2009. "Interactions of Cl- and OH Radical in Aqueous Solution." Journal of Physical Chemistry A 113(31):8823-8825. Abstract Fundamental understanding of ion-radical interactions in aqueous solutions is of significant relevance to many environmentally important applications. An important example can be found in the problem involving the excess production of molecular chlorine in marine layer, where interactions between OH radical and Cl- species have been implicated as the main reason for the unexpectedly high concentration of Cl2. Current understanding of this process is hindered due to uncertainty regarding the nature of the [OHCl]- complex in aqueous phase.
2009. "Functional Representation for the Born-Oppenheimer Diagonal Correction and Born-Huang Adiabatic Potential Energy Surfaces for Isotopomers of H3." Journal of Physical Chemistry A 113(16):4479-4488. Abstract Multireference configuration interaction (MRCI) calculations of the Born-Oppenheimer diagonal correction (BODC) for H3 were performed at 1397 symmetry-unique configurations using the Born-Handy approach; isotopic substitution leads to 4041 symmetry-unique configurations for the DH2 mass combination. These results were then fit to a functional form that permits calculation of the BODC for any combination of isotopes. Mean unsigned fitting errors on a test grid of configurations not included in the fitting process were 0.14, 0.12, and 0.65 cm−1 for the H3, DH2, and MuH2 isotopomers, respectively. This representation can be combined with any Born-Oppenheimer potential energy surface (PES) to yield Born-Huang (BH) PESs; herein we choose the CCI potential energy surface, the uncertainties of which (~0.01 kcal/mol) are much smaller than the magnitude of the BODC. FORTRAN routines to evaluate these BH surfaces are provided. Variational transition state theory calculations are presented comparing thermal rate constants for reactions on the BO and BH surfaces to provide an initial estimate of the significance of the diagonal correction for the dynamics.
2009. "Thermodynamics and Kinetics of Nanoclusters Controlling Gas-to-Particle Nucleation." Journal of Physical Chemistry C 113(24):10354-10370. Abstract Nucleation of new particles from vapor-phase molecular precursors is an important process in the synthesis of nanomaterials and in the formation of aerosols in the atmosphere. Vapor-to-particle nucleation is a macroscopic process controlled by nanoscale particles (e.g., molecular clusters). Computational approaches to nucleation have been limited by the lack of a consistent theory of the process and by the lack of efficient approaches to simulate the properties of clusters relevant to nucleation. In this article, we focus on two advances that allow nucleation to be treated in a rigorous manner for molecular systems: Dynamical Nucleation Theory permits a consistent treatment of the nucleation kinetics and aggregation-volume-bias Monte Carlo simulations using self-adaptive umbrella sampling combined with histogram reweighting provides an efficient approach to evaluate the thermodynamics of molecular clusters important in nucleation. The combination of these two approaches positions molecular computational approaches to make significant advances in our understanding of the mechanisms of nucleation, particularly in multiple component systems that play crucial roles in nanoscience applications and in the atmosphere. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE.
2009. "The muonic He atom and a preliminary study of the He-4 mu + H-2 reaction ." Physica B Condensed Matter 404(5-7):946-949. Abstract The muonic atom 4Heu has the composition a++u-e-, and is formed by stopping negative muons in He doped with a small amount of NH3 (or Xe). It may be regarded as a unique heavy H-atom isotope with a mass of 4.1 amu. As such, the study of its chemical reaction rates and comparison with those of the well-known light Mu atom (0.113amu) allows unprecedented tests of kinetic isotope effects over a range of 36 in mass. As a first example, and one which is of most fundamental interest, we have begun kinetics studies of the Heu + H2 - HeuH + H reaction in the gas phase. The first measurements, at 295K, give a rate constant of kHei = 4:1 - 0:7 x 10-16 cm3 molec-1 s-1. In comparison, variational transition state calculations give a value of 2:46 x 10-16 cm3 molec-1 s-1, some what below the measurement, despite the large error bar, raising the possibility that the calculations, on a nessentially exact potential energy surface, have underestimated the amount of quantum tunneling involved, even for this heavyH-atom isotope.
2008. "Molecular Theory of Mass Transfer Kinetics and Dynamics at Gas/Water Interface." Fluid Dynamics Research 40(7-8):459-473. Abstract The mass transfer mechanism across gas/water interface is studied with molecu- lar dynamics (MD) simulation. The MD results provide a robust and qualitatively consistent picture to previous studies about microscopic aspects of mass transfer, including interface structure, free energy profiles for the uptake, scattering dynamics and energy relaxation of impinging molecules. These MD results are quantitatively compared with experimental uptake measurements, and we found that apparent inconsistency between MD and experiment could be partly resolved by precise de- composition of the observed kinetics into elemental steps. Remaining issues and future perspectives toward constructing a comprehensive mutli-scale description of interfacial mass transfer are summarized.
