Search

Research has been conducted on the fluoromethyllithium and chloromethyllithium carbenoids. The monomers and dimers of these carbenoids have been investigated in the gas phase and in dimethyl ether solvent in studying carbenoids' solvation effect and the use of quantum mechanical calculations is described.

Research has been conducted on the effects of quantum mechanics in enzyme kinetics. The topics of interest include atoms' electronic structure in the catalytic center, the nature of the quantum mechanical vibrational energies and the multidimensional tunneling.

A study of the parametrization using compounds in the data set that do not contain bond between SI and O at the HF/MIDI!6D,HF/6-31G*,HF/6-31(sub +)G*,HF/cc-pVDZ,BPW91/MIDI!,BPW91/MIDI!6D,BPW91/DVZP,B3LYP/MIDI!,AMI,and PM3 levels of theory is presented. The results show that the mean unsigned errors over the eight differential free energies of solvation and three absolute free energies for these levels of theory are in the range of 0.48-0.53kcal/mol.

A new algorithm is presented for carrying out large-curvature tunneling (LCT) calculations. It is tested on five hydrogen-atom transfer reactions and it is found that more than an order of magnitude can reduce the cost of LCT calculations.

Methods for incorporation of quantum mechanical effects into enzyme kinetics simulations in which the enzyme is an explicit part of the model are discussed with emphasis on three aspects. The first is the use of quantum mechanical electronic structure methods such as molecular orbital theory and density functional theory, usually in conjunction with molecular mechanics. The second is treating vibrational motions quantum mechanically in an instantaneous harmonic approximation, or by path integrals, or by a three-dimensional wave function coupled to classical nuclear motion. The third is incorporation of multidimensional tunneling approximations into reaction rate calculations.

The solvent-dependent transition states for decarboxylations are interpreted.

The hybrid Hartree-Fock density functional theory is compared to ab initio approaches for locating the saddle point geometries and calculating barrier heights on a Born-Oppenhiemer potential energy surface. The database is dominated by hydrogen-atom transfer reactions of neutral species in the gas phase.

The characterization of alternative mechanisms for the reductive dechlorination of hexachloroethane (HCA) to perchloroethylene is discussed. It was possible to obtain theoretical predictions accurate to within 23 mV for four aqueous reduction potentials relevant to HCA.

A set of effective solvent descriptors was created using a database of 440 molecules, that characterize the organic carbon component of soil. Solute atomic surface tension parameters were combined with this set of effective solvent descriptors. The article is broken down into an introduction, methodology and theory of the study, and the results.

Research is done on two classes of semiclassical methods for studying multielectronic-state dynamics, the trajectory surface-hopping (TSH) method and the time-dependent self-consistent field (TDSCF) method. In conclusion, several aspects of various nonadiabetic trajectory methods are shown pictorially.

Research is presented on the computation of kinetic isotope effects and Swain-Schaad exponents in enzymatic reactions by using combined quantum mechanical and molecular mechanical potential energy function in quantum dynamics simulations.

A multi-coefficient correlation method is studied based on quadratic configuration interaction with single and double excitations (MC-QCISD) and basis sets using segmented contraction and having the same exponential parameters in the s and p spaces. It is concluded that the new method should be very efficient for computing geometries of open-shell transition states.

Research designed to create a systematic database of classical barrier heights that can play a role in testing electronic structure methods for use in forecasting chemical reactivity is presented. A new hybrid Hartree-Fock-density functional theory is suggested.

It has been possible to apply the SM5.4, SM5.0R and SM5.2R solvation models to determine the vapour pressure of 156 molecules. The SM5 series of solvation models predicts free energies of self-solvation to better than 1 kcal/mol.

It has been possible to parametrize the SM5.42R solvation model for the intermediate neglect of differential overlap for spectroscopy method. The parametrization is based on 2184 free energies of solvation of 49 ions and 275 neutral solutes in water and 90 organic solvents.

Multidimensional transition state theory is used as the basis for an investigation of the Grote-Hynes theory of nonequilibrium solvation effects on reaction kinetics. It has been possible to derive intermediate quantities that give an insight into the ability of Grote-Hynes theory to capture relevant physical features of the reaction dynamics.

The performance of the ROMP2, UQCISD, UQCISD(T), RUCCSD, UCCSD, RCCSD(T), UCCSD(T) and RUCCSD(T) many-electron formulations has been tested with two one-electron basis sets on three hydrogen abstraction reactions.

