Polyatomic canonical variational theory for chemical reaction rates. Separable-mode formalism with application to OH+H2→H2O+H

Abstract

A formalism for the application of variational transition-state theory and semiclassical vibrationally adiabatic transmission coefficients to bimolecular reactions involving an arbitrary number of atoms is presented. This generalizes previous work on atom-diatom reactions. We make applications in this paper to the reactions OH + H,--H,o + Hand OH + D,--HDO + D using the Schatz-Elgersma fit to the Walch-Dunning ab initio potential energy surface. For both reactions we find large differences between conventional and variational transition-state theory and large effects of anharmonicity on the calculated rate constants. The effect of reaction-path curvature on the calculated transmission coefficients and rate constants is also large. The final calculated values of the kinetic isotope effects are in good agreement with experiment at high temperature but too large at room temperature.

High-Level ab Initio Studies of Hydrogen Abstraction from Prototype Hydrocarbon Systems

“…More sophisticated approaches may seek to find transition states on enthalpy or free-energy surfaces determined at the appropriate temperature. 101 For present purposes, such results simply confirm that the reaction barriers are very low, if they exist at all.…”

Semiclassical and Quantum Mechanical Calculations of Isotopic Kinetic Branching Ratios for the Reactionof O(3P) with HD

“…This com parison of semiclassical fixed-energy three-dimensional results with accurate ones is consistent with expectations based on previous comparisons of ther mally averaged semiclassical rate coefficients with ap proximate quantum ones [37] and with comparisons of semiclassical collinear thermal rate constants with accurate quantal ones in the same dimensionality [3]. Since the semiclassical calculations are applicable to systems with many degrees of freedom [29, [55][56][57][58][59][60][61] their good agreement with accurate quantal results bodes well for our ability to predict quantum effects on kinetic isotope effects involving tunneling in many systems of interest.…”

“…The In quantized VTST, the reaction path is defined as the minimum energy path through mass-scaled coor dinates [4, 56,65], which is called the MEP, and quan tized energy levels for the degrees of freedom orthogo nal to the MEP are computed. At the threshold energy of microcanonical transition state theory, the varia tional transition state is located at the maximum VAG of the vibrationally adiabatic ground-state potential curve F G(s) defined by [6, 7,10, 11, 17]…”

Quantifying the non-RRKM effect in the H + O2 ? OH + O reaction

“…The TSDS's are taken to be perpendicular to the reaction coordinate, and each of the family of N 1 (E,s) is evaluated using a classical approximation for rotations and the quantized independent normal mode approximation for vibrations (classical rotation, independent normal mode or CRINM (see refs. [24] and [54] for more details). For the O ϩ OH reaction there is no intrisic barrier and quantum mechanical effects on the reaction coordinate (e.g., tunneling) are not important and therefore excluded.…”