2008. "The Impact of Molecular Interactions on Atmospheric Aerosol Radiative Forcing." Advances in Quantum Chemistry 55:429-447. doi:10.1016/S0065-3276(07)00220-1 Abstract We review the chemical physics of nucleation and its connection to atmospheric aerosol radiative forcing. The scientific community has demanded a comprehensive investigation of aerosol radiative forcing of climate. Particular emphasis has been placed on gaining the fundamental knowledge necessary to accurately predict aerosol formation and growth and their subsequent radiative affects on climate. The chemical physics of the molecular clusters underlying nucleation will be outlined and future developments towards modeling multi-component nucleation in the atmosphere discussed. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences of the U.S. Department of Energy (DOE) and calculations were performed in part using the Molecular Science Computing Facility in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL) at the Pacific Northwest National Laboratory. The EMSL is funded by the DOE Office of Biological and Environmental Research. Battelle operates Pacific Northwest National Laboratory for DOE.
2008. "Activation Energies and Potentials of Mean Force for Water Cluster Evaporation." Journal of Chemical Physics 128(6):Art. No. 064306. doi:10.1063/1.2837282 Abstract Activation energies for water cluster evaporation are of interest in many areas of chemical physics. We present the first computation of activation energies for small waters clusters using the formalism of Dynamical Nucleation Theory (DNT). To this end, individual evaporation rate constants are computed for water clusters (H2O)i, where i = 2 to 10 for temperatures ranging from 243 to 333K. These calculations employ a parallel sampling technique utilizing the Global Arrays Toolkit developed at PNNL. The resulting evaporation rate constants for each cluster are then fit to Arrhenius equations to obtain activation energies. We discuss DNT evaporation rate constants and their relation to potentials of mean force, activation energies, and how to account for non-separability of the reaction coordinate in the reactant state partition function. This work was supported by the PNNL Computational Science and Engineering LDRD Program and the Chemical and Material Sciences Division, Office of Basic Energy Sciences, Department of Energy. The Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
2008. "Many-Body Decomposition of the Binding Energies for OH•(H2O)2 and OH•(H2O)3 Complexes." Journal of Chemical Physics 128(8):Art. No. 084307. doi:10.1063/1.2828522 Abstract We use ab initio electronic structure methods to calculate the many-body decomposition of the binding energies of the OH(H2O)n (n=2,3) complexes. We employ MP2 and CCSD(T) levels of theory with aug-cc-pVDZ and aug-cc-pVTZ basis sets and analyze the significance of the non-pairwise interactions between OH radical and the surrounding water molecules. We also evaluate the accuracy of our newly developed potential function, the modified Thole-type model (mTTM), for predicting the many-body terms in these complexes. Our analysis of the many-body contributions to the OH(H2O)n binding energies clearly shows that they are just as important in the OH interactions with water as they are for interactions in pure water systems. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences of the U.S. Department of Energy (DOE) and was performed in part using the Molecular Science Computing Facility in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL) at the Pacific Northwest National Laboratory. The EMSL is funded by the DOE Office of Biological and Environmental Research. Battelle operates Pacific Northwest National Laboratory for DOE. The authors thank Sotiris Xantheas, Jun Li, Tzvetelin Iordanov, and Jun Cui for helpful discussions and assistance.
2008. "Self-consistent polarization neglect of diatomic differential overlap: Application to water clusters." Journal of Chemical Physics 128(16):119-137 (Art. no. 164111). doi:10.1063/1.2905230 Abstract Semiempirical SCF methods such as MNDO, AM1, and PM3 have the ability to treat the formation and breaking of chemical bonds but have been found to poorly describe hydrogen bonding and weak electrostatic complexes. In contrast, most empirical potentials are not able to describe bond-breaking and formation, but have the ability to add missing elements of hydrogen bonding using classical electrostatic interactions. We present a new method which combines aspects of both NDDO-based SCF techniques and classical descriptions of polarization to describe the diffuse nature of the electronic wavefunction in a self-consistent manner. We develop the “self-consistent polarization neglect of differential diatomic overlap” (SCP-NDDO) theory with the additional description of molecular dispersion developed as a second-order perturbation theory expression. The current study seeks to model water-water interactions as a test case. To this end, we have parameterized the SCP-NDDO model to the accurate MP2/CBS estimates of small water cluster binding energies of Xantheas et al.[S. S. Xantheas, C. J. Burnham, and R. J. Harrison, J. Chem. Phys. 116, 1493 (2002); S. S. Xantheas and E. Aprà, J. Chem. Phys. 120, 823 (2004)]. Overall agreement with the ab initio binding energies (n = 2 – 6, 8) is achieved with an RMS error of 0.20 kcal/mol. We achieve noticeable improvements in the structure, vibrational frequencies, and energetic predictions of water clusters (n ≤ 21) relative to standard NDDO-based methods.
2007. "Variational Transition State Theory in the Treatment of Hydrogen Transfer Reactions." Chapter 27 in Hydrogen-Transfer Reactions, vol. 2, ed. JT Hynes, JP Klinman, H-H Limbach, RL Schowen, pp. 833-874. WILEY-VCH, Weinheim, Germany. Abstract We review variational transition state theory with optimized multidimensional tunneling as a well validated method for calculating rate constants of reactions with significant quantum mechanical effects, such as proton, hydrogen atom, and hydride transfer reactions.
