Scientific Publications 2004
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A
2004. "Electronic Structure, Isomerism, and Chemical Bonding in B7- and B7 ." Journal of Physical Chemistry A 108(16):3509-3517. Abstract The electronic structure and chemical bonding of B7- and B7 have been investigated using photoelectron spectroscopy and ab initio calculations. Complicated, but well-resolved, photoelectron spectra were obtained for B7- at several photon energies and were shown distinctly to contain contributions from different B7- isomers. The structures of the global minima and low-lying isomers were identified using extensive ab initio calculations. Two almost degenerate pyramidal structures I (C6v, 3A1) and II (C2v, 1A1) were the most stable for B7-. The triplet structure-I is slightly more stable than the singlet structure at our highest level of theory [RCCSD(T)/6-311+G(2df)] by 0.7 kcal/mol only. The next lowest singlet structure V (C2v, 1A1) was perfectly planar and was 7.8 kcal/mol higher in energy at RCCSD(T)/6-311+G(2df) level. The observed photoelectron spectra can only be explained when contributions from all these three low-lying isomers were considered; the observed spectral features were in good agreement with the calculated detachment transitions from the three isomers. Analyses of the molecular orbitals and chemical bonding revealed evidence that the triplet pyramidal C6V structure-I has a twofold (п and σ) aromaticity, the singlet pyramidal C2V structure-II has σ-aromaticity and п-antiaromaticity, and the singlet planar C2V structure V has a twofold (п and σ) antiaromaticity.
2004. "Structure of the NaxC1x+1- (x=1-4) Clusters via AB Initio Genetic Algorithm and Photoelectron Spectroscopy." Journal of Chemical Physics 121(12):5709-5719. Abstract The first application of the ab initio genetic algorithm with an embedded gradient has been carried out for the elucidation of global minimum structures of a series of anionic sodium chloride clusters, NaxClx+1- (x = 1-4), produced in the gas-phase using electrospray ionizaion and studied by photoelectron spectroscopy. These are all superhalogen species with extremely high electron binding energies. The adiabatic electron binding energies for NaxClx+1- were measured to be 5.64, 6.22, 6.3, and 6.9 eV, for x = 1-4, respectively. Our genetic algorithm program detected the linear global minima of NaCl2- and Na2Cl3- and tree-dimensional structures for the larger species. Na3Cl4- was found to have C3v symmetry, which can be viewed as a Na4Cl4 cube missing a corner Na+ cation, whereas Na4Cl5- was found to have C4v symmetry, close to a 3x3 planar structure. Further accurate ab initio calculations were carried out for the elucidated global minimum structures. Excellent agreement between the theoretically calculated and the experimental spectra was observed, confirming the obtained structures and demonstrating the power of the developed genetic algorithm technique.
2004. "Molecular Wheel B82- as a New Inorganic Ligand. Photoelectron Spectroscopy and ab Initio Characterization of LiB8-." Inorganic Chemistry 43(12):3552-3554. Abstract The bare B8 cluster was previously reported to be a D1h molecular wheel with a triplet group state. The B82- dianion was predicted to be a closed-shell singlet and double aromatic D1h molecular wheel. Here we report the experimental observation of B82- stabilized by a Li+ cation in LiB8- and its experimental characterization using photoelectron spectroscopy. Theoretical searches lead to a C-1V LiB8- global minimum structure, and its calculated photodetachment transitions are in good agreement with the experimental values. Except for a small out-of-plane distortion due to the asymmetric Li+ capping, the B82- unit in LiB8- is nearly identical to the bare B82-, suggesting it is a robust and stable structural unit and may be used as a new ligand and building block in chemistry.
2004. "Carbon-Nanotube-Reinforced Polymer-Derived Ceramic Composites." Advanced Materials 16(22):2036-2040. Abstract Carbon nanotube reinforced ceramic composites were synthesized by using recently developed polymer-derived ceramics as matrices. Multi-wall carbon nanotubes, treated with a surfactant, were first dispersed in a liquid polymer precursor by sonication and mechanical stirring. The solution was then converted to fully dense ceramic composites with pressure-assist pyrolysis technique. Microstructural observation revealed that nanotubes were homogeneously dispersed throughout the ceramic matrix. Significant increases in mechanical and thermal properties were observed by adding only ~6vol% nanotubes. Strong nanotube pullout revealed by SEM observation suggested that the composites could possess high fracture toughness.
