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The small curvature method is used for calculating the variational transition-state theory constants with multidimensional tunneling contributions for the C[F.sub.3]C[H.sub.2]C[H.sub.3] + OH reaction. The reliability of the structure activity relationship that is used for estimating the rate constant values for OH reactions with hydrofluorocarbons is discussed.

The lipid peroxidation is studied using a nonadenine model of linoleic acid. The oxidation is found to be terminal involving the mono-allylic methylene groups at the end for hydrogen abstraction.

Two mechanisms are proposed for the racemization reaction catalyzed by Aquifex pyrophillus glutamate racemase enzyme. The results have shown that the behavior of an enzyme depends on the ionization state of its active site residues and justifies the calculation of their ionization probabilities to understand the enzyme catalytic mechanism.

A fully microscopical simulation of the rate-limiting hydrogen abstraction catalyzed by soybean lipoxygenase-1 (SLO-1) was carried out and the rate constants and kinetic isotope effect (KIE) were calculated. The results show that the compression of the wild-type active site enzyme results in the huge contribution of tunneling to the rate of the hydrogen abstraction and the calculated KIE for the wild-type enzyme has an important dependence on the barrier width.

Computer simulations on a QM/MM potential energy surface are carried out to gain insights into the catalytic mechanism of glutamate racemase (Murl). Studies suggest two possible roles for Murl as a catalyst, which are achieved without resorting to covalent bond formulation between the enzyme or cofactor and the transition state.

Variational transition state theory is used to calculate the kinetic isotope effects affecting product ratios in the reaction between 1O2 and d6-tetramethylethylene. The calculated KIE at 263 K turns out to be 1.126 and analogously, a H/T KIE of 1.17 at the same temperature is found for the reaction of (super 1)O(sub 2) with the tritiated derivative of tetramethylethylene.

The first variational transition-state theory rate constant calculations including multidimensional tunneling corrections of the OH + CH3SH reaction and several isotopic variants is presented. It is found that the reaction takes place by forming first a weakly bound complex that presents two hydrogen bonds between the hydroxyl radical and methanethiol from which two reaction paths diverge.

The variational transition-phase rate constant computation for the OH + CH3COCH(sub3)-> P reaction is presented. The potential energy surface is described by low level calculations at the B3LYP/6-31G level jointly with higher-level calculations applying the multilevel CBS-RAD method.

Research has been conducted on the cyanobenzyl alkyl ethers' radical anions. The study of the fragmentation reaction of these anions carried out electrochemically is presented.

The presence of a negative slope of the chemical shift as a function of the temperature can be used to identify hydrogen bonds as low-barrier hydrogen bonds in gas phase and even in inert solvents. This was gleaned from an analysis of the nuclear magnetic resonance chemical shifts of the proton involved in the intramolecular hydrogen bond in hexabenzyloxymethyl-XDK monoanion and hydrogen oxalate anion. It was found that the chemical shift decreases as the temperature rises for hexabenzyloxymethyl-XDK monoanion, while the opposite is true for the hydrogen oxalate anion.

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.

Quantum mechanical and thermodynamic ab initio calculations were performed to characterize and compare the gas-phase properties of the intramolecular hydrogen bond in the maleic monoanion, otherwise known as hydrogen maleate anion, with hydrogen bonds in the monoanion of maleic monoamide. The results provided evidence concerning its status as a low-barrier hydrogen bond, one of which relates to the short distance between the hydrogen-donor and hydrogen-acceptor atoms.

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.

Molecular Dynamics simulations with a Molecular Mechanics (MM) force field and the quantum mechanics (QM)/MM potential energy surfaces are preformed to analyze the D-glutamate -> L-glutamate reaction catalyzed by Bacillus subtilis glutamate racemase. The studies have shown that the whole process has involved four successive proton transfers occurring in three different steps.

The mechanism for the atmospheric oxidation of methanesulfinic acid (MSIA) is studied. Results show that as the temperature increases the CH3S(O)2 radical decomposes faster, rapidly increasing the methanesulfonic (MSA) yield.

Variational transition state theory including semiclassical multidimensional tunneling corrections is used to study the hydrogen abstraction reaction from (-)-epicatechin (EC) by a methylperoxyl radical as the peroxolipidic radical model. Quantum-mechanical tunneling has increased the corresponding rate constant value in such a way that catechins can trap the lipid peroxyl radicals in a dominant competition with the very damaging free-radical chain-lipid peroxidation reaction.

The reaction mechanism of the racemization of mandelate substrate by mandelate racemase enzyme is studied and a quantum mechanics/molecular mechanics (QM/MM) potential energy surface is used to compute the free energy profiles associated with the reaction. The results show that enzyme stabilized the reactants and the products through local hydrogen bond interactions and short and long range electrostatic contributions.

A synthetic improvement of the known reaction of polyfluoroalkoxylation is described by using an excess of nucleophile/base, a change that transforms the mechanism from a slow polar S(sub N)Ar to a very fast chain radical one. Polyfluoroalkoxylation of aromatics can be carried by mild substitution of a nitro in nitroaromatic compounds in DMF, under conditions that include a relatively high concentration of substrate, nucleophile, and base.