2007. "Variational Transition State Theory with Multidimensional Tunneling." Chapter 3 in Reviews in Computational Chemistry, vol. 23, ed. Kenneth B. Lipkowitz, Thomas R. Cundari, pp. 125-232. Wiley-VCH, Hoboken, NJ. Abstract This review describes the application of variational transition state theory (VTST) to the calculation of chemical reaction rates. In 1985 two of us, together with Alan D. Isaacson, wrote a book chapter on this subject entitled “Generalized Transition State Theory” for the multi-volume series entitled Theory of Chemical Reaction Dynamics.1 Since that time, variational transition state theory has undergone important improvements due mainly to the ability of this theory to adapt to more challenging problems. For instance, the 1985 chapter mainly describes the application of VTST to bimolecular reactions involving 3–6 atoms, which were the state-of-the-art at that moment. The study of those reactions by VTST dynamics depended on the construction of an analytical potential energy surface (PES). Nowadays, thanks to the development of more efficient algorithms and more powerful computers, the situation is completely different, and most rate calculations are based on “on the fly” electronic structure calculations, which together with hybrid approaches, like combined quantum mechanical molecular mechanical methods (QM/MM), allow researchers to apply VTST to systems with hundreds or even tens of thousands of atoms. Three other major advances since 1985 are that transition state dividing surfaces can now be defined much more realistically, more accurate methods have been developed to include multidimensional quantum mechanical tunneling into VTST, and the theory has also been extended to reactions in condensed phases.
2007. "Hybrid Approach for Free Energy Calculations with High-Level Methods: Application to the S(N)2 Reaction of CHCl3 and OH- in Water." Journal of Chemical Physics 127(5):51102 (1-4). Abstract We present an approach for potential of mean force calculations within quantum mechanical molecular mechanics framework using hierarchy of approximations involving density functional and high level coupled cluster theories. As an example application we study the reaction process of CHC1₃ with OH¯ in aqueous solution.
2007. "The Critical Role of Anharmonicity in Aqueous Ionic Clusters Relevant to Nucleation." Journal of Physical Chemistry C 111(13):4977-4983. doi:10.1021/jp067468u Abstract Calculating accurate free energies of aqueous ionic clusters is important in many areas of chemical physics. A common practice used in ab initio and other methods to determine the cluster free energy is to use a single energetically favorable configuration and then assume the harmonic approximation. In the present study, we compare and contrast harmonic free energies to those determined from anharmonic configurational sampling and how both compare to experiment. It is found that anharmoncity plays a critical role in determining accurate free energies for the clusters underlying ion-induced nucleation. Consequently, energies, geometries, and frequencies obtained using a single or multiple configurations within the harmonic approximation lead to incorrect aqueous cluster thermodynamics.This work was supported by the Office of Basic Energy Sciences of the Department of Energy, in part by the Chemical Sciences Program. The Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
2007. "Comment on "Quantum Nature of the Sign Preference in Ion-Induced Nucleation"." Physical Review Letters 98(10):Art.no.109603. Abstract A reliable molecular-level treatment of nucleation requires (i) accurate descriptions of the interaction potentials, (ii) a consistent theoretical approach that connects the interaction potential to the nucleation rate, and (iii) appropriate statistical mechanics to evaluate quantities of interest. In a recent PRL Nadykto et al. [1] “attack the fundamental problem of the sign preference by employing a higher level of theory such as quantum mechanics”, where quantum mechanics in this context refers to electronic structure calculations of interaction energies. They concluded that “The strong effect of the chemical nature of the core ion on the conversion of vapor molecules to clusters is essentially quantum in nature, and, thus, systematic accounting for the actual core species is impossible without taking into account the actual electronic structures of the core ions.” The purpose of this Comment is to point out conceptual errors concerning the critical importance of items (ii) and (iii) above and to clarify misrepresentations of our previous work on this topic [2]. Any consistent theory of nucleation must describe the relevant regions of configuration space that govern the kinetics and thermodynamics of cluster formation through evaporation and condensation processes – e.g., Dynamical Nucleation Theory (DNT) [3].
2007. "Ab initio and analytical intermolecular potential for ClO-H2O." Journal of Chemical Physics 126(11):146-155. doi:10.1063/1.2566537 Abstract In recent years, the ClO free radical has been found to play an important role in the ozone removal processes in the atmosphere. In this work, we present a Potential Energy Surface (PES) Scan of the ClO•H2O system with high-level ab initio methods. Because of the existence of low-lying excited states of the ClO•H2O complex, and their potential impact on the chemical behavior of the ClO radical in the atmosphere, we perform a PES scan at CCSD(T)/aug-cc-pVTZ level of both the first excited and ground states in order to model the physics of the unpaired electron in the ClO radical. Analytical potentials for both ground and excited states, with internal molecular coordinates held fixed, were built based on a Thole Type Model. The two minima of the ClO•H2O complex are recovered by the analytical potential. This work was supported by the Office of Basic Energy Sciences of the Department of Energy, in part by the Chemical Sciences program and in part by the Engineering and Geosciences Division. The Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.