A study was conducted on the development of mixed quantal-classical techniques for explaining the dynamics of electronically nonadiabatic molecular processes. It extended a previous experiment where four semiclassical approaches were tested against precise quantal findings for three atom-diatom reactions involving conical intersections. This study tested the same four semiclassical methods against precise quantal findings for an atom-diatom reaction where the diabatic potential surfaces do not cross but are roughly parallel in the entrance channel.

Continuum solvation models that are based on microscopic parameters provide a highly-efficient and accurate framework for assessing solvation phenomena in qualitative terms. Compared to conventional models, continuum solvation platforms reduces the degree of freedom involved in the same, thereby making solvation measurements more accurate. Continuum models also provide an efficient and optimally precise means of assessing strong, long-range electrostatic forces.

A study relating to ab initio calculations of electronic structure and molecular energies proposes an empirical multi-coefficient technique for simultaneously extrapolating both the different components of the correlation energy and the basis set. It was prompted by two extrapolation methods proposed in previous research. The proposed technique is very appropriate for calculating potential energy surfaces. Experimental methods, findings and results are discussed.

Moller-Plesset perturbation theory and coupled-cluster theory are used in deriving 12 general parametrizations of the scaling-all-correlation (SAC) method for the semiempirical estimation of electronic structure calculations. The computed F parameters are better than previous ones in solving thermochemical properties such as bond energies, potential curves, potential surfaces and barrier heights for chemical reaction. All calculations were done using the Gaussian94 software.

Quantum mechanical cumulative reaction probabilities (CRPs) for the reaction Cl + H2 -> HCl + H and its reverse are calculated and converged. These CRPs are then analyzed for the properties of quantized dynamical bottlenecks that control microcanonical-ensemble rate constants for the Cl + H2 reaction and its reverse. Analysis of the total and state-specific rate constants provides compelling evidence that quantized transition states exist.

The reaction rate and kinetic isotope effects (KIE) during the conversion of 2-phospho-D-glycerate (2-PGA) to phosphoenolpyruvate (PEP) by the yeast enolase during glucose fermentation have been calculated using semiclassical variational transition state theory. Enolase is a glycolytic enzyme that catalyzes the reversible dehydration of 2-PGA to PEP. Primary and secondary KIEs strongly point to the presence of a stepwise carbanionic mechanism in the early formation of a carbanion intermediate.

Potential energy surface calculations yield physical insight into the structure of intermediates and the dynamics of molecular collisions, and they are the first step toward molecular simulations that provide physical insight into energy transfer, reaction, and dissociation probabilities. The potential energy surface for high-energy collisions of N2 with N can be used for modeling chemical dynamics and energy transfer in atmospheric shock waves. Here we present an analytic ground-state. (4A′′) potential energy surface for N3 that governs electronically adiabatic collisions of N2(1Σ +g) with N(4S). The fitted surface consists of a pairwise potential based on an accurate diatomic potential energy curve plus a connected permutationally invariant polynomial (PIP) in mixed-exponential-Gaussian bond order variables (MEGs) for the three-body part. The three-body fit is based on multireference complete active space second order perturbation theory (CASPT2) calculations. The quality of the quartet N3 fit is comparable to that for a previous fit of the NO2 potential. We characterize two local minima of N3, two tight transition structures, two van der Waals geometries, and the noncollinear reaction path for the symmetric exchange reaction. The nonreactive approach of an N atom to N2 along the perpendicular bisector is more repulsive than the collinear reproach, but plots of the force on the bond versus the potential energy at the distance of closest approach allow us to infer that vibrational energy transfer should occur much more readily in high-energy collinear collisions than in high-energy perpendicular-bisector collisions. [ABSTRACT FROM AUTHOR]

The effects of para substitution on the electronic properties of phenylnitrenium ions in the gas phase and aqueous solution were investigated via quantum chemical computations. Linear free energy relationships between phenylnitrenium ion S-T gaps and three model substituents for para-substitution effects were also examined. Experimental results revealed trends in singlet-triplet splittings, singlet aromatic stretching frequencies and singlet C-N+ bond lengths, which are associated with linear free energy relationships.

