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

Abstract

The dynamics of the reactions CH 3 + HBr → CH 4 + Br and HO + HBr → H 2 O + 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.

Theoretical dynamics studies of the CH3 + HBr → CH4 + Br reaction: integral cross sections, rate constants and microscopic mechanism

“…For more details, the reader is referred to ref. 48. The interesting observation that the excitation function switches from activated (initially zero and rising starting from a threshold energy) to capture-type (diverging with decreasing collision energy) that can be drawn from the figures obtained by Wang et al , 29 which show that the extra attraction can even override the presence of a positive potential barrier.…”

“…In ref. 48 it was found that the vibrational amplitude of the H 0 Br molecule is very large already in the vibrational ground state. When the breaking bond is stretched long, the H 0 atom is so far from Br that an attractive force arises between the C and H 0 atoms as if the H 0 -Br bond was partially broken.…”

Shining light on the microscopic resonant mechanism responsible for cavity-mediated chemical reactivity

“…Our conclusions may be viewed in the light of altering the vibrational contribution to Polanyi’s rule 47 , 48 . For a reaction A+BC → AB+C, a transition-state close to the product, as present on our PES, implies a strong dependence on the vibrational energy along the reactive path.…”

Experimental and Theoretical Study of the Kinetics of the CH3 + HBr → CH4 + Br Reaction and the Temperature Dependence of the Activation Energy of CH4 + Br → CH3 + HBr

“…The calculations were performed using the potential energy surface function developed by Czakó corrected as described in ref . The standard QCT technology was used, described in more detail in refs − . The calculations were performed using an extensively modified version of the trajectory code VENUS 88 .…”

“…The quoted numbers refer to the analytical potential energy surface that is based on the high-accuracy energy values derived by Czakó characterized by an estimated error of 0.6 kJ/mol.) The reaction dynamical studies − performed using the quasiclassical trajectory (QCT) method (validated by reduced-dimensionality quantum scattering calculations) indicate that the long-range attraction induces a capture-type behavior: At low collision energies, the excitation function (the reaction cross section as a function of collision energy) diverges as the collision energy is reduced. This kind of excitation function is associated with rate coefficients that, at low temperatures, increase as the temperature decreases, which is in agreement with the experimental observations.…”