2006. "Sensitivity analysis of thermodynamic properties of liquid water: A General Approach to Improve Empirical Potentials ." Journal of Physical Chemistry A 110(2):762-771. Abstract A sensitivity analysis of bulk water thermodynamics is presented in an effort to understand the relation between details of molecular potentials and the properties that they predict. The analysis is incorporated in molecular dynamics simulation and investigates the sensitivity of the Helmholtz free energy, internal energy, entropy, heat capacity, pressure, thermal pressure coefficient, and static dielectric constant to details of the potential. The sensitivities of the properties are calculated with respect to the van der Waals repulsive and the attractive parts, plus short and long range Coulomb parts of three four site empirical water potentials: TIP4P, Dang-Chang and TTM2R. The polarization sensitivity is calculated for the polarizable Dang-Chang and TTM2R potentials. The analysis indicates that all investigated properties are most sensitive to the van der Waals repulsive, the short range Coulomb and the polarization components of the potentials. The sensitivity of the Helmoltz free energy, internal energy, and entropy due to polarizaion is almost 30% of total electrostatic sensitivity. In addition the similarities in the trends of the observed sensitivities for nonpolarizable and polarizable potentials lead to the conclusion that the complexity of the model is not of critical importance for the calculation of these properties for bulk water. The van der Waals attractive and the long range Coulomb sensitivities are relatively small for the entropy, heat capacity, thermal pressure coefficient and the static dielectric constant, while small changes in any of the potential contributions will significantly effect the pressure. The analysis suggests a procedure for modification of the potentials and their improved prediction of thermodynamic properties. Based on the proposed procedure the water cluster potential TTM2R was adapted for simulation of bulk water properties.
2006. "Molecular simulations of the transport of molecules across the liquid/vapor interface of water." Chemical Reviews 106(4):1355-1374. Abstract The transport of molecules across the liquid/vapor interface of water is a fundamental process that is important in a number of areas. Uptake of trace molecules by aqueous droplets is important in the atmosphere as it enables heterogeneous new chemical pathways that are not available in the gas phase. The uptake of molecules by water and partitioning to the air/water surface is important in aquatic environmental systems. The summary of molecular simulations presented in this work supports a picture of solute transport across the water’s vapor/liquid interface in which a hydrophilic solute molecule impinging on the surface is rapidly equilibrated, sticks to the surface of the interface with nearly unit probability, then diffuses into the bulk liquid on an free energy surface. Analysis of a large number of experimental observations of uptake, using a variety of techniques, supports a view of mass transport that is diametrically opposed – most solute molecules that collide with the surface return to the vapor rather than being absorbed into the liquid. This discrepancy stresses the need for greater understanding of the important process of uptake. We provide a perspective on molecular-scale simulations and evaluate the type of accuracy we should expect for interfacial properties, particularly those associated with the update of molecular at the surface. We briefly review computational methods commonly used in studies of aqueous interfaces, including functional forms of the molecular interaction potentials, descriptions of the simulation methodologies, and descriptions of the molecular models of the interface. We present the results of simulations of select properties of the air/water interface and properties of molecular interactions at interfaces and present an overview of modeling approaches to the macroscopic process of uptake. In particular, we present detailed descriptions of the uptake process in flow tube experiments with the help of fluid dynamics calculations toward critical comparison between the uptake experiments and the molecular simulations.
2006. "The OH Radical-H2O Molecular Interaction Potential." Journal of Chemical Physics 124:224318 (15). doi:10.1063/1.2200701 Abstract The OH radical is one of the most important oxidants in the atmosphere due to its high reactivity. The study of hydrogen-bonded complexes of OH with the water molecules is a topic of significant current interest. In this work, we present the development of a new analytical functional form for the interaction potential between rigid OH radical and H2O molecules. To do this we fit a selected functional form to a set of high level ab initio data. Since there are low-lying excited states for the H2O•OH complex, the impact of the excited states on the chemical behavior of the OH radical can be very important. We perform a Potential Energy Surface (PES) scan using the CCSD(T)/aug-cc-pVTZ level of electronic structure theory for both excited and ground states. To model the physics of the unpaired electron in the OH radical, we develop a tensor polarizability generalization of the Thole Type all-atom polarizable rigid potential for the OH radical, which effectively describes the interaction of OH with H2O for both ground and excited states. The stationary points of (H2O)n•OH clusters were identified as a benchmark of the potential. Battelle operated PNNL for the USDOE.
2006. "On NO3-H2O interactions in aqueous solutions and at interfaces." Journal of Chemical Physics 124(6):066101 (3). doi:10:1063/1.2171375 Abstract Constrained molecular dynamics technique was employed to investigate the transport of a nitrate ion across the water liquid/vapor interface. We developed the nitrate ion-water polarizable potential capable of describing well the solvation properties of the hydrated nitrate ion. The computed free energy profile for the transfer of the nitrate ion across the air/water interface increases monotonically as the nitrate ion approaches the Gibbs dividing surface from the bulk liquid side. The computed density profiles of 1M KNO3 salt solution slab indicate that the nitrate and potassium ions are both found below the aqueous interface. Upon analyzing the results, we can conclude that the probability of finding the nitrate anion at the aqueous interface is quite small.
2005. "Variational transition state theory." Chapter 5 in Theory and Applications of Computational Chemistry: The First 40 Years, ed. C. Dykstra, G. Frenking, K. Kim and G. Scuseria, pp. 84-87. Elsevier Science, Amsterdam, Netherlands. Abstract We present an overview of variational transition state theory (VTST) from the perspective of the dynamical formulation of the theory. This formulation provides a firm classical mechanical foundation for a quantitative theory of reaction rate constants, and it provides a sturdy framework for the consistent inclusion of corrections for quantum mechanical effects and the effects of condensed phases. A central construct of the theory is the dividing surface separating reaction and product regions of phase space. We focus on the robust nature of the method offered by the flexibility of the dividing surface, which allows the accurate treatment of a variety of systems from activated and barrierless reactions in the gas phase, reactions in rigid environments, and reactions in liquids and enzymes.
