), U. S. Department of Energy, Grant No. DE-FG03-02ER63470 (JHM), (iii) Polish State Committee for Scientific Research (KBN) Grant DS/8221-4-0140-6 (MG), (iv) European Social Funds (EFS) ZPORR/2.22/II/2.6/ARP/U/2/05 (M.H.). M.H. holds the Foundation for Polish Science (FNP) award for young scientists. The calculations were performed at the Academic Computer Center in Gdańsk (TASK) and at the Molecular Science Computing Facility (MSCF) in the William R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the U.S. Department of Energy's Office of Biological and Environmental Research and located at the Pacific Northwest National Laboratory, which is operated by Battelle for the US Department of Energy. The MSCF resources were available through a pilot project.
2007. "Bound Anionic States of Adenine ." Proceedings of the National Academy of Sciences of the United States of America 104(12):4804-4807. doi:10.1073/pnas.0609982104 Abstract The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Anionic states of nucleic acid bases are involved in DNA damage by low-energy electrons and in charge transfer through DNA. Previous gas phase studies of free, unsolvated nucleic acid base parent anions probed only dipole-bound states, which are not present in condensed phase environments, but did not observe valence anionic states, which for purine bases are thought to be adiabatically unbound. Contrary to this expectation, we have demonstrated that some thus far ignored tautomers of adenine, which result from enamine-imine transformations, support valence anionic states with electron vertical detachment energies as large as 2.2 eV, and at least one of these anionic tautomers is adiabatically bound. Moreover, we predict that the new anionic tautomers should also dominate in solutions and should be characterized by larger values of electron vertical detachment energy than the canonical valence anion. All of the newfound anionic tautomers might be formed in the course of dissociative electron attachment followed by a hydrogen atom attachment to a carbon atom, and they might affect the structure and properties of DNA and RNA exposed to low-energy electrons. The new valence states observed here, unlike the dipole-bound state, could exist in condensed phases and might be relevant to radiobiological damage. The discovery of these valence anionic states of adenine was facilitated by the development of (i) an experimental method for preparing parent anions of nucleic acid bases for photoelectron experiments, and (it) a combinatorial/quantum chemical approach for identification of the most stable tautomers of organic molecules.
2007. "Bound anionic states of adenine." Proceedings of the National Academy of Sciences of the United States of America 104(12):4804-4807. doi:10.1073/pnas.0609982104 Abstract Anionic states of nucleic acid bases are involved in DNA damage by low-energy electrons and in charge transfer through DNA. Previous gas phase studies of free, unsolvated nucleic acid base parent anions probed only dipole-bound states, which are not present in condensed phase environments, but did not observe valence anionic states, which for purine bases, are thought to be adiabatically unbound. Contrary to this expectation, we have demonstrated that some thus far ignored tautomers of adenine, which result from enamine-imine transformations, support valence anionic states with electron vertical detachment energies as large as 2.2 eV, and at least one of these anionic tautomers is adiabatically bound. Moreover, we predict that the new anionic tautomers should also dominate in solutions and should be characterized by larger values of electron vertical detachment energy than the canonical valence anion. All of the new-found anionic tautomers might be formed in the course of dissociative electron attachment followed by a hydrogen atom attachment to a carbon atom, and they might affect the structure and properties of DNA and RNA exposed to low-energy electrons. The discovery of these valence anionic states of adenine was facilitated by the development of: (i) a new experimental method for preparing parent anions of nucleic acid bases for photoelectron experiments, and (ii) a new combinatorial/ quantum chemical approach for identification of the most stable tautomers of organic molecules. The computational portion of this work was supported by the: (i) Polish State Committee for Scientific Research (KBN) Grants: DS/8000-4-0140-7 (M.G.) and N204 127 31/2963 (M.H.), (ii) European Social Funds (EFS) ZPORR/2.22/II/2.6/ARP/U/2/05 (M.H.), and (iii) US DOE Office of Biological and Environmental Research, Low Dose Radiation Research Program (M.G.). M.H. holds the Foundation for Polish Science (FNP) award for young scientists. The calculations were performed at the Academic Computer Center in Gdańsk (TASK) and at the Molecular Science Computing Facility (MSCF) in the William R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the U.S. DOE OBER and located at PNNL, which is operated by Battelle for the US DOE. The MSCF resources were available through a Computational Grand Challenge Application grant. The experimental material in this paper (K.H.B.) is based upon work supported by the National Science Foundation under Grant No. CHE-0517337.
2007. "Bound Anionic States of Aadenine." Proceedings of the National Academy of Sciences of the United States of America 104(12):4804-4807. doi:10.1073/pnas.0609982104 Abstract The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Anionic states of nucleic acid bases are involved in DNA damage by low-energy electrons and in charge transfer through DNA. Previous gas phase studies of free, unsolvated nucleic acid base parent anions probed only dipole-bound states, which are not present in condensed phase environments, but did not observe valence anionic states, which for purine bases are thought to be adiabatically unbound. Contrary to this expectation,wehave demonstrated that some thus far ignored tautomers of adenine, which result from enamine-imine transformations, support valence anionic states with electron vertical detachment energies as large as 2.2 eV, and at least one of these anionic tautomers is adiabatically bound. Moreover, we predict that the new anionic tautomers should also dominate in solutions and should be characterized by larger values of electron vertical detachment energy than the canonical valence anion. All of the newfound anionic tautomers might be formed in the course of dissociative electron attachment followed by a hydrogen atom attachment to a carbon atom, and they might affect the structure and properties of DNA and RNA exposed to low-energy electrons. The new valence states observed here, unlike the dipole-bound state, could exist in condensed phases and might be relevant to radiobiological damage. The discovery of these valence anionic states of adenine was facilitated by the development of (i) an experimental method for preparing parent anions of nucleic acid bases for photoelectron experiments, and (ii) a combinatorial/quantum chemical approach for identification of the most stable tautomers of organic molecules.
2006. "On the unusual stability of valence anions of thymine based on very rare tautomers. A computational study." Journal of Physical Chemistry B 110(48):24696-24707. doi:10.1021/jp065666f Abstract We characterized valence-type and dipole-bound anionic states of thymine using various electronic structure methods, with the most accurate results obtained at the CCSD(T)/aug-cc-pVDZ level of theory followed by extrapolations to complete basis set limits. We found that the most stable anion in the gas phase is related to neither the canonical 2,4-dioxo nor a rare imino-hydroxy tautomer. Instead, it is related to an imino-oxo tautomer, in which the N1H proton is transferred to the C5 atom. This valence anion, , is characterized by an electron vertical detachment energy (VDE) of 1251 meV and it is adiabatically stable with respect to the canonical neutral by 2.4 kcal/mol. It is also more stable than the dipole-bound ( ) and valence anion ( ) of the canonical tautomer. The latter is adiabatically unbound with respect to by 0.1 kcal/mol and this instability is smaller than the uncertainty of the computational model used. The VDE values for and are 55 and 457 meV, respectively. Another, anionic, low-lying imino-oxo tautomer with a VDE of 2458 meV has a proton transferred from N3H to C5 ( ). It is less stable than by 3.2 kcal/mol. The mechanism of formation of anionic tautomers with the carbons C5 or C6 protonated may involve intermolecular proton transfer or dissociative electron attachment to the canonical neutral tautomer followed by a barrier-free attachment of a hydrogen atom to C5. The six-member ring structure of anionic tautomers with carbon atoms protonated might be unstable upon an excess electron detachment. Indeed, the neutral systems resulting from electron detachment from and evolve, along barrier-free decomposition pathways, to a linear or a bicyclo structure, respectively, which might be viewed as lesions to DNA.
2006. "Gaseous Arginine Conformers and Their Unique Intramolecular Interactions." Journal of Physical Chemistry A 110(44):12282-12291. doi:10.1021/jp0645115 Abstract Extensive ab initio calculations were employed to characterize stable conformers of gaseous arginine, both the canonical and zwitterionic tautomers. Step-by-step geometry optimizations of possible single-bond rotamers at the B3LYP/6-31G(d), B3LYP/6-31++G(d,p), and MP2/6-31++G(d,p) levels yield numerous structures that are more stable than any known ones. The final electronic energies of the conformers were determined at the CCSD/6-31++G(d,p) level. The lowest energies of the canonical and zwitterionic structures are lower than the existing values by 2.0 and 2.3 kcal/mol, respectively. The relative energies, rotational constants, dipole moments, and harmonic frequencies of the stable conformers remain for future experimental verification. The conformational distributions at various temperatures, estimated according to thermodynamic principles, consist almost exclusively of the newly found structures. One striking feature is the occurrence of blueshifting hydrogen bonds in all six of the most stable conformers. A unique feature of important conformations is the coexistence of dihydrogen and blue- and red-shifting hydrogen bonds. In addition to the hydrogen bonds, the stereoelectronic effects were also found to be important stabilization factors. The calculated and measured proton affinities agree within the theoretical and experimental uncertainties, affirming the high quality of our conformational search. The theoretical gas-phase basicity of 245.9 kcal/mol is also in good agreement with the experimental value of 240.6 kcal/mol. The extensive searches establish firmly that gaseous arginine exists primarily in the canonical and not the zwitterionic form.