Density functional theory electronic structure calculations of the homolytic/heterlytic aniomesolytic C-O fragmentations in the gas phase of a series of radical anions of substituted-phenyl benzyl ethers and substituted-benzyl phenyl ethers are carried out. The result showed that along the series, the electron-withdrawing strength of the substituents increases.

The first theoretical study of the oxidation reaction of glycolaldehyde by the OH radical is presented. The dual-level interpolation algorithms are used to calculate the variational transition-state theory rate constants with multidimensional tunneling corrections (VTST-MT).

In many cases, variational transition states for a chemical reaction are significantly displaced from a saddle point because of zero-point and entropic effects that depend on the reaction coordinate. Such displacements are often controlled by the competition between the potential energy along the minimum-energy reaction path and the energy requirements of one or more vibrational modes whose frequencies show a large variation along the reaction path. In calculating reaction rates from potential-energy functions we need to take account of these factors and—especially at lower temperatures—to include tunneling contributions, which also depend on the variation of vibrational frequencies along a reaction path. To include these effects requires more information about the activated complex region of the potential-energy surface than is required for conventional transition-state theory. In the present article we show how the vibrational and entropic effects of variational transition-state theory and the effective potentials and effective masses needed to calculate tunneling probabilities can be estimated with a minimum of electronic structure information, thereby allowing their computation at a higher level of theory than would otherwise be possible. As examples, we consider the reactions OH+H2, CH3+H2, and Cl+CH4 and some of their isotopic analogs. We find for Cl+CH4→HCl+CH3 that the reaction rate is greatly enhanced by tunneling under conditions of interest for atmospheric chemistry. [ABSTRACT FROM AUTHOR]

The isotopic fractionation factor in the monohydrated gas-phase cluster Cl-(H2O), i.e., the equilibrium constant for Cl-(H2O)(g)+D2O(g)⇄Cl-(D2O)(g) +H2O(g), is used to test models used for solution-phase simulations and to test semiempirical and ab initio molecular orbital theory for their detailed structural and vibrational predictions for both the solute–solvent bond properties and the interaction-induced intramolecular changes in water itself. The isotope effect is studied at a consistent level of vibration–rotation theory, i.e., the harmonic-oscillator-rigid-rotator approximation, using four different kinds of approach, namely extended-basis-set ab initio electronic structure calculations, both (i) with and (ii) without electron correlation, (iii) semiempirical molecular orbital theory at the level of neglect of diatomic differential overlap, and (iv) analytic force fields based on site–site interactions. The calculations of type (i) show good convergence and are compared both to experiment, which presumably tests the harmonic approximation, and to the results of efforts (ii), (iii), and (iv), which presumably tests the structural and vibrational properties predicted by these more approximate approaches. We find significant effects of anharmonicity and electron correlation; semiempirical molecular orbital theory does remarkably well; and there is a wide variation in predictions among the 25 analytic force fields tested. Finally we combine the well converged ab initio results for the properties of the well, the vibrational red and blue shifts, and the isotope effect on the equilibrium constant with the previous ab initio calculations of Dacre for the repulsive interactions to obtain a new interaction potential for chloride ion with nonrigid water that also predicts a more accurate enthalpy of complexation and binding energy than do the chloride–water potentials available in the literature. [ABSTRACT FROM AUTHOR]

Studies the activation energy trends for the C[sub2]H[sub4]+OH to C[sub2]H[sub4]OH reaction using canonical variational transition state theory. Location of the stationary points; Parameterization of the reaction path in the vicinity of the transition state.

Research into the racemization of vinylglycolate catalyzed by the enzyme mandelate racemase is presented. It was possible to identify three different reaction mechanisms that avoid any dianionic species or allow that stabilization.

A study theoretically investigated how the solvent and the counterion modify the symmetry of the hydrogen bond in the monoanion of a dicarboxylic acid by examining the intramolecular hydrogen bond of hydrogen phthalate anion in water and in chloroform as a solvent of low polarity, including the counterion effect. Findings showed that the hydrogen bond is symmetric in the gas phase. The polarity of the solvent tends to stabilize more the asymmetric structures, although the symmetric structure still has less energy for both chloroform and water.

A study on the free radical association reaction H + C2H4 calculated 21 kinetic isotope effects for the addition of the hydrogen atom to ethylene using canonical variational-state theory with multidimensional tunneling contributions. Results of the experiment demonstrate the significance of the tunneling effect for the primary kinetic isotope effects and reaction rate increases in the series deuterium-protium-muonium. Experimental methods, calculations and findings are discussed.

Electronic structure calculations and canonical variational transition state theory were used to study the gas-phase HO + H2O to H2O + OH reaction and three associated isotope exchange reactions. Results indicated that the reactions occur through several dynamical bottlenecks. Moreover, it was observed that tunneling is very important in the hydrogen transfer processes at and over 300 degrees Kelvin.

The convenience of a diabatic or an adiabatic interpolation of frequencies in variational transition state calculations involving interpolation methodologies is discussed. The gas phase proton transfer between butanone and a hydroxide anion has been used to illustrate the theoretical discussion. For example, it has been shown that if vibrational normal mode crossings exist the simple adiabatic interpolation can produce incorrect entropy contributions and, as a consequence, a spurious displacement of the generalized transition state. [ABSTRACT FROM AUTHOR]