2005. "Molecular dynamics simulations of atmospheric oxidants at the air--water interface:Solvation and Accommodation of OH and O3." Journal of Physical Chemistry B 109(33):15876-15892. Abstract A comparative study of OH, O3, and H2O equilibrium aqueous solvation and gas phase accommodation on liquid water at 300 K is performed using a combination of ab initio calculations and molecular dynamics simulations. Polarizable force fields are developed for the interaction potential of OH and O3 with water. The free energy profiles for transfer of OH and O3 from the gas phase to the bulk liquid exhibit a pronounced minimum at the surface, but no barrier to solvation in the bulk liquid. The calculated surface excess of each oxidant is comparable to calculated and experimental values for short chain, aliphatic alcohols. Driving forces for the surface activity are discussed in terms of the radial distribution functions and dipole orientation distributions for each molecule in the bulk liquid and at the surface. Simulations of OH, O3, and H2O impinging on liquid water with a thermal impact velocity are used to calculate thermal accommodation, S, and mass accommodation, α, coefficients. The values of S for OH, O3, and H2O are 0.95, 0.90, and 0.99, respectively. The approaching molecules are accelerated towards the liquid surface when they are approximately 5 Å above it. The molecules that reach thermal equilibrium with the surface do so within 2 ps of striking the surface, while those that do not, scatter into the gas phase with excess translational kinetic energy in the direction perpendicular to the surface. The time constants for absorption and desorption range from approximately 35 – 140 ps and the values of α for OH, O3, and H2O are 0.83, 0.047, and 0.99, respectively. The results are consistent with previous formulations of gas phase accommodation from simulations, in which the process occurs by rapid thermal and structural equilibration followed by diffusion on the free energy profile. The implications of these results to atmospheric chemistry are discussed.
2005. "Ion-Induced Nucleation: The Importance of Chemistry." Physical Review Letters 94(11):116104. Abstract Experiments have shown that ions can substantially increase vapor-to-liquid nucleation rates. However, interpretation of these experiments is complicated by ambiguities arising from the manner in which the ions are produced. Several studies have concluded that water has a general preference for anions over cations. We show that specification of ion’s sign alone is insufficient to provide an understanding of the aqueous ionic cluster thermodynamics and that Classical Ion-Induced Nucleation Theory does not treat the cluster physics properly to describe ion-induced nucleation accurately.
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. "DFT calculations for the structure and properties of polychlorodibenzo-para-dioxine anion-radicals." Journal of Structural Chemistry 46(4):591-595. Abstract Borisov, YA., BC. Garrett, VA. Mazunov, and YS. Nekrasov. 2005. “ DFT calculations for the structure and properties of polychlorodibenzo-para-dioxine anion-radicals,” Journal of Structural Chemistry 46 (4): 591-595. http://www.springerlink.com/content/d361t549815667u7/fulltext.pdf
2004. "Hydroxyl radical at the air-water interface." Journal of the American Chemical Society 126(50):16308-16309. Abstract The free energy profile for transfer of OH across the air/water interface at 300 K was calculated using classical molecular dynamics computer simulation with polarizable potential. The experimental hydration free energy (∆Gs ) is satisfactorily reproduced by the present force field. The free energy profile exhibits a minimum at the air/water interface, with the free energy of adsobtion (∆Gs ) being about 1 kcal/mol larger than the hydration free energy. The propensity of the OH radical for the air/water interface was further explored in simulation, in which OH radicals were originally placed inside the bulk liquid region of an infinite water slab about 30 Å thick. During 1 ns propagation, the OH radicals were observed to diffuse through the interior of the water slab, however, they were predominantly located in either of the two interfacial regions. Collisions of OH radical with the water surface were also investigated in a series of scattering simulations. From 250 initial conditions of a gas phase OH radical approaching the surface of liquid water with a thermal impact velocity, the thermal and mass accommodation coefficients at 300 K were determined to be 0.95 and 0.90, respectively. The partitioning of OH radicals between the bulk water and the interface was observed. The enhancement in the surface concentration of OH relative to the concentration in the aqueous phase, resulting from the surface activity of the hydroxyl radical, suggests that important OH chemistry may be occurring in the interfacial water layer of the water droplets, aqueous aerosol particles, and thin water films adsorbed on solid surfaces. This has profound consequences for modelling heterogeneous atmospheric chemical processes.
2004. "Multicomponent Dynamical Nucleation Theory And Sensitivity Analysis ." Journal of Chemical Physics 120(19):9133-9141. Abstract Vapor to liquid multi-component nucleation is a dynamical process governed by a delicate interplay between condensation and evaporation. Since the population of the vapor phase is dominated by monomers at reasonable supersaturations, the formation of clusters is governed by monomer association and dissociation reactions. Since no potential energy barrier exists the formation of a cluster is impeded by a free energy of activation that is entropic in nature. Dynamical Nucleation Theory provides a framework in which equilibrium evaporation rate constants can be calculated and the corresponding condensation rate constants determined from detailed balance. The nucleation rate can then be obtained by solving the kinetic equations. The rate constants governing the multi-step kinetics of multi-component nucleation including sensitivity analysis and the influence of possible contaminants will be presented and discussed.