2006. "Gaseous Arginine Conformers and Their Unique Intramolecular Interactions." Journal of Physical Chemistry A 110(44):12282-12291. doi:10.1021/jp0645115 Abstract Extensive ab initio calculations were employed to characterize stable conformers of gaseous arginine, both canonical and zwitterionic tautomers. Step-by-step geometry optimizations of possible single-bond rotamers at the B3LYP/6-31G(d), B3LYP/6-31++G(d,p) and MP2/6-31++G(d,p) levels yield numerous structures that are more stable than any known ones. The final electronic energies of the conformers were determined at the CCSD/6-31++G(d,p) level. The lowest energies of the canonical and zwitterionic structures are lower than the existing ones by 2.0 and 2.3 kcal/mol, respectively. The relative energies, rotational constants, dipole moments and harmonic frequencies of the stable conformers were given for future experimental verifications. The conformational distributions at various temperatures, estimated based upon the thermodynamic principles, consist almost exclusively of the newly found structures. One striking feature is the occurrence of the blue-shifting hydrogen bonds in all the six most stable conformers. A unique feature of important conformations is the coexistence of dihydrogen, blue- and red-shifting hydrogen bonds. In addition to the hydrogen bonds, the stereoelectronic effects were also found to be important stabilization factors. The calculated and measured proton affinities agree within the theoretical and experimental uncertainties, affirming high quality of our conformational search. The theoretical gas phase basicity of 245.9 kcal/mol is also in good agreement with the experimental value of 240.6 kcal/mol. The extensive searches establish firmly that gaseous arginine exists primarily in the canonical and not the zwitterionic form. Computing resources were available through a Computational Grand Challenge Application grant from the Molecular Sciences Computing Facility in the Environmental Molecular Sciences Laboratory. PNNL is operated by Battelle for the U.S. DOE under Contract DE-AC06-76RLO 1830.
2006. "Hydrogen Gets Onboard." Chemistry World (March 2006):, doi:www.rsc.org/chemistryworld/Issues/2006/March Abstract In this brief review we update progress in research efforts for on-board hydrogen storage for fuel cell powered vehicles. In addition to economic targets, the technological challenges are bounded by volumetric and gravimetric constraints. Specifically, an amoiunt of 4 kg of H2, required to propel a highly fuel efficient automobile for 500 kilometers, must fit into the space of a conventional gasoline tank. The volumetric constraints rule out compressed and liquefied H2 and teach us that hydrogen must be stored as a solid material, either by physi-sorption to high surface area materials or chemically bond (covalent or ionic) to light weight elements. Hydrogen stored on high surface area materials is weakly bound and general requires low temperatures to stabilize the hydrogen. On the other end, hydrogen covalently bound to light metals requires high temperatures to release the hydrogen. One interesting alterative is chemical hydrogen storage (CHS). CHS covers a broad range of materials but is defined as a process whereby the hydrogen is released by a chemical reaction. The reaction could be induced by hydrolysis, a reaction with water, or by thermolysis, heating to moderate temperatures to release hydrogen. The spent material can then be reprocessed or regenerated off-board. Battelle operates the Pacific Northwest National Laboratory for the US Department of Energy.
2005. "Electronic Structure Differences in ZrO2 vs. HfO2." Journal of Physical Chemistry A 109(50):11521-11525. Abstract While ZrO2 and HfO2 are, for the most part, quite similar chemically, subtle differences in their electronic structures appear to be responsible for differing MO2/Si (M = Zr, Hf) interface stabilities. In order to shed light on the electronic structure differences between ZrO2 and HfO2, we have conducted joint experimental/theoretical studies. Since electron affinities are a sensitive probe of electronic structure, we have measured them by conducting photoelectron spectroscopic experiments on ZrO2- and HfO2-. The electron affinity of HfO2 was determined to be 2.14 0.03 eV, while that of ZrO2 was determined to be 1.64 0.03 eV. Concurrently, electronic structure calculations were conducted to determine electron affinities, vibrational frequencies, and geometries of these systems. The calculated electron affinities of HfO2 and ZrO2 were found to be 2.05 and 1.62 eV, respectively. The molecular results confirm earlier predictions from solid phases that HfO2 is more ionic than ZrO2. The excess electron in MO2- occupies an sd-type hybrid orbital localized on the M atom (M=Zr, Hf). The structural parameters of ZrO2 and HfO2 were found to be very similar. The difference in geometries between the neutral and the anion is along the symmetrical stretching and bending modes. Together, these studies unveil significant differences in the electronic structures of ZrO2 and HfO2.
2005. "AT Base Pair Anions vs. (9-methyl-A)(1-methyl-T) Base Pair Anions." Journal of the American Chemical Society 127(17):6443-6450. doi:10.1021/ja050724g Abstract The anionic base pairs of adenine and thymine, (AT)-, and 9-methyladenine and 1-methylthymine, (MAMT)-, have been investigated both theoretically and experimentally in a complementary, synergistic study. Calculations on (AT)- found that it had undergone a barrier-free proton transfer (BFPT) similar to that seen in other dimer anion systems and that its structural configuration that was neither Watson-Crick (WC) nor Hoogsteen (HS). The vertical detachment energy (VDE) of (AT)- was determined by anion photoelectron spectroscopy and found to be in agreement with the VDE value predicted by theory for the BFPT mechanism. An AT pair in DNA is structurally immobilized into the WC configuration, in part, by being bonded to the sugars of the double helix. This circumstance was mimicked by methylating the sites on both A and T where these sugars would have been tied, viz., 9-methyladenine and 1-methylthymine. Calculations found no BFPT in (MAMT)- and a resulting (MAMT)- configuration that was either HS or WC, with the configurations differing in stability by ca. 2 kcal/mol. The photoelectron spectrum of (MAMT)- occurred at a completely different electron binding energy than had (AT)-. Moreover, the VDE value of (MAMT)- was in agreement with that predicted by theory. The configuration of (MAMT)- and its lack of electron-induced proton transfer are inter-related. While there may be other pathways for electron-induced damage, BFPT in the WC/HS configurations of (AT)- is not feasible.
2005. "Low-temperature Polymorphs of ZrO2 and HfO2. A Density Functional Theory Study ." Physical Review. B, Condensed Matter and Materials Physics 72(14):144107. Abstract We present density functional calculations of the total energies and equations of state of the monoclinic, tetragonal, cubic, orthorhombic-I (Pbca) and orthorhombic-II (cotunnite)-structure phases of zirconia and hafnia in the local density (LDA) and generalized-gradient (GGA) approximations. The accuracy of the LDA approximation is not sufficient and GGA corrections are critical to obtain low-temperature phase transitions under pressure that are consistent with experiment, i.e., (monoclinic‡ orthorhombic-I ‡ cotunnite). The GGA values of the bulk modulus of the cotunnite phase were found to be 251 and 259 GPa for ZrO2 and HfO2, respectively. We developed a new population analysis scheme in which atomic radii are adapted to the actual charge distribution in the material. The results indicate that the effective atomic radius of Hf is smaller than that of Zr, which is a drastic manifestation of the relativistic lanthanide contraction. The population analysis results demonstrate that ionicity: (i) increases from the monoclinic to the cotunnite phase, and (ii) is larger for HfO2 than for ZrO2. This variable ionicity may be the reason why LDA fails to describe the relative stability of different polymorphs. The bandgap and heat of formation are also larger for monoclinic HfO2 than for ZrO2 by 0.6 eV and 0.60 eV/formula unit, respectively. The tetragonal phase, which often exists as a metastable phase at ambient conditions, has a bandgap larger than the monoclinic phase by 0.35 and 0.65 eV for ZrO2 and HfO2, respectively.