This article presents a perspective on Kohn-Sham density functional theory (KS-DFT) for electronic structure calculations in chemical physics. This theory is in widespread use for applications to both molecules and solids. We pay special attention to several aspects where there are both concerns and progress toward solutions. These include: 1. The treatment of open-shell and inherently multiconfigurational systems (the latter are often called multireference systems and are variously classified as having strong correlation, near-degeneracy correlation, or high static correlation; KS-DFT must treat these systems with broken-symmetry determinants). 2. The treatment of noncovalent interactions. 3. The choice between developing new functionals by parametrization, by theoretical constraints, or by a combination. 4. The ingredients of the exchange-correlation functionals used by KS-DFT, including spin densities, the magnitudes of their gradients, spin-specific kinetic energy densities, nonlocal exchange (Hartree-Fock exchange), nonlocal correlation, and subshell-dependent corrections (DFT+U). 5. The quest for a universal functional, where we summarize some of the success of the latest Minnesota functionals, namely MN15-L and MN15, which were obtained by optimization against diverse databases. 6. Time-dependent density functional theory, which is an extension of DFT to treat time-dependent problems and excited states. The review is a snapshot of a rapidly moving field, and--like Marcel Duchamp--we hope to convey progress in a stimulating way. [ABSTRACT FROM AUTHOR]

In the past decade, a number of approaches have been developed to fix the failure of (semi)local density-functional theory (DFT) in describing intermolecular interactions. The performance of several such approaches with respect to highly accurate benchmarks is compared here on a set of separation-dependent interaction energies for ten dimers. Since the benchmarks were unknown before the DFT-based results were collected, this comparison constitutes a blind test of these methods. [ABSTRACT FROM AUTHOR]

We present quasiclassical trajectory simulations of the high-energy collision-induced O(³P) + N2 reaction. The simulation results show that reactive scattering may proceed on both the ³A′ and ³A" potential energy surfaces, but the reaction on ³A" surface overwhelmingly dominates. At total energies of collision slightly above the threshold, the NO(X²Π) product is found to be scattered primarily in the forward direction, and at higher collision energies, the fraction of NO products that scatter in the backward direction increases. Two-dimensional joint distributions of the center-of-mass differential cross section and translational energy have been examined to unravel the details of the scattering as functions of collision energy. [ABSTRACT FROM AUTHOR]

Metal-organic frameworks (MOFs) have many potential uses for separations, storage, and catalysis, but their use as intercalation hosts for batteries has been scarce. In this article, we examine the mechanism of Li insertion in a MOF to provide guidance to future design efforts in this area. As a model system, we choose UiO-66, a MOF with the formula (Zr6O4(OH)4)4 (1,4-benzenedicarboxylate)6, as an electrode material for lithium-ion batteries; this MOF is of special interest because the zirconium is not redox active. We report both quantum mechanical characterization of the mechanism and experimental studies in which the material is synthesized as nanoparticles to reduce diffusion lengths for lithium ions and increase the contact area with a conductive carbon phase. The calculated changes in the IR spectra of UiO-66 and lithiated UiO-66 are consistent with the experimental FTIR results. We found experimentally that this MOF can maintain a specific discharge capacity of at least 118 mAh/g for 30 lithiation and delithiation cycles at a rate of C/5, exhibiting good cyclability. Density functional electronic structure calculations show that the charge transfer during lithiation is mainly from Li to node oxygens and carboxylate oxygens, that is, it involves anions rather than cations or aromatic rings, and they provide a mechanistic understanding of the potential for increased Li capacity because the theoretical capacity of UiO-66 with Li at the oxygens in the metal oxide nodes and the carboxylate linkers is more than 400 mAh/g. The lithiation process greatly decreases the bandgap of UiO-66, which is expected to increase its electronic conductivity. The electrode material was also characterized by X-ray diffraction and scanning electron microscopy, which were consistent in confirming that smaller particle sizes were obtained in lower-temperature syntheses. [ABSTRACT FROM AUTHOR]

We recently proposed the dipole-quadrupole (DQ) method for transforming adiabatic electronic states to diabatic states by using matrix elements of the dipole and quadrupole operators, and we applied the method to 3-state diabatizations of LiH and phenol. Here we extend the method to also include the electrostatic potential, and we call the resulting method the DQΦ method, which denotes the dipoleâ€"quadrupoleâ€"electrostatic-potential diabatization method. The electrostatic potential provides extra flexibility, and the goal of the present work is to test and illustrate the robustness of the methods for producing diabatic potential energy curves that tend to the adiabatic curves away from crossings and avoided crossings and are smooth in regions of crossings and avoided crossings. We illustrate the generality of the methods by an application to LiH with four states and by two-state diabatizations of HCl, (H2)2, O3, and the reaction Li + HF â?' LiF + H. We find thatÂ--if enough states are includedÂ--the DQ method does not have a significant dependence on the parameter weighting the quadrupole moment, and a geometryindependent value of 10 a0-2 is adequate in all cases tested. We also find that the addition of the electrostatic potential improves the diabatic potentials in some cases and provides an additional property useful for increasing the generality of the method for diabatization. [ABSTRACT FROM AUTHOR]

