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Dispersion forces are ubiquitous in chemistry, yet they are often misunderstood. This article provides background into why they are called dispersion forces and explains how to describe them in terms of time-independent quantum mechanics. The article also describes the breakdown of the multipole series that is often used to describe dispersion forces, and it comments on the representation of dispersion forces in molecular mechanics. In general, the article tries to provide a proper framework for understanding and describing dispersion forces.

The increasing use of computational quantum chemistry in the modeling of environmentally important processes is described. The employment of computational quantum mechanics for the prediction of oxidation-reduction potential for solutes in an aqueous medium is discussed.

A general example of a delocalization system associated with a higher energy than the localized one, which suggests that it is wrong to consider delocalization as equivalent to resonance stabilization, is presented. The meaning of resonance energy as it appears in valence bond theory is described as the lowering of the calculated ground-state electronic energy when one improves a quantum mechanical model containing a single valence bond structure in the wave function to include other significance valence bond structure.

Presents a parallel treatment of the same phenomena by two approaches (steady-state approximation and approximate solution of the appropriate diffusion equation) to clarify the physical meaning of the important radiation boundary condition in the diffusional derivation. (JN)

The chemistry of several polyatomic reaction kinetics and dynamics is studied to demonstrate the importance of the C[H.sub.5] potential energy surfaces. These surfaces are found to have similar features that help them to efficiently support the reactions.

Density functional theory was used to examine the solvation number and aggregation state of several alkyllithium compounds in clusters with tetrahydrofuran molecules coordinated to each lithium atom. The number of coordinated solvent molecules in the monomer and dimer show significant temperature dependence, as does the aggregation state of the alkyllithiums.

The theoretical and computational modeling of bimolecular reactions especially with generally applicable methods for kinetics is presented. The model is derived from improved methods for generating and using reactive potential energy surfaces, especially implicit potential energy surfaces generated by direct dynamics.

Simulations of hydride and deuteride transfer catalyzed by dihydrofolate reductase from the hyperthermophile Thermotoga maritima (TmDHFR) are presented using ensemble-averaged variational transition theory with multidimensional tunneling. The results suggest that quantum mechanical tunneling makes a significant contribution to the reaction rate at all temperatures and dimerization is important for thermal stability of TmDHFR and its catalytic activity.

Potential of mean force calculations were performed on the wild-type medium-chain acyl-CoA dehydrogenase (MCAD) and two of its mutant forms. The study demonstrates that significant insights into the catalytic mechanism can be obtained from simulation studies, and it is possible to design novel mechanistic probes for the enzyme with the help of the result.

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.

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 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.

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.

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.

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.

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.

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.

Carbenes occur widely as reactive intermediates in many organic reactions and they are extensively studied. The isoelectronic analog of a carbene with a negative charge is a borenide anion. A study has been undertakent to characterize how a charge on a divalent atom affects reactivity, through the calculation of barrier heights to 1,2-hydrogen migration, for single states of methylphenylborenide, methylphenylnitrenium and methylphenylcarbene. Energy splittings are also examined.

The reaction rate constant of the trans-Rh(PHsub3)sub3C1(etasuper2-CHsub4) to RH(PHsub3)sub2-CIH(CHsub3) rearrangement, where the C-H bond of methane is activated, is calaculated. Density functional theory based on the direct dynamics approach was used to obtain Born-Oppenheimer potential energies, reaction-path geometries and vibrational frequences.

The rate constants and deuterium kinetic isotope effects (KIEs) for the competing SN2 and E2 reactions of ClO- and C2H5Cl were determined. Dual-level generalized transition state theory and statistical calculations based on correlated electronic calculation were used in the study. The results showed that the KIEs at room temperature were normal for the E2 reaction and inverse for the SN2 reaction. The temperature dependences of the KIEs were also observed.