2005. "Valence and dipole-bound anions of the most stable tautomers of guanine." Journal of the American Chemical Society 127(2):699-706. Abstract Anionic states of guanine, which is the only nucleic acid base the anions of which have not yet been studieded in either photoelectron spectroscopic (PES) or Rydberg electron transfer (RET) experiments, have been characterized for the four most stable tautomers of neutral guanine using a broad spectrum of electronic structure methods from density functional theory with the B3LYP exchange-correlation functional to coupled cluster method with single, double, and perturbative triple excitations. Both valence and dipole-bound anionic states were addressed. We identified some of the difficulties facing future PES or RET experiments on the anion of guanine. Even if guanine is successfully transferred to the gas phase without thermal decomposition, it is critical to have the canonical amiono-oxo, and both amino-hydroxy tautomers in the beam, not only the most stable, a non-canonical, amino-oxo tautomer, as the latter does not support a bound anionic state. We also suggested a scheme for enrichment of gas phase guanine with the canonical tautomer, which is not the most stable in the gas phase, but which is of main interest due to its biological relevance. Only the tautomers GN7H and GHN7H support vertically bound valence anionic states with the CCSD(T) value of vertical detachment energy of +0.21 and +0.39 eV, respectively. These anionic states are, however, adiabatically unbound. The vertical electronic stability of these valence anionic states is accompanied by serious “buckling” of the molecular skeleton, in particular in the region of the 5-member ring, where the excess electron is primarily localized. The G and GHN7H tautomers support dipole-bound states with the CCSD(T) values of adiabatic electron affinity of 65 and 36 meV, respectively. A contribution from higher-then-second-order correlation terms represents, respectively, 48 and 68% of the total vertical electron detachment energy determined at the CCSD(T) level.
2005. "Stabilization of very rare tautomers of 1-methylcytosine by an excess electron." Journal of Physical Chemistry A 109(50):11495-11503. Abstract We characterized valence anionic states of 1-methylcytosine using various electronic structure methods. We found that the most stable valence anion is related to neither the canonical amino-oxo nor a rare imino-oxo tautomer, in which a proton is transferred from the N4 to N3 atom. Instead, it is related to an imino-oxo tautomer, in which the C5 atom is protonated. This anion is characterized by an electron vertical detachment energy (VDE) of 2.12 eV and it is more stable than the anion based on the canonical tautomer by 1.0 kcal/mol. The latter is characterized by a VDE of 0.31 eV. Another unusual low-lying imino-oxo tautomer with a VDE of 3.60 eV has the C6 atom protonated and is 3.6 kcal/mol less stable than the anion of the canonical tautomer. All these anionic states are adiabatically unbound with respect to the canonical amino-oxo neutral, with the instability of 5.8 kcal/mol for the most stable valence anion. The mechanism of formation of anionic tautomers with carbon atoms protonated may involve intermolecular proton transfer or dissociative electron attachment to the canonical neutral tautomer followed by a barrier-free attachment of a H atom to the C5 or C6 atom. The six-member ring structure is unstable upon an excess electron detachment. It collapses without a barrier to a linear or a bicyclo structure. Within the PCM hydration model, the anions become adiabatically bound with respect to the corresponding neutrals, and the tautomer with C6 protonated becomes the most stable, being followed by the tautomer with C5 protonated.
2005. "Intermolecular proton transfer in anionic complexes of uracil with alcohols." Journal of Physical Chemistry B 109(27):13383-13391. Abstract A series of eighteen alcohols (ROH) has been designed with an enthalpy of deprotonation (HDP) in a range of 13.8-16.3 eV. The effects of excess electron attachment to the binary alcohol-uracil (ROH…U) complexes have been studied at the density functional level with a B3LYP exchange-correlation functional and at the second order Møller-Plesset perturbation theory level. The photoelectron spectra of anionic complexes of uracil with three alcohols (ethanol, 2,2,3,3,3-pentafluoroethanol and 1,1,1,3,3,3-hexafluoro-2-propanol) have been measured with 2.54 eV photons. For ROHs with deprotonation enthalpies larger than 14.8 eV only the ROH...U- minimum exists on the potential energy surface of the anionic complex. For alcohols with deprotonation enthalpies in a range of 14.3-14.8 eV two minima might exist on the anionic potential energy surface, which correspond to the RO-…HU. and ROH...U- structures. For ROHs with deprotonation enthalpies smaller than 14.3 eV, the excess electron attachment to the ROH…U complex always induces a barrier-free proton transfer from the hydroxyl group of ROH to the O8 atom of U, with the product being RO-…HU.. A driving force for the intermolecular proton transfer is to stabilize the excess negative charge localized on a * orbital of uracil. Therefore, these complexes with proton transferred to the anionic uracil are characterized by larger values of electron vertical detachment energy (VDE). The values of VDE for anionic complexes span a range from 1.0 to 2.3 eV and roughly correlate with the acidity of alcohols. However, there is a gap of ~0.5 eV in the values of VDE, which separates the two families, ROH...U- and RO-…HU., of anionic complexes. The energy of stabilization for the anionic complexes spans a range from 0.6 to 1.7 eV and roughly correlates with the acidity of alcohols. The measured photoelectron spectra are in good agreement with the theoretical predictions.
2005. "Finding Adiabatically Bound Anions of Guanine through a Combinatorial Computational Approach." Angewandte Chemie International Edition 44(40):6585-6588. doi:10.1002/anie.200501671 Abstract In summary, guanine supports many adiabatically bound valence anions, which result from enamine-imine transformations of the most stable neutral tautomers. These stable anionic tautomers have been found using combinatorial-computational prescreening at the B3LYP level of theory followed by CCSD(T)/aug-cc-pVDZ calculations. The new anionic tautomers contradict a common opinion that guanine has the lowest electron affinity among nucleobases. The new anionic tautomers might be formed in the course of dissociative electron attachment followed by a hydrogen atom attachment to a carbon atom. They might affect the structure and properties of DNA and RNA exposed to low-energy electrons. Chemical transformations of DNA triggered by the new anionic tautomers will be explored in our future studies.
2005. "Nano-Scaffold Mediates Hydrogen Release and Reactivity of Ammonia Borane." Angewandte Chemie International Edition 44(23):3578-3582. Abstract One of the imposing barriers to realizing the promise of an energy economy based on hydrogen is onboard hydrogen storage for fuel-cell-powered vehicles. New materials that enable the release of dense, plentiful and pure hydrogen at temperatures less than 85 ºC are necessary to move the world from an oil-based economy to a hydrogen economy. We report a novel approach in which we deposit a hydrogen-rich material into a nanoporous scaffold. The role of the scaffold is to impose a nano-phase structure on the hydrogen-rich material thus providing an additional handle on the kinetics and thermodynamics of hydrogen release. We demonstrate on the example of ammonia borane infused in the nanoporous silica that the kinetics of hydrogen release is improved while the purity of hydrogen is increased in comparison with the release from bulk ammonia borane. These findings suggest that hydrogen rich materials infused in nanoscaffolds offer the most promising approach to date for onboard hydrogen storage
2005. "Role of Water in Electron-Initiated Processes and Radical Chemistry: Issues and Scientific Advances." Chemical Reviews 105(1):355-389. doi:10.1021/cr030453x Abstract An understanding of electron-initiated processes in aqueous systems and the subsequent radical chemistry these processes induce is significant in such diverse fields as waste remediation and environmental cleanup, radiation processing, nuclear reactors, and medical diagnosis and therapy. We review the state of the art in the physical chemistry and chemical physics of electron-initiated processes in aqueous systems and raise critical research issues and fundamental questions that remain unanswered.
2005. "Thermodynamic Properties of Molecular Borane Amines and the [BH4-][NH4+] salt for Chemical Hydrogen Storage Systems from Ab Initio Electronic Structure Theory." Journal of Physical Chemistry A 109(23):5129-5135. doi:10.1021/jp0445627 Abstract The heats of formation for the boron amines BH3NH3, BH2NH2, and HBNH, tetrahedral BH4-, and the BN molecule have been calculated by using ab initio molecular orbital theory. Coupled cluster calculations with perturbative triples (CCSD(T)) were employed for the total valence electronic energies. Correlation consistent basis sets were used, up through the augmented quadruple zeta, to extrapolate to the complete basis set limit. Core/valence, scalar relativistic, and spin-orbit corrections were included in an additive fashion to predict the atomization energies. Geometries were calculated at the CCSD(T) level up through at least aug-cc-pVTZ and frequencies were calculated at the CCSD(T)/aug-cc-pVDZ level. The heats of formation at 0K in the gas phase are ΔHf(BH3NH3) = -9.1, ΔHf(BH2NH2) = -15.9, ΔHf(BHNH) = 13.6, ΔHf(BN) = 146.4, and ΔHf(BH4-) = -11.6 kcal/mol. The reported experimental value for ΔHf(BN) is clearly in error. The heat of formation of the salt [BH4-]NH4+] (s) has been estimated by using an empirical expression for the lattice energy and the calculated heats of formation of the two component ions. The calculations show that both BH3NH3(g) and [BH4-]NH4+](s) can serve as good hydrogen storage systems which release H2 in a slightly exothermic process. The hydride affinity of BH3 is calculated to be 72.2 kcal/mol in excellent agreement with the experimental value at 298K of 74.2 ± 2.8 kcal/mol.
