Effect of a complex formation on the calculated low-pressure rate constant of a bimolecular gas-phase reaction governed by tunneling

Abstract

Many important bimolecular hydrogen‐transfer processes that take place in the atmosphere proceed via a potential energy minimum (hydrogen‐bonded complex) that precedes along the minimum energy path the unique saddle point of the reaction, the one corresponding to the hydrogen transfer. It is clear that the one‐step low‐pressure rate constant of such a reaction does not depend on the existence of any complex along the minimum energy path below the reactant if the reaction takes place by thermal activation over a transition state that lies quite above the reactants (for instance 10 kcal/mol). However, we have quantitatively shown in this article that the scenario notoriously changes if the reaction involves significant tunneling. In this work, we have theoretically calculated the rate constants and their temperature dependence for the reaction HO+HOH→HOH+OH by means of a canonical variational transition state theory and a canonical unified statistical theory (when necessary). Multidimensional tunneling effects have been included with a semiclassical transmission coefficient. Two kinds of modified potential energy surfaces (PESs), obtained from an original ab initio potential energy surface, previously calculated by us, have been used. The Eckart‐modified PESs serve to model the hydrogen‐abstraction profiles with no complexes along the path, while the Gaussian‐modified PESs model the energy profiles with two complexes along the path symmetrically distributed at each side of the abstraction saddle point. Our results show that the existence of those complexes reduces the thickness of the classically forbidden region for energies below the adiabatic barrier, and then tunneling is promoted and the reaction is accelerated. The effect of the complex formation in several kinetic magnitudes, as the Arrhenius parameters and the kinetic isotope effect has also been analyzed. © 1999 John Wiley & Sons, Inc. J Comput Chem 20: 1685–1692, 1999

Shock Tube Measurements of the tert-Butanol + OH Reaction Rate and the tert-C4H8OH Radical β-Scission Branching Ratio Using Isotopic Labeling

“…Kinetic simulations were also used to verify that conversion of 18 OH to 16 OH through the reaction 18 OH + H 2 16 O ↔ 16 OH + H 2 18 O does not perturb the inferred values of 18 k ′ or 16 k ′. The rate constant for this reaction was computed using the “Thermo” code in the MultiWell software suite with structures, frequencies, and energies from theoretical calculations by Uchimaru et al Tunneling corrections were included, and the estimated rate constant from this study is in good agreement with theoretical calculations by Masgrau et al…”

Variational Transition-State Theory Rate Constant Calculations of the OH + CH3SH Reaction and Several Isotopic Variants

“…On the other hand, the MCCM-CCSD(T) method has been proven to give very good rate constants for similar abstraction reactions. 17,18 A search for the stationary points involved in the CH 3 SH + OH reaction was carried out on the MP2(full)/cc-pVDZ potential energy surface (PES). 19,20 First and second energy derivatives were also calculated at the same level of theory.…”

Modeling the Kinetics of Bimolecular Reactions

“…It has been pointed out that a reactive surface should have a qualitatively correct well in about the right place so that the repulsive interaction energy decreases to about the right value at about the right place . The width of the energy barrier depends on the location of van der Waals well, and thus the correct calculation of the tunneling probabilities, especially at low energy, is sensitive to the quality of modeling this feature. , …”