The calculated kinetic isotopic effect (KIE) and their temperature dependence for gas-phase S(sub N)2 reactions X(super -) + CH3Y gives CH3X + Y(super -)(X, Y = Cl, Br, I) are consistent with the experimentally established data. Small alterations of the transition state theory from conventional transition state occurs. The KIE is susceptible to vibrational frequencies. Anharmonicity have substantial effect on the experimental results. Rate constants exhibit negative correlation below 500 K.

Theoretical studies help develop a quantum mechanical solvation model to evaluate solvation free energies in the nonpolar solvent n-hexadecane. Transferable parameters of the solvation model are Hamiltonian-independent. This model utilizes the CM1 charge models and a set of both short and long-range surface tensions. Solvation effect significantly influences the size of the primary solvation shell. Application of this model to 306 compounds produces an rms errors of 0.41 kcal mol.

Theoretical studies help examine gas-phase S(N)2 reactions between F(-)(H2O) and CH3Cl, and the deuterium substituted analogs. Electronic structure calculations help define geometries and evaluate energies and vibrational frequencies of reactants, products and saddle points of the reactions. The data helps determine transition state theory rate constants and secondary kinetic isotope effects for CD3 and D2O substitution.

Aqueous solvation of all possible conformers of 1,2-ethanediol are analyzed by the combination of correlated, extended basis set, gas-phase free energies with the quantum statistical SMx models. Aqueous equilibrium population and free energy of solvation for the solute are determined and are in agreement with the experimental results. Free energies of solvation between conformers with and without hydrogen bonds are calculated and show increased trans C-C conformations in aqueous solutions compared to the gas phase.

Extended-basis-set electronic structure computations with high levels of electron correlation for heterocyclic tautomerizations were reinforced by an elaborate study of the aqueous solvation of free energy differences, including electronic and geometric relaxation in aqueous solution. The Austin Model 1-solvation model 2 (AM1-SM2) formed the basis of the attempts.

A study used a semiclassical technique to estimate the kinetic isotope effect of the reaction Fe3 + CD3H to understand the tunneling paths for hydrogen atom transfer. The neglect of diatomic differential overlap (NDDO) molecular orbital theory with semiempirical specific-reaction parameters (SRP) are used. With variational transition state theory and multidimensional semiclassical tunneling computations including corner cutting, this technique can be used to model to intricate quantum mechanical dynamics of organic reactions.

Researchers used a quantum chemical solvation model drawn from a continuum treatment of dielectric polarization and solvent accessible surface area to assess the aqueous solvation influences on the anomeric and conformational equilibria of D-glucopyranose. The solvation model places the comparative sequencing of the hydroxymethyl conformers in accordance with the experimentally identified sequencing of populations. The study offers proof of intra-glucose hydrogen bond conjugation in aqueous solution.

The kinetic isotope effect for the (1,5) sigmatropic rearrangement reaction of cis-1,3-pentadiene was studied. The direct dynamics method was employed in conjunction with a new computer program to simulate the reaction process. All dynamic calculations were performed based on the cannonical variational transition-state theory. The results show excellent agreement with available experimental data.

Analyses of Claisen rearrangement of carboxymethylated derivative, possible methoxy-substituted derivatives, a pair of alkylated derivatives and allyl vinyl ether confirm that electric polarization and first hydration shell hydrophilic effects are responsible for the aqueous acceleration of the rearrangement. Responsiveness to substitution pattern is very high.

We present a new local density functional, called M06-L, for main-group and transition element thermochemistry, thermochemical kinetics, and noncovalent interactions. The functional is designed to capture the main dependence of the exchange-correlation energy on local spin density, spin density gradient, and spin kinetic energy density, and it is parametrized to satisfy the uniform-electron-gas limit and to have good performance for both main-group chemistry and transition metal chemistry. The M06-L functional and 14 other functionals have been comparatively assessed against 22 energetic databases. Among the tested functionals, which include the popular B3LYP, BLYP, and BP86 functionals as well as our previous M05 functional, the M06-L functional gives the best overall performance for a combination of main-group thermochemistry, thermochemical kinetics, and organometallic, inorganometallic, biological, and noncovalent interactions. It also does very well for predicting geometries and vibrational frequencies. Because of the computational advantages of local functionals, the present functional should be very useful for many applications in chemistry, especially for simulations on moderate-sized and large systems and when long time scales must be addressed. [ABSTRACT FROM AUTHOR]