2005. "Interaction with Glycine Increases Stability of a Mutagenic Tautomer of Uracil. A Density Functional Theory Study." Journal of the American Chemical Society 127(7):2238-2248. Abstract The most stable structures for complexes of minor tautomers of uracil (U) with glycine (G) were characterized at the density functional B3LYP/6-31++G** level of theory. These are cyclic structures stabilized by two hydrogen bonds. The relative stability of isolated tautomers of uracil was rationalized by using thermodynamic and structural arguments. The stabilization energies for complexes between tautomers of U and G result from interplay between stabilizing two-body interaction energies and destabilizing one-body terms. The latter are related to the energies of: (i) tautomerization of the unperturbed moieties and (ii) distortions of the tautomers in the complex. The two-body term is related to the interaction energy between distorted tautomers. The two-body interaction energy term correlates with perturbations of length of the Y-H proton-donor bonds as well as with deprotonation enthalpies and proton affinities of the appropriate monomers sites. It was demonstrated that the relative instability of rare tautomers of uracil is diminished due to their interactions with glycine. In particular, the instability of the third most stable tautomer is decreased from 11.9 kcal/mol for an isolated uracil to 6.7 kcal/mol in a complex with the zwitterionic tautomer of glycine. A decrease of instability by 5.2 kcal/mol in the free enthalpy scale could result in an increase of concentration of this tautomer by almost five orders of magnitude. Such a concentration falls in the mutationally significant range. The increased stability of the third tautomer of uracil, and probably thymine, can be relevant for point mutations because its proton donor and acceptor sites are complementary with those of guanine rather than these of adenine.
2005. "DNA strand breaks induced by concerted interaction of H radicals and low-energy electrons. A computational study of the nucleotide of cytosine." European Physical Journal. D, Atomic, molecular and optical physics. 35(2):429-435. Abstract We propose a mechanism of DNA single strand breaks induced by low-energy electrons. Density functional theory calculations have been performed on a nucleotide of cytosine to identify barriers for the phosphate-sugar O-C bond cleavage. Attachment of the first excess electron induces intermolecular proton transfer to cytosine. The resulting neutral radical of hydrogenated cytosine binds another excess electron, and the excess charge is localized primarily on the C6 atom. A barrier based of free enthalpies of 4.2 kcal/mol is encountered for proton transfer from the C2’ atom of the adjacent sugar unit to the C6 atom of cytosine. The proton transfer is followed by a barrier-free sugar-phosphate C-O bond cleavage. The rate of the C-O bond cleavage in the anion of hydrogenated nucleotide of cytosine is estimated to be in a range 5x108 to 2x1010 s-1, which makes the proposed mechanism probable to take place in DNA. The process proceeds through bound anionic states, not through metastable states with finite lifetimes and discrete energy positions with respect to the neutral target. The results suggest that even very low-energy electrons may cleave the DNA backbon
2005. "Stabilization of very rare tautomers of uracil by an excess electron." Physical Chemistry Chemical Physics. PCCP 7(10):2116-2125. Abstract We characterized valence-type and dipole-bound anionic states of uracil using various electronic structure methods. We found that the most stable anion is related to neither the canonical 2,4-dioxo nor a rare imino-hydroxy tautomer. Instead, it is related to an imino-oxo tautomer, in which the N1H proton is transferred to the C5 atom. This valence anion, , is characterized by an electron vertical detachment energy (VDE) of 1267 meV and it is adiabatically stable with respect to the canonical neutral by 3.93 kcal/mol. It is also more stable by 2.32 and 5.10 kcal/mol than the dipole-bound ( ) and valence anion ( ), respectively, of the canonical tautomer. The VDE values for and are 73 and 506 meV, respectively. Another, anionic, low-lying imino-oxo tautomer with a VDE of 2499 meV has a proton transferred from N3H to C5 ( ). It is less stable than the neutral canonical tautomer by 1.38 kcal/mol. The mechanism of formation of anionic tautomers with the carbon C5 protonated may involve intermolecular proton transfer or dissociative electron attachment to the canonical neutral tautomer followed by a barrier-free attachment of a hydrogen atom to C5. The six-member ring structure of anionic tautomers with carbon atoms protonated might be unstable upon an excess electron detachment. Indeed, the neutral systems resulting from electron detachment from and evolve along barrier-free decomposition pathways to a linear or a bicyclo structure, respectively, which might be viewed as lesions to RNA. Within the PCM hydration model, the low-lying valence anions become adiabatically bound with respect to the canonical neutral; remains the most stable, being followed by , and .
2005. "Anion of the Formic Acid Dimer as a Model for Intermolecular Proton Transfer Induced by a [pi]* Excess Electron." Journal of Chemical Physics 122(20):article #204304. Abstract The neutral and anionic formic acid dimers have been studied at the second order Møller-Plesset and coupled cluster level of theory with single, double, and perturbative triple excitations with augmented, correlation consistent basis sets of double- and triple-zeta quality. Scans of the potential energy surface for the anion were performed at the density functional level of theory with a hybrid B3LYP functional and a high quality basis set. Our main finding is that the formic acid dimer is susceptible to intermolecular proton transfer upon an excess electron attachment. The unpaired electron occupies a * orbital, the molecular moiety that accommodates an excess electron “buckles”, and a proton is transferred to the unit where the excess electron is localized. In consequence of these geometrical transformations the electron vertical detachment energy becomes substantial, 2.35 eV. The anion is barely adiabatically unstable with respect to the neutral at 0 K. However, at standard conditions and in terms of Gibbs free energy, the anion is more stable than the neutral by 0.04 eV. The neutral and anionic dimers display different IR characteristics. In summary, the formic acid dimer can exist in two quasidegenerate states (neutral and anionic), which can be viewed as “zero” and “one” in the binary system. These two states are switchable and distinguishable.
2004. "The SrTiO3/Si(001) epitaxial interface: A density functional theory." Physical Review. B, Condensed Matter 70:165319. Abstract The Sr and SrO monolayers on Si(001) and the SrTiO3/Si(001) interface have been simulated by means of total energy minimization within DFT (Density Functional Theory) formalism and GGA (Generalized Gradient Approximation) form of exchange-correlation potential. It has been found that the first SrO layer restores a 1´1 structure of the substrate thus providing a template for subsequent epitaxy of the SrTiO3 layers. The calculated densities of states are in good agreement with recent XPS and UPS valence band spectra. The role of the “buffer layer” in forming electronic structure is discussed and illustrated with an example of SrO monolayer at the SrTiO3 – Si(001) interface.
2004. "First-principles study of noncommutative band offsets at [alpha]-Cr2O3/[alpha]-Fe2O3(0001) interfaces." Physical Review. B, Condensed Matter and Materials Physics 69(20):205106. Abstract Using first-principles density functional theory, we have modeled the atomic, electronic and magnetic structure of epitaxial interfaces between alpha-hematite and alpha-chromia (corundum structure) in the hexagonal (0001) basal plane. Our model was a superlattice with a period of about 27.5Å, corresponding to the shortest-period superlattice considered in a recent series of experiments (Chambers et al., Phys. Rev. B 61, 13223 (2000)). Two different epitaxial interface structures were studied: (i) an oxygen plane separating an Fe double layer from a Cr double layer, or (ii) a metal double layer split between Fe and Cr. We found that these two structures are close in total energy but have distinct spin structure and different valence band offsets (chromia above hematite by 0.4 and 0.6 eV for (i) and (ii) respectively), possibly explaining the experimental non-commutative band offset seen in this system (0.3±0.1 eV for hematite grown atop chromia, and 0.7±0.1 eV for the reverse).