2004. "Ions at the Air/Water Interface." Science 303(5661):1146-1147. Abstract In the conventional picture of simple salt solutions, atomic ions shun the air/water interface and are more likely to be found in the bulk of the liquid. Hence, simple inorganic salts such as sodium halides should be excluded from the water surface. However, recent computational and experimental studies show that atomic ions such as halides can be present in the surface region, in some cases even at enhanced concentrations. Halide ions at the surfaces of atmospheric aerosol particles may plan an important role in controlling oxidant levels in the marine boundary layer of the atmosphere.
2004. " Molecular mechanism of water and ammonia uptake by the liquid/vapor interface of water ." Chemical Physics Letters 385(3-4):309-313. Abstract Constrained molecular dynamics techniques were used to investigate the mechanism of the transfer of water and ammonia molecules from the gas phase to the liquid phase of water. The computed potentials of mean force or transfer free energies were nearly constant when the solute was more than several angstrom from the Gibbs dividing surface and decreased with no substantial minimum free energy as the solute molecules crossed the interface from the vapor to liquid phase. The computed free energy of solvation for water, estimated from the potential of mean force, was in excellent agreement with the experimental measurement while the corresponding computed solvation free energy for the ammonia molecule somewhat over estimated the experimental value. Possible reasons for the discrepancy are discussed.
2003. "Thermochemistry and kinetics of evaporation and condensation for small water clusters." Chapter 2 in Water in Confining Geometries, ed. V. Buch and J. P. Devlin, pp. 25-51. Springer-Verlag, New York, NY. Abstract The evaluation of thermochemical properties of small water clusters (e.g., consisting of 2 - 10 water molecules) is complicated by their dissociative nature. At temperatures for which dissociation of the cluster into molecular fragments has significant probability, an "operational definition" of a cluster is required to restrict the phase space and evaluate a finite value for the partition function or cluster property of interest. We will review a theoretical approach, Dynamical Nucleation Theory (DNT), for evaluating rate constants for cluster evaporation and condensation, which allows for a unique definition of clusters and their thermochemical properties. We will also discuss the relevance of DNT to homogeneous vapor-to-liquid nucleation of water.
2003. "Reduced Mass in the One-Dimensional Treatment of Tunneling." Journal of Physical Chemistry A 107(19):4006-4007. Abstract This comment points out that the conclusions of Gonzalez et al. in a recent paper this journal [J. Phys. Chem. A 2001, 105, 11034] arise from an inconsistent treatment of the coordinates and their associated reduced masses. We show this by discussing the issues involved in associating a reduced mass or effective mass with a reaction coordinate.
2003. "Generalized Transition State Theory in Terms of the Potential of Mean Force." Journal of Chemical Physics 119(12):5828-5833. doi:10.1063/1.1597477 Abstract The relationship between the free energy of activation and the potential of mean force is derived for reaction coordinates that are arbitrary functions of all the coordinates defining a system. The general result is illustrated for rectilinear reaction coordinates, bond distance reaction coordinates, atom transfer reaction coordinates, synchronous double atom transfer reaction coordinates, and energy gap reaction coordinates.
2003. "H + H2 Thermal Reaction: A Convergence of Theory and Experiment ." Physical Review Letters 91(6):Art. No. 063201. doi:10.1103/PhysRevLett.91.063201 Abstract New experimental and theoretical rate constants for two isotopologs of the simplest chemical reaction, H + H2 H2 + H, are presented. The theoretical results are obtained using accurate quantum dynamics with a converged Born-Oppenheimer potential energy surface and include non-Born-Oppenheimer corrections. The new experiments are carried out using a shock tube and complement earlier investigations over a very large T range, 167 to 2112 K. Experiment and theory now agree perfectly, within experimental error, bringing this 75-year-old scientific problem to completion.
2003. "H + H2 thermal reaction: A convergence of theory and experiment." Physical Review Letters 91(6):Art. No. 063201. doi:10.1103/PhysRevLett.91.063201 Abstract New experimental and theoretical rate constants for two isotopologs of the simplest chemical reaction, D+H2 HD +H, and H + D2 HD + D, are presented. The experiments are carried out using a shock tube and complement earlier investigations over a very large temperature range, 167 to 2112 K. The theoretical results are obtained using accurate quantum dynamics with a converged Born-Oppenheimer potential energy surface and non-Born-Oppenheimer corrections. Experiment and theory now agree perfecty, within experimental error, bringing to completion this seventy-five year old scientific problem.
2003. "Potentials of Mean Force With Ab Initio Mixed Hamiltonian Models of Solvation ." Journal of Molecular Structure - Theochem 632(1-3):173-183. Abstract We give an account of a computationally tractable and efficient procedure for the calculation of potentials of mean force using mixed Hamiltonian models of electronic structure where quantum subsystems are described with computationally intensive ab initio wavefunctions. The mixed Hamiltonian is mapped into an all-classical Hamiltonian that is amenable to a thermodynamic perturbation treatment for the calculation of free energies. A small number of statistically uncorrelated (solute-solvent) configurations are selected from the Monte Carlo random walk generated with the all-classical Hamiltonian approximation. Those are used in the averaging of the free energy using the mixed quantum/classical Hamiltonian. The methodology is illustrated for the micro-solvated SN2 substitution reaction of methyl chloride by hydroxide. We also compare the potential of mean force calculated with the above protocol with an approximate formalism, one in which the potential of mean force calculated with the all-classical Hamiltonian is simply added to the energy of the isolated (non-solvated) solute along the reaction path. Interestingly the latter approach is found to be in semi-quantitative agreement with the full mixed Hamiltonian approximation.