Using Feynman path integrals, a molecular partition function can be written as a double integral with the inner integral involving all closed paths centered at a given molecular configuration, and the outer integral involving all possible molecular configurations. In previous work employing Monte Carlo methods to evaluate such partition functions, we presented schemes for importance sampling and stratification in the molecular configurations that constitute the path centroids, but we relied on free-particle paths for sampling the path integrals. At low temperatures, the path sampling is expensive because the paths can travel far from the centroid configuration. We now present a scheme for importance sampling of whole Feynman paths based on harmonic information from an instantaneous normal mode calculation at the centroid configuration, which we refer to as harmonically guided whole-path importance sampling (WPIS). We obtain paths conforming to our chosen importance function by rejection sampling from a distribution of free-particle paths. Sample calculations on CH4 demonstrate that at a temperature of 200 K, about 99.9% of the free-particle paths can be rejected without integration, and at 300 K, about 98% can be rejected. We also show that it is typically possible to reduce the overhead associated with the WPIS scheme by sampling the paths using a significantly lower-order path discretization than that which is needed to converge the partition function. [ABSTRACT FROM AUTHOR]

We present a global ground-state triplet potential energy surface for the N2O2 system that is suitable for treating high-energy vibrational-rotational energy transfer and collision-induced dissociation. The surface is based on multi-state complete-active-space second-order perturbation theory/minimally augmented correlation-consistent polarized valence triple-zeta electronic structure calculations plus dynamically scaled external correlation. In the multireference calculations, the active space has 14 electrons in 12 orbitals. The calculations cover nine arrangements corresponding to dissociative diatom-diatom collisions of N2, O2, and nitric oxide (NO), the interaction of a triatomic molecule (N2O and NO2) with the fourth atom, and the interaction of a diatomic molecule with a single atom (i.e., the triatomic subsystems). The global ground-statepotential energy surface was obtained by fitting the many-body interaction to 54?889 electronic structure data points with a fitting function that is a permutationally invariant polynomial in terms of bond-order functions of the six interatomic distances. [ABSTRACT FROM AUTHOR]

The SMR.0R model for computing solvation energies employing only geometry-dependent atomic surface tensions is extended to organic solvents. The technique has a mean unsigned error of approximately 0.4 kcal/mol when used with gas-phase geometries computed at either the Hartree-Fock level with a heteroatom-polarized valence-double-zeta basis set or when using with semi-empirical molecular orbital gas-phase geometries.

A study based on variational transition state theory was conducted on the temperature dependence of the dynamical bottleneck location for the prototype olefin addition process H + C2H4. A method called variable scaling of external correlation which uses a scale factor was developed. The findings indicate that tunneling effects are quantitatively responsible for the curvature of the Arrhenius plots.

Variational transition state calculations and tunneling calculations for a new class of reactions with high-frequency vibrations strongly linked to the reaction coordinate, (Cl-)(CH3NH3+) -> ClCH3(NH3) and CICH3 + NH3, and CH3Cl(H2O) + NH3(H2O) -> (CH3NH3+)(Cl-)(H2O)2. The calculations used the new RODS algorithm based on optimizing the orientation of the dividing surface at each point along the reaction path. It has proven useful in performing stable calculations of the reactions and for speeding up convergence on several other problems.

The integrated molecular orbital method with harmonic cap (IMOHC) is a new technique for geometry optimization in integrated calculations, which can be used for QM:MM and dual-level-QM methods. The approach, which is an extension to the original integrated dual-level methods, has strong potential for determining gradients, Hessians and optimized geometries of large molecules. IMOHC, which has been found to be simple and does not require additional computational cost, is based on a harmonic restoring potential for the capping atom in the capped subsystem.

A study was conducted relating to research on trajectory surface hopping as a model for electronically nonadiabatic processes in molecules. The experiment reports on a test of two of the most general trajectory surface hopping algorithms as well as two variants against accurate quantum dynamics for a three-dimensional reactive collision process. In particular, the study considers a model system where nonreactive quenching competes with reaction.