2004. "Structural Criteria for the Rational Design of Selective Ligands: Convergent Hydrogen Bonding Sites for the Nitrate Anion." Journal of the American Chemical Society 126(25):7925-7934. Abstract Molecular hosts for anion complexation are often constructed by combining two or more hydrogen bonding functional groups, D–H. The deliberate design of complementary host architectures requires knowledge of the optimal geometry for the hydrogen bonds formed between the host and the guest. Herein, we present a detailed study of the structural aspects of hydrogen bonding interactions with the NO3– anion. A large number of crystal structures are analyzed to determine the number of hydrogen bond contacts per anion and to further characterize the structural aspects of these interactions. Electronic structure calculations are used to determine stable geometries and interaction energies for NO3– complexes with several simple molecules possessing D–H groups, including water, methanol, N-methylformamide, and methane. Theoretical results are reported at several levels of density functional theory, including BP86/DN**, B3LYP/TZVP, and B3LYP/TZVP+, and at MP2/aug-cc-pVDZ. In addition, MP2 binding energies for these complexes were obtained at the complete basis set limit by extrapolating from single point energies obtained with larger correlation-consistent basis sets. The results establish that NO3– has an intrinsic hydrogen bonding topography in which there are six optimal sites for proton location. The structural features observed in crystal structures and in the optimized geometries of complexes are explained by a preference to locate the D–H protons in these positions. For the strongest hydrogen bonding interactions, the N–O•••H angle is bent at an angle of 115 ± 10°, and the hydrogen atom lies in the NO3– plane giving O–N–O•••H dihedral angles of 0 and 180°. In addition, the D-H vector points towards the oxygen atom, giving D–H•••O angles that are near linear, 170 ± 10°. Due to steric hindrance, simple alcohol O–H and amide N–H donors form 3:1 complexes with NO3–, with H•••O distances of 1.85 ± 0.5 Å. Thus, the optimal cavity radius for a tridentate host, defined as the distance from the center to the D–H hydrogen atoms, is 2.65 ± 0.15 Å.
2004. "Excess Electron Attachment Induces Barrier-Free Proton Transfer in Anionic Complexes of Thymine and Uracil with Formic Acid ." Journal of Physical Chemistry B 108: 6919-6921. Abstract The anionic complexes of formic acid with uracil and thymine reveal broad features in photoelectron spectroscopy (PES) experiments with maxima at 1.7 and 1.1 eV, respectively. The results of quantum chemical calculations suggest that electron vertical detachment energies (VDE) of 1.6-1.9 eV correspond to anionic structures in which a proton has been transferred from the carboxylic group of the formic acid to the O8 atom of uracil or thymine. Smaller values of VDE (0.8 to 1.3 eV) correspond to chemically untransformed complexes, in which anionic uracil or thymine interacts through two hydrogen bonds with the carboxylic group of the intact formic acid. The recorded spectra and the results of quantum chemical calculations suggest that both nucleic acid bases undergo barrier-free proton transfer in anionic complexes with formic acid. The difference in experimental spectra of UF- and TF- provides an indication that the methyl group of thymine could make a difference in the intermolecular proton transfer.
2004. "Effect of Hydrogen Bonding on Barrier-Free Proton Transfer in Anionic Complexes of Uracil with Weak Acids: (U…HCN)- versus (U…H2S)-." Israel Journal of Chemistry 44(1-3 ):157-170. Abstract A photoelectron spectrum is reported for an anionic complex of uracil (U) with HCN. The effects of electron attachment to a complex of U with HA (A = CN, NC) have been studied at the density functional theory level with 6-31++G** basis sets and the B3LYP and MPW1K exchange correlation functionals. Critical anionic structures have been reexamined at the MP2/6-31++G** level. The B3LYP gas phase deprotonation enthalpies are equal to 14.56, 15.13, and 15.12 eV for HNC, HCN, and H2S, respectively. The experimental deprotonation enthalpies are 15.1000.008 and 15.2140.125 eV for HCN and H2S, respectively. Hence, HCN and H2S have very similar deprotonation enthalpies. The photoelectron spectra of anionic complexes of uracil with HCN and H2S are, however, very different. The (UHCN)- spectrum reveals a broad feature with a maximum between 1.2-1.4 eV, whereas the main feature of the (UH2S)- spectrum has a maximum between 1.7 and 2.1 eV. We suggest that barrier-free proton transfer (BFPT) [Eur. Phys. J. D 2002, 20, 431-439; J. Phys. Chem. B 2003, 107, 7889-7895] occurs in the (UH2S)- complex but not in (UHCN)-. Critical factors for the occurrence of BFPT have been analyzed. The difference between the (UHCN)- and (UH2S)- complexes is attributed to differences in hydrogen bonds formed by HCN and H2S with uracil.
2004. "Barrier-Free Proton Transfer in Anionic Complex of Thymine with Glycine." Physical Chemistry Chemical Physics. PCCP 6(17):4351-4357. Abstract We report the photoelectron spectrum of the thymine-glycine anionic complex (TG-) recorded with low energy photons (2.540 eV). The spectrum reveals a broad feature with a maximum between 1.6-1.9 eV. The measured electron vertical detachment energy is too large to be attributed to a complex in which an anion of intact thymine is solvated by glycine, or vice versa. The experimental data are paralleled by electronic structure calculations carried out at the density functional theory level with 6-31++G** basis sets and the B3LYP and MPW1K exchange-correlation functionals. The critical structures are further examined at the second order Møller-Plesset level of theory. The results of calculations indicate that the excess electron attachment to the complex induces an intermolecular barrier-free proton transfer from the carboxylic group of glycine to the O8 atom of thymine. As a result, the four most stable structures of the thymine-glycine anionic complex can be characterized as a neutral radical of hydrogenated thymine solvated by an anion of deprotonated glycine. The calculated vertical electron detachment energies for the four most stable anionic complexes lie in a range 1.6-1.9 eV, in excellent agreement with the maximum of the photoelectron peak.
2004. "Barrier-Free Intermolecular Proton Transfer Induced by Excess Electron Attachment to the Complex of Alanine with Uracil ." Journal of Chemical Physics 120(13):6064-6071. Abstract The photoelectron spectrum of the uracil-alanine anionic complex (UA)- has been recorded with 2.540 eV photons. This spectrum reveals a broad feature with a maximum between 1.6-2.1 eV. The vertical electron detachment energy is too large to be attributed to an (UA)- anionic complex in which an intact uracil anion is solvated by alanine, or vice versa. The neutral and anionic complexes of uracil and alanine were studied at the B3LYP and second order Møller-Plesset level of theory with 6-31++G** basis sets. The neutral complexes form cyclic hydrogen bonds and the three most stable neutral complexes are bound by 0.72, 0.61 and 0.57 eV. The electron hole in complexes of uracil with alaninie is localized on uracil, but the formation of a complex with alanine strongly modulates the vertical ionization energy of uracil. The theoretical results indicate that the excess electron in (UA)- occupies a p* orbital localized on uracil. The excess electron attachment to the complex can induce a barrier-free proton transfer (BFPT) from the carboxylic group of alanine to the O8 atom of uracil. As a result, the four most stable structures of the uracil-alanine anionic complex can be characterized as the neutral radical of hydrogenated uracil solvated by the anion of deprotonated alanine. Our current results for the anionic complex of uracil with alanine are similar to our previous results for the anion of uracil with glycine [Eur. Phys. J. D 20, 431 (2002)], and together they indicate that the BFPT process is not very sensitive to the nature of the amino acid's hydrophobic residual group. The BFPT to the O8 atom of uracil may be relevant to the damage suffered by nucleic acid bases due to exposure to low energy electrons.
2004. "Experimental Determination of Valence Band Maxima for SrTiO3, TiO2, and SrO and the Associated Valence Band Offsets with Si(001)." Journal of Vacuum Science and Technology B--Microelectronics and Nanometer Structures 22(4):2205-2215. Abstract Abstract We address the issue of accurate determination of the energy at the top of the valence band for SrTiO₃(001) single crystals, as well as TiO₂(001) anatase and SrO epitaxial films. These measurements are of critical importance in determining valence band offsets for oxides in heterojunctions involving these materials. Three different methods are presented and compared: (1) fitting Gaussian broadened theoretical densities of states to x-ray excited valence band spectra, (2) finding the intersection of a regression line that spans the linear portion of the x-ray excited valence band leading edge with the background between the valence band maximum and the Fermi level, and, (3) determining the energy at which high-resolution ultraviolet photoemission intensity of the leading edge goes to zero. We find that method 1 is not reliable due to limitations in the accuracy of density functional theory when applied to these oxides. In contrast, methods 2 and 3 give physically reasonable results that are in good mutual agreement. The difference in VBM between method 1 and methods 2 & 3 is 0.4 – 0.6 eV, depending on the oxide. The true valence band maximum can be directly and accurately measured using methods 2 & 3 provided the experiment is carried out with adequate energy resolution. PACS numbers: 79.60.Jv a)Electronic mail: sa.chambers@pnl.gov
2004. "Accurate Valence Band Maximum Determination for SrTiO₃(001) ." Surface Science 554(2-3):81-89. Abstract We reexamine a well-established method for determining valence band maxima in semiconductors based on fitting photoemission spectra to theoretical densities of states. This technique is inaccurate for certain oxides because electronic structure methods predict too sharp a leading edge rise, which in turn appears to stem from an underestimation of the extent of metal-oxygen hybridization at the top of the valence band. In contrast, extrapolating the x-ray excited leading edge to the energy axis, in combination with the energy at which the UV-excited leading edge intensity goes to zero, yields consistent and reliable results that are useful for accurate band offset determinations.