In the present study, we comparatively assessed the newly developed M05 functional against a data set of reaction energies for transition-metal chemistry. The functionals to which we compare are BLYP, B3LYP, B97-2, MPWLYP1M, TPSS, and TPSSh. We draw the following conclusions: (1) TPSS gives the best performance for calculating the binding energies of three transition-metal dimers (Sc2, Ni2, and V2) that have severe multireference character, (2) B97-2 gives the best performance for calculating the binding energies of the nine metal-ligand diatomics (three monohydrides, three monoxide, and three monofluorides), and (3) M05 gives the overall best performance for all 18 data in the assessment, and it has a mean unsigned error 55% lower than the popular B3LYP functional. Since the M05 functional also gives good performance for main-group thermochemistry, for noncovalent chemistry, and for calculating barrier heights, M05 can be applied to a wide range of problems where nonhybrid functionals or functionals designed for kinetics fail. [ABSTRACT FROM AUTHOR]

Several liquid-phase density functional theory calculations are conducted to explain the different concerted and nonconcerted [alpha],[beta]-elimination reactions of esters, as well as thioesters. The compounds are shown to prefer the anti transition state more, as compared to the syn state.

A new model is described for predicting branching ratios of chemical reactions when a branching of the reaction path occurs after the dynamical bottleneck, including the case where it occurs after an intermediate. The new model is based on combining nonstatistical phase space theory for the direct component of a reaction with variational transition-state theory for an indirect component of reaction and the competition between direct and indirect processes is treated by the unified statistical model.

4-Oxalocrotonate tautomerase enzyme (4-OT) has catalyzed the isomerization of 2-oxo-4-hexenedioate and the isomerization is studied by using the combined quantum mechanical and molecular mechanical method. The studies have shown that proton transfer process is coupled to a conformational change of the substrate and the conformation change is promoted by the changes in the electron distribution of the substrate taking place as the proton transfer has occurred.

The melting transition of metal nanoparticles is examined with a realistic many-body potential developed from quantum mechanical calculations on nanoparticles. The studies have shown that [Al.sub.n] nanoparticles have a wide temperature window of coexistence of solid and liquid states and this coexistence regime is called the slush state.

The multiconfigurational electronic structure calculations are employed to study the relation between the hydrogen atom transfer (HAT) and proton-coupled electron transfer (PCET) mechanisms. The conical intersection seam is shown to be a major topographical feature, which affects the reaction between phenol and radicals.

An overview of the algorithms associated with the computation of free energies of solvation in the SM8 model is presented. The accuracies of the SM8 model compared to other continuum solvation models to compute ionic solvation, that is free energies, oxidation and reduction potentials, and p [K.sub.a] values are also discussed.

The article discusses the design and preparation of new MO6-class density functionals that are widely used in computational chemistry. These functionals are found to be depended on the spin densities and spin density gradients and are found to be extremely effective and cheap.

The characterization of thermodynamic properties of neural [Al.sub.n] (2 (less than or equal to) n (less than or equal to) 65) particles which provide insight into size-selective reactivity and catalytic activity of metal nanoparticles is reported.

An intramolecular proton-transfer mechanism is described for the carbocationic cyclization of farnesyl pyrophosphate (FPP) to (+)-aristolochene catalyzed by aristolochene synthase. The results have shown that the mutations of either residue have led to pronounced generation of aborted cyclization products.

The rate constants for the OH + [H.sub.2]S -> [H.sub.2]O + HS reaction are calculated by direct dynamics with the M06-2X density functional theory. The rate constants are important for both atmospheric chemistry and combustion, while the temperature dependence is accounted for even in the context of a simple abstraction reaction.