The Role of Zero-Point Vibration and Reactant Attraction in Exothermic Bimolecular Reactions with Submerged Potential Barriers: Theoretical Studies of the R + HBr → RH + Br (R = CH3, HO) Systems.

Benjámin Csorba,Péter Szabó,György Lendvay,Szabolcs Góger

doi:10.1021/acs.jpca.1c05839

Abstract

The dynamics of the reactions CH3 + HBr → CH4 + Br and HO + HBr → H2O + Br have been studied using the quasiclassical trajectory method to explore the interplay of the vibrational excitation of the breaking bond and the potential energy surface characterized by a prereaction van der Waals well and a submerged barrier to reaction. The attraction between the reactants is favorable for the reaction, because it brings together the reactants without any energy investment. The reaction can be thought to be controlled by capture. The trajectory calculations indeed provide excitation functions typical to capture: the reaction cross sections diverge when the collision energy is reduced toward zero. Excitation of reactant vibration accelerates both reactions. The barrier on the potential surface is so early that the coupling between the degrees of freedom at the saddle point geometry is negligible. However, the trajectory calculations show that when the breaking bond is stretched at the time of the encounter, an attractive force arises, as if the radical approached a HBr molecule whose bond is partially broken. As a result, the dynamics of the reaction are controlled more by the temporary “dynamical”, vibrationally induced than by the “static” van der Waals attraction even when the reactants are in vibrational ground state. The cross sections are shown to drop to very small values when the amplitude of the breaking bond’s vibration is artificially reduced, which provides an estimate of the reactivity due to the “static” attraction. Without zero-point vibration these reactions would be very slow, which is a manifestation of a unique quantum effect. Reactions where the reactivity is determined by dynamical factors such as the vibrationally enhanced attraction are found to be beyond the range of applicability of Polanyi’s rules.

Highlights

Hydrogen abstraction reactions by halogen atoms and by OH radicals from small molecules are prototypes of bimolecular atom-transfer reactions
For reactions of CH3 and HO radicals with HBr, the quasiclassical trajectory (QCT) calculations revealed unexpectedly large reactivity: the reaction cross sections diverge swiftly when the collision energy is reduced
The capturetype excitation functions observed for this reaction repeat what has been seen for the HO + HBr reaction

Summary

Introduction

Hydrogen abstraction reactions by halogen atoms and by OH radicals from small molecules are prototypes of bimolecular atom-transfer reactions. As they lend themselves to detailed experimental and theoretical reaction dynamical studies, they serve as testing grounds of theories on the general rules of reaction dynamics. In the class of reactions of hydrogen halides with alkyl and other radicals, those involving Br atoms attracted attention because of their importance in atmospheric chemistry[12,13] and flame retardation.[14] The experimental studies of H atom-abstraction reactions from HBr were instrumental in the determination of heats of formation of the radicals.[15−18] For the reactions of HBr with methyl radicals

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