2003. "Consequences of Proton Transfer in Guanidine." Journal of Physical Organic Chemistry 16(2):91-106. Abstract Consequences of proton-transfer reactions in guanidine in the solid state, solution and gas phase are discussed. Y-delocalization, resonance and symmetry strongly influence the basicity of guanidine in the gas phase. These effects are however insufficient to explain the basicity of guanidine which in aqueous solution is stronger than that of trisubstituted alkylamines and proton sponge (DMAN). The intrinsic (gas-phase) basicity of guanidine is close to that of triethylamine. The large difference between the basicity of amines and guanidine in solution is attributed to the important role played by effects such as polarizability, internal and external solvation.
2003. "Excess electron attachment induces barrier-free proton transfer in binary complexes of uracil with H2Se and H2S but not with H2O." Journal of Physical Chemistry B 107(31):7889-7895. Abstract The photoelectron spectrum of the uracil-H2S anionic complex (UH2S)- has been recorded with 2.540 eV photons. Unlike the (uracil-H2O)- spectrum, which displays a broad feature with maximum at about 0.9 eV, the (UH2S)- spectrum reveals a broad feature with a maximum between 1.7 and 2.1 eV. The latter vertical detachment energy value is too large to be attributed to an (UH2S)- complex in which an intact uracil anion is solvated by H2S. The effects of electron attachment to the UH2A complexes (A Se,S, O) have been studied at the density functional theory level with the B3LYP and MPW1K exchange correlation functionals as well as at the second-order Møller-Plesset perturbation theory level. The three acids cover a broad range of acidity with calculated gas-phase deprotonation enthalpies being equal to 14.8, 15.1, and 16.9 eV for H2Se, H2S, and H2O, respectively. In the case of H2Se and H2S, electron attachment is predicted to induce a barrier-free proton transfer (BFPT) from the acid to the O8 atom of uracil, with the product being the radical of hydrogenated uracil bound to AH-. No BFPT is predicted for the anion of uracil with H2O. Critical factors for the occurrence of BFPT have been analyzed, and the role of the stabilizing interaction between the hydrogenated uracil and the deprotonated acid has been discussed. Four structures have been considered for every UH2A complex, and their relative stabilities are different for the neutral and anionic species. The increased stabilities of anionic complexes that undergo BFPT can be related to the properties of the second hydrogen bond (C5HâââA or N1(3)HâââA). In comparison with the case of neutral structures, this bond is weakened for anionic structures without BFPT and strengthened for those with BFPT.
2003. "Formation of the c(1X1) Cu Monolayer on CaO(100): A Theoretical Study." Physical Review. B, Condensed Matter 6815(15):, doi:10.1103/PhysRevB.68.155411 Abstract The Cu overlayer adsorbed on CaO(100) was studied at the density functional level of theory. Following an experimental suggestion, we examined a complete c(1X1) structure with copper atoms adsorbed on every hollow site of CaO(100). The binding energy/atom for this structure is 3.19 eV. However, another c(1X1) structure proposed by us, with copper atoms on every surface Ca and O atoms, displayed a larger binding energy of 3.37 eV. When the copper coverage is reduced by half, the preferred adsorption site fo rhte Cu overlayer is above the O site and the dominant mechanism for the metal adhesion is rehybridization of the metal 3d and surface O 2p bands. Our results suggest a delicate balance between the formation of a monolayer film and 3D particles on the substrate.
2002. "Electron Binding Energies of Dipole-Bound Anions at the Coupled Cluster Level with Single, Double, and Triple Excitations: HCN- and HNC-." Journal of Chemical Physics 116(8):3297-3299. Abstract The electron binding energies for the weak dipole-bound anions HCN- and HNC- were found to be 13.2 and 35.7 cm-1, respectively, at the coupled cluster level of theory with single, double, and triple excitations [CCSDT]. A more approximate approach, in which the triples contribution is treated perturbatively [CCSD(T)], provides an electron binding energy which is underestimated for HCN- by 25% and overestimated for HNC- by 19%. The new results provide benchmarks for model potentials aiming to reproduce dynamical correlation effects in electron-molecule interactions.
2002. "Thermodynamic Stability of High-K Dielectric Metal Oxides ZrO2 and HfO2 in Contact with Si and SiO2." Applied Physics Letters 80(11):1897-1899. Abstract We present theoretical and experimental results regarding the thermodynamic stability of the high-k dielectrics MO2 (M = Zr and Hf) in contact with Si and SiO2. The HfO2/Si interface is found to be stable with respect to formation of silicides whereas the ZrO2/Si interface is not. The MO2/SiO2 interface is marginally unstable with respect to formation of silicates. Cross-sectional transmission electron micrographs expose formation of nodules, identified as silicides, across the polysilicon/ZrO2/Si interfaces but not for the interfaces with HfO2. For both ZrO2 and HfO2, the X-ray photoemission spectra illustrate formation of silicate-like compounds in the MO2/SiO2 interface.
2002. "Solvated Electrons in Very Small Clusters of Polar Molecules: (HF)(3)(-): art. no. 143001." Physical Review Letters 88(14):28-31. Abstract A cluster of polar molecules can host an excess electron in at least two ways. First, the excess electron can be tethered to the cluster by its interaction with the cluster?s dipole moment. , Second, the electron can localize inside the cluster, bulk analogs being the hydrated and ammoniated electrons. - While the structural reorganization of the cluster, due to attachment of an excess electron, is typically small for dipole-bound electrons (dbe), it is usually quite significant for "solvated electrons" (se), since the solvation occurs at the expense of breaking of pre-existing hydrogen bonds. The se structures, however, provide more contact interactions between the polar molecules and the excess electron. For these reasons, it is often assumed that dbe's dominate for small polar clusters, whereas large clusters form se's. Here we show that dbe's and se's coexist in as small a cluster as (HF)3-. The stability of these anions with respect to the neutral cluster results not only from the excess electron binding energy but also from favorable entropic effects, which reflect the greater "floppiness" of the anionic structures.
2002. "Computational Study of Hydrogen-Bonded Complexes Between the Most Stable Tautomers of Glycine and Uracil." Journal of Physical Chemistry A 106(32):7423-7433. Abstract Twenty-three hydrogen bonded complexes between the lowest energy tautomers of uracil and glycine have been characterized at the density functional level of theory with a hybrid B3LYP exchange-correlation functional and 6-31++G** basis sets. The most stable complexes are formed when the carboxylic group of glycine is bound through two hydrogen bonds with a NH proton donor and an O proton acceptor of uracil, and stabilization energies for these cyclic structures span a range of 12.3-15.6 kcal/mol. Interplay between the topological match among proton donor and acceptor sites involved in cyclic structures and their preference to form single hydrogen bonds, measured by the values of proton affinity and deprotonation enthalpy, has been discussed. Upon formation of a uracil-glycine complex, the elongations of proton donor bonds and vibrational red shifts for proton donor stretching modes can reach 0.05 ? and 650 cm-1, respectively. These perturbations of proton donor bonds correlate with the magnitude of two-body interaction energy terms. A qualitative correlation was demonstrated between the values of proton affinity and deprotonation enthalpy of the sites involved in hydrogen bonds and the values of both the two-body interaction energy term and elongation of the proton donor bond.
2001. "Quasidegeneracy of Zwitterionic and Canonical Tautomers of Arginine Solvated by an Excess Electron ." Journal of the American Chemical Society 123(44):11073-11074. Abstract We demonstrated that the 3.12 kcal/mol instability of the zwitterion form of neutral arginine in the gas phase relative to the canonical tautomer could be suppressed by attaching an excess electron. This solvation by an excess electron provides and extra stabilization of 7.3 kcal/mol for the zwitterion structure and only 3.7 kcal/mol for the canonical structure. Hence, the anions based on the zwitterionic and canonical structures become quasidegenerate with their electronic energies, determined at the CCSD level of theory and corrected for zero-point vibrational energies determined at the B3LYP level, differing by less than 0.4 kcal/mol. Higher-order treatment of electron correlation effects and more complete one electron basis sets may be required to identify the global minimum for the anion. Moreover, thermal effects will contribute significantly to the stability of different anionic tautomers and rotamers. The effect of electron attachment/detachment on tautomerization reactions is currently investigated.