2004. "Noble Gas-Uranium Coordination and Intersystem Crossing for the CUO(Ne)x(Ng)n (Ng = Ar, Kr, Xe) Complexes in Solid Neon." New Journal of Chemistry 28(2):289-294. Abstract Atomic uranium excited by laser ablation reacts with CO in excess neon to produce the novel CUO molecule, which forms weak complexes CUO(Ne)m with neon and stronger complexes CUO(Ne)x(Ng)n (Ng = Ar, Kr, Xe) when the heavier noble gas atoms are present. The heavier CUO(Ne)m-1(Ng) complexes are identified through the effects of CO isotopic and Ng substitution on the neon matrix infrared spectra and by comparison to DFT frequency calculations on model complexes CUO(Ng) (Ng = Ne, Ar, Kr, Xe). The U-C and U-O stretching frequencies of CUO(Ne)m-1(Ng) complexes are slightly red shifted from 1047 and 872 cm-1 frequencies for the 1Sigma+ CUO ground state neon complex, which identifies singlet ground state CUO(Ne)m-1(Ng) complexes in solid neon. The next singlet CUO(Ne)x(Ng)2 complexes in excess neon follow in like manner. However, stretching modes and the isotopic shifts of the higher CUO(Ne)x(Ng)n complex approach those of the pure argon matrix CUO(Ar)n complex, which characterizes triplet ground state complexes by comparison to DFT frequency calculations.
2004. "Thermochemistry of Aqueous Hydroxyl Radical from Advances in Photoacoustic Calorimetry and ab Initio Continum Solvation Theory." Journal of the American Chemical Society 126(12):3680-3681. Abstract Photoacoustic signals from dilute (~30 mM) solutions of H2O2 were measured over the temperature range from 10-45 °C to obtain the reaction enthalpy and volume change for H2O2(aq) → 2 OH(aq) from which we ultimately determined ∆f G°, ∆f H° and partial molal volume, v°, of OH (aq). We find ∆r H*=*46.8 ± 1.4 kcal/mol, which is 4 kcal/mol smaller than the gas phase bond energy, and ∆ Vr = 6.5 ± 0.4 mL/mol. The v° for OH in water is 14.4 ± 0.4 mL/ml: smaller than the v° of water. Using ab intio continuum theory, the hydration free energy is calculated to be –3.9 ± 0.3 kcal/mol (for standard states in number density concentration units) by a novel approach devised to capture in the definition of the solute cavity the strength and specific interactions of the solute with a water solvent molecule. The shape of the cavity is defined by “rolling” a 3 dimensional electron density isocontour of water on the ab initio water-OH minimum interaction surface. The value of the contour is selected to reproduce the volume of OH in water. We obtain for OH(aq): ∆f H° = -0.2 ± 1.5 and ∆f G° = 5.8 ± 0.4 kcal/mol that are inagreement with literature values. The results provide confidence in the pulsed PAC technique for measuring aqueous thermochemistry of radicals and open the way to obtaining thermochemistry for most radicals that can be formed by reaction of OH with aqueous substrates while advancing the field of continuum solvation theory towards ab initio-defined solute cavities
2004. "X-ray Photoelectron Spectroscopy Studies of Oxidized and Reduced Ce₀․₈Zr₀․₂O₂(111)." Surface Science Spectra 11(1-4):82-90. doi:10.1116/11.20050202 Abstract We have studied the electronic structure of oxidized and reduced Ce₀․₈Zr₀․₂O₂(111) using x-ray photoelectron spectroscopy (XPS). The 50 nm thick Ce₀․₈Zr₀․₂O₂(111) film was grown on a YSZ(111) substrate using oxygen assisted molecular beam epitaxy (OPA-MBE). This film has been characterized using in-situ RHEED (reflection high energy electron diffraction) and ex-situ XRD (x-ray diffraction), HRTEM (high energy resolution transmission electron spectroscopy) and RBS (Rutherford backscattering spectroscopy). Surfaces of the Ce₀․₈Zr₀․₂O₂(111) film used in this study is found to be unreconstructed and exhibits the structure of bulk CeO₂(111) where Zr atoms occupy the lattice sites of Ce in the fluorite structure of ceria. The extent of surface reduction as a result of vacuum annealing has been reported here in addition to the electronic structure of defect-free Ce₀․₈Zr₀․₂O₂(111) surface.