2003. "On the Relationship between the Enthalpy of Formation of Carbenes upon Cleavage of the Double Bond in Fluoroolefins and the Electron Density on the pi Bond: An Ab Initio Study." Doklady. Physical Chemistry 392(1-3):212-216. doi:10.1023/A:1025778018991 Abstract In this study, we established a correlation between the enthalpy (of cleavage of the C=C bond in fluorine-substituted olefins giving rise to two carbenes in the electronic ground state and the distribution of the electron density on this bond.
2002. "Equilibrium Constant for Water Dimerization: Analysis of the Partition Function for a Weakly Bound System." Journal of Physical Chemistry A 106(8):1557-1566. Abstract The treatment of dissociative states in the calculation of the partition function of a weakly bound system, such as the water dimer, is discussed. For a dissociative system, the number of phase-space configurations that contribute to the total partition function from energies above the dissociation energy depend upon the system volume. For a sufficiently large system volume, entropy from these confiurations will dominate over the energy contribution of the local minimum and contributions from dissociative states will dominate the total partition function. The calculation of the dimer partition function requires limiting the phase space of the cluster or providing a definition of those phase space points that correspond to a dimer. Since there is no unique procedure to constrain the phase space of a dimer, we provide an analysis of the dimer partition function using different constraints. For the water dimer at temperatures in the range 200-500 K, the value of the dimer partition function changes by up to 3 orders of magnitude with variations of the constraint.
2002. "Dynamical benchmarks of the nucleation kinetics of water." Journal of Chemical Physics 116(10):4275-4280. Abstract Recently a theory of vapor-to-liquid phase nucleation was developed that was based on the kinetics of cluster formation and decomposition. The new method used variational transition state theory (VTST) to obtain the evaporation and condensa-tion rate constants needed in the kinetic model of nucleation. VTST provides a means to systematically improve estimates for rate constants involved in the nucleation process. In the current work we perform dynamical simulations of the condensation process, estimating the effective reactive cross-section using a definition of a cluster that is determined from VTST. These calculations allow us to characterize dynamical corrections to the VTST rate constants. We find that for water cluster sizes rang-ing from 10-40 waters, VTST estimates of the condensation and evaporation rate constants using a spherical dividing surface require dynamical corrections that are approximately a factor of two.
2002. "A hierarchical family of global analytic Born-Oppenheimer potential energy surfaces for the H + H2 reaction ranging in quality from double-zeta to the complete basis set limit." Journal of Chemical Physics 116(10):4142-4161. Abstract A hierarchical family of analytical Born-Oppenheimer potential energy surfaces has been developed for the H + H2 system. Ab initio calculations of near full CI quality (converged to within ? 1 mEh) were performed for a set of 4067 configurations with the aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ basis sets. The complete basis set (CBS) limit energies were obtained using a highly accurate many-body basis set extrapolation scheme. Surfaces were fitted for the estimated complete basis set (CBS) limit, as well as for the aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ basis sets using a robust new functional form. The mean unsigned fitting error for the CBS surface is a mere 0.0023 kcal/mol, and deviations for data not included in the fitting process are of similarly small magnitudes. Highly accurate calculations of the saddle point and van der Waals minimum configurations were performed using basis sets as large as aug-mcc-pV7Z, and these data show excellent agreement with the results of the extrapolated potential surface. The remaining errors from fitting, correlation treatment, and basis set incompleteness for the new CBS-limit surface are lower by over an order of magnitude compared to any prior analytic surface, and are all now significantly smaller than non-Born-Oppenheimer effects. We expect that this new family of potential surfaces will prove useful in studies elucidating the sensitivity of dynamical quantities to the quality of the potential surface.
2002. "Understanding the Sensitivity of Nucleation Kinetics: A Case Study on Water." Journal of Chemical Physics 116(12):5046-5057. Abstract Small atomic or molecular clusters provide the bridge between vapor and liquid phases. Nucleation is a rare event process by which clusters of a new phase are produced. This process is inherently dynamic and as such the new phase cannot exist until an activation barrier is surmounted. Dynamical Nucleation Theory (DNT) utilizes variational transition state theory (VTST) to provide a framework in which cluster evaporation and condensation rate constants can be determined directly. To date, the fundamental nature regarding the intrinsic instability of the kinetics of the nucleation process has eluded theoretical efforts. In this paper we present a sensitivity analysis of the homogeneous nucleation rate on kinetic parameters used in DNT. Moreover, several classical interaction potentials for water exist; most of which have been parameterized to reproduce some bulk properties of water at ambient conditions. Thus, an analysis was undertaken to explore what effects the different water potentials have on the dynamical quantities relevant to nucleation. The implication of these results on the direction of future work will be discussed.