2001. "Non-Ionic and Zwitterionic Forms of Neutral Arginine - an ab initio Study." Chemical Physics Letters 337(1-3):143-150. Abstract The properties of six carefully selected structures of arginine have been studied using electronic structure methods. The relative energies were determined at several levels of theory including the CCSD/6-31++G(d,p)+5(sp)//MP2/6-31++G(d,p)+5(sp) level and corrected for zero-point vibrational contribution determined at the B3LYP level. Two new non-ionic structures were identified, one of which is lower (by 1.75 kcal/mol) than any previously reported structure. Two new conformers of the zwitterion were identified and are lower in energy than any previously reported zwitterion. The lowest non-ionic structure is lower in energy than the lowest zwitterion by 2.8 kcal/mol at our highest level of theory. The energy orderings of these structures vary somewhat with the basis set and level of theory, but for no basis or theory level is a zwitterion structure suggested to be the global minimum. Somewhat surprisingly, dipole moments of ca. 9 D are displayed by the lowest energy zwitterion and one of the non-ionic structures, but the Koopmans? vertical electron attachment energy is 2.4 times larger for the former than for the latter.
2001. "Ab Initio Electronic Structure of HCN- and HNC- Dipole-Bound Anions and a Description of Electron Loss upon Tautomerization." Journal of Chemical Physics 114(17):7443-7449. Abstract The binding of an excess electron to HCN and HNC was studied at the coupled cluster level of theory with single, double, and noniterative triple excitations and with extended basis sets to accommodate the loosely bound excess electron. The HCN molecule, with a dipole moment of 3.05 Debye, binds an electron by 10 cm-1, whereas the HNC tautomer possesses a similar dipole moment (3.08 Debye) and binds the electron by 43 cm-1. The electronic stability of the anionic system along the minimum energy HCN<->HNC tautomerization path has been investigated, and it was concluded that the excess electron autodetaches during the tautomerization. Unusually large electron correlation energy contributions to the total electron binding energy were found and are discussed.
2001. "Photoinduced Non-Adiabatic Dynamics in Quartet Na3 and K3 Formed Using Helium Nanodroplet Isolation." Journal of Chemical Physics 115(22):10265-10274. Abstract Helium nanodroplet isolation is used to produce van der Waals-bound quartet state alkali trimers (Na3 and K3) selectively over the corresponding chemically bound doublet trimers. Frequency resolved excitation and emission spectroscopy reveals the presence of non-adiabatic spin flip processes in the electronically excited states. A total of four quartet to quartet electronic transitions are observed: the 24E*,14E *14A2* transitions of Na3 and the 14A1**,24E**14A2* transitions of K3. Time resolved spectroscopy reveals that intersystem crossing times in Na3 decrease from 1.4 ns after excitation of the 0-0 band to approximately 400 ps for the higher vibronic levels (3, 5/2). Analysis of the resonant quartet fluorescence reveals that the excited electronic state cools vibrationally on a timescale that is comparable with, but slower than the intersystem crossing time.
2001. "Low Energy Tautomers and Conformers of Neutral and Protonated Arginine." Journal of the American Chemical Society 123(47):11695-11707. Abstract The relative stabilities of zwitterionic and canonical forms of neutral arginine and of its protonated derivative were studied using ab initio electronic structure methods. Trial structures were first identified at the PM3 level of theory using a genetic algorithm to systematically vary geometrical parameters. Further geometry optimizations of these structures were performed at the MP2 and B3LYP levels of theory with basis sets of the 6-31++G** quality. The final energies were determined at the CCSD/6-31++G** level and corrected for thermal effects determined at the B3LYP level. Two new non-zwitterionic structures of the neutral were identified, and one of them is the lowest energy structure found so far. The five lowest energy structures of neutral arginine are all non-zwitterionic in nature and are clustered within a narrow energy range of 2.3 kcal/mol. The lowest energy zwitterion structure is less stable than the lowest non-zwitterion structure by 4.0 kcal/mol. For no level of theory is a zwitterion structure suggested to be the global minimum. The calculated proton affinity of 256.3 kcal/mol and gas-phase basicity of 247.8 kcal/mol of arginine are in good agreement with the measured values of 251.2 and 240.6 kcal/mol, respectively. The calculated vibrational characteristics of the low-energy structures of neutral arginine provide an alternative interpretation of the IR-CRLAS spectrum (Chapo et al., J. Am. Chem. Soc. 1998, 120, 12956-12957).
2001. "An ab initio Study of the Betaine Anion - Dipole-Bound Anionic State of a Model Zwitterion System." Journal of Chemical Physics 114(24):10673-10681. Abstract The electron binding to a model zwitterionic molecule, betaine, is studied at the coupled cluster level of theory with single, double, and noniterative triple excitations as well as at the density functional theory level. Our results indicate that there is only one stable conformer of betaine with a dipole moment of 11.5 D. This dipole moment supports a relatively strongly bound anionic state and the vertical electron detachment energy was found to be 2261 cm-1. Furthermore, we predict the excess electron binding energy for the betaine analog with a blocked canonical structure (N,N-dimethylglycine methyl ester) to be less than 100 cm-1. This significant difference in electron binding energies suggests that the gas phase instability of zwitterions of some common naturally occurring amino acids with respect to their canonical forms may be reversed be the excess electron attachment. The calculated Franck-Condon factors for the betaine's anion/neutral pair suggest that the photoelectron spectrum of the anion will display only a very weak vibrational structure.
2000. "Modeling of Structure and Reactivity of Model Oxide Surfaces from All-Electron Density Functional Theory Calculations with Periodic Boundary Conditions." In Recent Research Developments in Physical Chemistry, vol. 4 (2000), Part II, ed. S.G. Pandalai, pp. 195-218. Transworld Research Network, Trivandrum, India. Abstract A primary objective in heterogeneous catalysis science is to correlate the atomic level properties of the catalysts to their observed macroscopic behavior. A wide variety of surface techniques and sophisticated theoretical approaches have been employed to realize this goal. Ab initio calculations based on a density-functional all electon approach using localized Gaussian basis sets have been used by us to study some model systems: the clean surfaces of MgO(100) and CaO(100), absorbed CO on MgO(100) and Ca(100), a simple model of supported metal catalysts consisting of alkali atom monolayer on MgO(100), and finally the interaction of CO with these model supported metal catalysts. In this article, we summarize the results obtained from these representatives case studies and discuss our contribution in the area of theoretical evaluation of the surface structure and reactivity of alkaline-earth metal oxides.
2000. "Periodic Density Functional LDA and GGA Study of CO Adsorption at the (001) Surface of MgO ." Journal of Physical Chemistry B 104(19):4717-4722. Abstract There is no abstract currently available for this item
2000. "LDA and GGA Calculations of Alkali Metal Adsorption at the (001) Surface of MgO." Journal of Chemical Physics 112(6):3014-3022. Abstract N/A
2000. "Adsorption of CO on MgO Supported Alkali Monolayers: A Periodic Density Functional Local Density Approximation and Generalized Gradient Approximation Study." Surface Science 445(2-3):495-505. Abstract N/A
2000. "On the Possibility of Binding of Two Electrons to Dipole Potentials." International Journal of Quantum Chemistry 76(2):197-204.
2000. "How to Choose a One-Electron Basis Set to Reliably Describe a Dipole-Bound Anion ." International Journal of Quantum Chemistry 80(4-5):1024-1038. Abstract The problem of choosing appropriate atomic orbital basis sets for ab initio calculations on dipole-bound anions has been examined. Such basis sets are usually constructed as combination of a standard valence-type basis, designed to describe the neutral molecular core, and an extra diffuse set designed to describe the charge distribution of the extra electron. As part of the present work, it has been found that the most commonly used valence-type basis sets (e.g., 6-31++G** or 6-311+G**) when so augmented, are subject to unpredictable under- or over-estimating electron binding energies for dipole-bound anions, whereas when the aug-cc-pVDZ, aug-cc-pVTZ (or other medium-size polarized (MSP) basis sets) are so augmented, more reliable binding energies respectively, especially when the electron binding energy is calculated at the CCSD(T) level of theory. The issue of designing and centering the extra diffuse basis functions for the excess electron has also been studied, and our findings are discussed here.
2000. "Excited Electronic States of the Anion of 7,7,8,8-tetracyanoquinodimethane (TCNQ)." Journal of Molecular Structure - Theochem 531:339-348. Abstract The doublet and quartet anionic states of TCNQ have been studied at the configuration interaction (CI/PM3)level. Our results support an experimental claim from the Brauman group that not only the ground anionic state but also some excited states of TCNQ$^-$ are electronically stable. The core excited $1^{2}B_{3u}$ state is separated from the ground $^{2}B_{2g}$ anionic state by only 1.2 eV at the CI/PM3 level.Two externally excited $^{2}A_{u}$ and $2^{2}B_{3u}$ anionic states are separated by ca. 2.3 eV from the ground $^{2}B_{2g}$ state and are close in energy to the ground electronic state of the neutral TCNQ. In addition, two quartet anionic states $^4B_{2g}$ and $^4B_{1g}$ are found to be electronically stable with respect to the lowest triplet $^3B_{1u}$ state of the neutral. All these anionic states have minima at $D_{2h}$ geometries. Our results suggest that the $1^{2}B_{3u}$ $\leftarrow$ $^{2}B_{2g}$ and $2^{2}B_{3u}$ $\leftarrow$ $^{2}B_{2g}$ transitions contribute to the electronic absorption and electron photodetachment spectra of TCNQ$^-$. This conclusion is based on calculated values of electronic transition energies, oscillator strengths, and excess electron binding energies.
2000. "A Bi-Dipole-Bound Dianion." Chemical Physics Letters 322(3-4):175. Abstract The possibility of binding one and two excess electrons by a molecule with two polar ends, each of which is capable of binding one electron, is studied using electronic structure methods. In the case of the singly charged anion, we find, as expected, a pair of nearly degenerate(unable to insert text) and (unable to insert text) states. For the dianion, stable open-shell triplet and singlet electronic states are reported, both of which have one electron on each polar terminus. Numerical results are presented for the mono- and di-anions of (LiCN)2...HCCH...(NCLi)2, and our estimation of the vertical electronic stability of ((LiCN)2...HGCCH...(NCLi)2)2- is 0.84 eV.
2000. "Comparison of Embedded-Atom Models and First-Principles Calculations for Al Phase Equilibrium ." Computational Materials Science 18(2):199-204. Abstract We have performed total energy calculations on Al in the fcc, bcc, ideal hcp and simple cubic crystal structures over a range of unit cell volumes. We employed density functional theory (DFT) in the local density approximation (LDA) and the generalized gradient approximation (GGA) as well as two different forms of the Embedded Atom Method (EAM) empirical atomistic potential with the aim of evaluating the predictive range of the EAM relative to first-principles methods. All four calculations correctly give the fcc structure as the preferred one at zero pressure, with the DFT results in good agreement with the experimental equation of state and the model potentials in exact agreement (by construction). The hcp structure is found to be fairly close in energy to the fcc structure in all cases, and the simple cubic structure is always found to be energetically very unfavorable. However, for the energetics of the bcc phase there is a serious disagreement between first-principles and atomistic calculations: the bulk modulus of bcc Al is much lower as predicted by the model potentials, with the result that its energy approaches that of the fcc phase as the volume is reduced. For the potential of Mishin et. al. an fcc e bcc phase transition is predicted at a pressure of 27.4 GPa, in disagreement with the experimental fact that fcc Al is stable to at least 220 GPa. Our DFT results are consistent with earlier DFT calculations and with experiment in predicting no transition over the density or pressure range considered.
2000. "On the Importance of Exchange Effects in Three-Body Interactions: The Lowest Quartet State of Na3." Journal of Chemical Physics 112(13):5751. Abstract N/A
2000. "Dipole-Bound Anions of Glycine Based on the Zwitterion and Neutral Structures." Journal of the American Chemical Society 122(41):10159-10162. Abstract The instability of the zwitterion structure of glycine is significantly reduced by the attachment of an excess electron and a local minimum develops on the anionic potential energy surface for the zwitterion structure. However, the global anionic minimum, that is lower by 9 kcal/mol, corresponds to a singly hydrogen-bonded non-zwitterion structure. The vertical electron detachment energies for these two dipole-bound zwitterion and non-zwitterion structures are 3175 and 668 cm$^{-1}$, respectively.
2000. "Bi-Dipole-Bound Anions ." International Journal of Mass Spectrometry and Ion Processes 201(1-3):245-252. Abstract N/A
2000. "(MgO)n-(n=1-5) Clusters: Multipole-Bound Anions and Photodetachment Spectroscopy." Physical Review Letters 85(15):3145-3148. Abstract Photoelectron spectra of (MgO)n- (n = 1-5) reveal a surprising trend: the electron binding energy decreases from n = 1 to 4, and then increases from 4 to 5. Ab initio calculations suggest this pattern is related to the electrostatic interaction between the extra electron and the charge distribution of the neutral cluster. This interaction is significant in MgO- and (MgO)5-, for which the lowest non-vanishing multipole moment (LNM) is a dipole; it is smaller for (MgO)2- and (MgO)3-, for which a quadrupole is the LNM; and it is the smallest for (MgO)4-, for which an octopole is the LNM. The cubic (MgO)4- is the first octopole-bound anion yet observed experimentally and characterized theoretically.
2000. "An Ab Initio Study of (H3B<- NH3)(-) - A Dipole-Bound Anion Supported by the Dative Charge Transfer Bond in the Neutral Host." Journal of Chemical Physics 113(20):8961-8968. Abstract The possibility of electron binding to the complex (H3BNH3) was studied at the coupled cluster level of theory with single, double, and noniterative triple excitations. The staggered conformation (minimum), with a dipole moment of 5.3 Debye, binds an electron by 984 cm-1, whereas the eclipsed conformer (saddle point) possesses a larger dipole moment (5.5 Debye) and binds an electron by 1014 cm-1. The neutral parent of the (H3BNH3)? anion involves a dative bond that is responsible for a significant polarization of the neutral species that generates the large dipole moment.
2000. "Opposite Rumpling of the MgO and CaO (100) Surfaces: A Density-Functional Theory Study." Physical Review. B, Condensed Matter 62(12):8318-8322. Abstract An all electron density-functional theory calculation using the periodic slab approach is carried out to investigate the relaxed structures of the regular MgO and CaO (100) surfaces. For MgO, the cations are displaced downwards relative to the oxygens with surface rumpling of 2.2%. The vertical displacements of the atoms in CaO are in the opposite direction, producing a rumple of -0.6%. The first and second interlayer spacings for both surfaces are essentially equal to the bulk value.
2000. "High-Coverage Adsorption of Alkali Metals at the CaO and MgO (100) Surfaces." Surface Science 466(1-3):111-118. Abstract Adsorption of alkali metals (Li and Na) on the defect free CaO and MgO(100) surface has been studied at the all-electron density functional level of theory using periodic boundary conditions. The binding energy was found to be the largest for the oxide binding sites and the lateral interaction among alkali atoms contributes significantly to the total binding energy. The predict that the 1ML film of Li wets the the MgO(100) surface and the small lattice mismatch between the substrate and the bcc Li indicates the possibility of creating a low energy interface between these two materials at proper growth conditions. The binding energy is weaker for alkali metals on CaO than on MgO due to smaller lateral stabilization among alkali atoms.
2000. "First-Principles Studies of Adsorption of CO on the Na(100) Surface ." Surface Science 453(1-3):130-136. Abstract We have investigated the interaction of CO with the Na(100) substrate in the framework of density functional theory formulated for periodic systems. We established that there is an attractive but weak chemisorption bond between CO and Na(100).
1999. "Electronic Structure of Dipole-Bound Anions." In Recent Research Developments in Physical Chemistry, vol. 3, pp. 245-260. Transworld Research Network, Kerala, India. Abstract There is no abstract currently available for this item.
1999. "Theoretical Study of the Dipole-Bound Anions (HPPH3)-." Journal of Chemical Physics 110(1):274. Abstract N/A
1999. "Mixed Valence/Dipole-Bound dianions." Journal of Chemical Physics 111(21):9469-9747. Abstract N/A
1999. "Dipole-Bound Anion of the HNNH3 Isomer of Hydrazine. An ab initio Study. ." Journal of Physical Chemistry A 103(5):625-631. Abstract There is no abstract currently available for this item
1999. "Ab Initio Study of the Dipole-Bound Anion (H2O...HCl)(-)." Journal of Chemical Physics 111(7):3004-3011. Abstract There is no abstract currently available for this item
1999. "Theoretical Study of the Quadrupole-Bound Anion (BeO)2- ." Chemical Physics Letters 303(1-2):65. Abstract N/A
1999. "Highly Accurate ab initio Calculation of the Interaction Potential for Two Sodium Atoms with Parallel Spins." Journal of Chemical Physics 110(10):4695-4698. Abstract There is no abstract currently available for this item
1999. "Favorable Performance of the DFT Methods in Predicting the Minimum-Energy Structure of the Lowest Triplet State of WF4." International Journal of Quantum Chemistry 73(4):369-375.
1999. "Electron Binding Energies in Linear Dipole-Bound (HCN)(n)(-) (n=2-5) Anions." Chemical Physics Letters 300(3-4):338. Abstract There is no abstract currently available for this item