2001. "Thermal and State-Selected Rate Coefficients for the O(3P) + HCI Reaction and New Calculations of the Barrier Height and Width." Journal of Physical Chemistry A 105:2298. Abstract Thermal and State-Selected Rate Coefficients for the O(3P) + HCI Reaction and New Calculations of the Barrier Height and Width
2001. "Variational transition state theory evaluation of the rate constant for proton transfer in a polar solvent." Journal of Chemical Physics 115:8460-8480. Abstract N/A
2001. "A Systematic Study of the Reactions of OH- with Chlorinated Methanes: 1. Benchmark Studies of the Gas-Phase Reactions." Journal of Physical Chemistry A 105(32):7724-7736. Abstract Nothing available at this timeNothing available at this timeNothing available at this timeNothing available at this timeNothing available at this timeNothing available at this time
2000. "Multidimensional Transition State Theory and the Validity of Grote-Hynes Theory." Journal of Physical Chemistry B 104(5):1069-1072.
2000. "The Quantum Vibrational Dynamics of Cl-(H2O)n Clusters." Journal of Chemical Physics 113(13):5171-5178. Abstract The centroid molecular dynamics technique is applied to the case of chloride- water clusters to estimate their finite temperature quantum vibrational structure. We employ the flexible RWK2 water potential [J.R. Reimers, R.O. Watts, and M.L. Klein, Chem. Phys. 64, 95 (1982)] and the parametrization of a chloride-water interaction potential of Dorsett, Watts and Xantheas.[J. Phys. Chem. A 103, 3351 (1999)] We then investigate the temperature-dependent vibrational structure (infrared spectra). We find that the centroid molecular dynamics technique is capable of recovering a majority of the redshift associated with hydrogen bonding.
2000. "Perspective on "The Transition State Method" by E. Wigner [Trans.Faraday Soc. 34, 29-41 (1938)]." Theoretical Chemistry Accounts 103(3-4):200-204.
2000. "Barrier for the H2CO --> H2 + CO reaction: A discrepancy between high-level electronic structure calculations and experiment." Journal of Chemical Physics 113(1):218-226. Abstract The unimolecular dissociation of formaldehyde to H2 + CO was studied using extended basis set calculations and a variety of medium-to-high accuracy correlation techniques, including second and fourth order perturbation theory, multireference configuration interaction wavefunctions, coupled cluster theory with perturbative triples and full iterative triples, and estimated full configuration interaction wavefunctions. The intrinsic error of the electronic structure methods was assessed by extrapolating total energies to the complete basis set limit. Our best estimate of the barrier height, including zero point vibrational effects, is 81.9 kcal/mol, almost 3 kcal/mol larger than the experimental value of 79.2 plus or minus 0.8 kcal/mol. This estimate includes corrections for the effects of finite basis set truncation (which is negligible at the quintuple zeta level), higher order correlation, core/valence correlation and scalar relativistic energy differences. Using the same theoretical approach, we estimate the exothermicity of the dissociation reaction to be -1.6 kcal/mol, compared to experimental values in the -0.4 to -2.2 kcal/mol range. New calculations of the unimolecular dissociation rate constants using a variety of techniques failed to reconcile theory and experiment.
1999. "Accommodation of Alcohols by the Liquid/Vapor Interface of Water: Molecular Dynamics Study." Journal of Physical Chemistry B 103(5):844-851.
1999. "Variational Transition State Theory of Vapor Phase Nucleation." Journal of Chemical Physics 110(16):7951-7959.
1999. "Dynamical Nucleation Theory: A New Molecular Approach to Vapor-Liquid Nucleation." Physical Review Letters 82(17):3484-3487.
1999. "The Utility of Many-Body Decompositions for the Accurate Basis Set Extrapolation of Ab Inito Data." Journal of Chemical Physics 111(9):3806-3811.
1999. "Dynamical Nucleation Theory: Calculation of Condenstion Rate Constants for Small Water Clusters." Journal of Chemical Physics 111(10):4688-4697.
1999. "Ab Initio Study of the Wolff Rearrangement of C6H40 Intermediate in the Gas Phase." Russian Chemical Bulletin 48(9):1642-1646. Abstract Ab initio calculations of the geometry and reactivity of 1,2-ketocarbene C6H4O as an intermediate in organic reactions were performed using the second-order Møller-Plesset (MP2) perturbation theory in the 6-311G* basis set. Only the singlet state of the intermediate was considered. An oxirene-like structure (6) with a six-membered ring and a ketene-like structure (5) with a five-membered ring were localized on the potential energy surface. Attempts to locate a quinone type structure characteristic of aliphatic ketocarbenes failed. The energy of structure5 is −70 kcal mol−1 lower than that of structure6. Harmonic frequencies and intensities of normal vibrations in the IR spectra of6 and5 were calculated. The activation energy of the Wolff rearrangement6→5 was estimated at 12.5 kcal mol−1. The geometry of the transition state of this reaction resembles the quinone-like structure.
1998. "Comparison of Classical and Quantum Statistical Mechanical Simulations of Aqueous Ionic Clusters." In Advances in Classical Trajectory Methods, Volume 3: Comparisons of classical and quantum dynamics , ed. William L. Hase, pp. 1-33. JAI Press, Inc., Stamford, CT. Abstract There is no abstract currently available for this item