Quasiclassical Trajectory Studies of the Dynamics of Bimolecular Reactions of Vibrationally Highly Excited Molecules

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

“…At low collision energies, maximum impact parameters as large as 14 Å were needed to get converged reaction probabilities and cross sections, while at large relative velocities, 4.5 Å is satisfactory. These values are similar to those observed for the H + H 2 O­( v stretch = 4) reaction. − Trajectories were integrated with the velocity–Verlet and the Runge–Kutta–Gill method for reactions and , respectively, with time steps 0.1 or 0.07 fs, ensuring energy conservation to better than 0.05 kJ/mol. All reactive collisions were included in the cross-section calculations, without any weighting.…”

“…Even for this kind of reaction, the “vibrationally induced” attraction does arise when the amplitude of the vibration of the breaking bond is large. The attraction facilitates the reaction and even induces enormous rate enhancement, such as that observed experimentally and explained by theory for the reaction of vibrationally highly excited water with H atoms. − In the previous sections we have seen that the same factors determine the reactivity when the potential barrier to reaction is early and is submerged so that it forms a reef next to a shallow prereaction potential well. Vibrational excitationin fact, already when it is relatively lowin such cases does promote the reaction, which might sound like a violation of Polanyi’s rule.…”

“…The trajectory calculations indicate that in reactions and , the trajectories leading to reaction follow a path similar to what was seen for some other reactions when the vibration of the breaking bond was highly excited. The earlier observations of the appearance of capture-type excitation functions include some highly endothermic hydrogen-abstraction reactions (H + H 2 O, H + HF) − with reactant vibrational quantum number above 3, and a close to thermoneutral reaction (H + HCl) . For the latter, the barrier is comparable in height to a vibrational quantum, and the extreme reactivity can be seen already at ν­(HCl) = 2.…”

“…The reaction is endothermic and has a late barrier, so in light of Polanyi’s rule, the fact that the rate increases by vibrational excitation of the reactant is not surprising, but the magnitude is. The explanation, the appearance of capture-type excitation functions at vibrational excitation by more than 2 quanta, was provided by the related theoretical studies. − The phenomenon was also observed for the very endothermic H + HF , and for the almost thermoneutral H + HCl hydrogen-abstraction reactions. According to quasiclassical trajectory calculations, the switch of the excitation function from activated to capture-type was found to take place as soon as the energy content of the HX vibration exceeded the potential barrier, which occurred at excitation by about 2.5 quanta for HF and below 2 quanta for HCl.…”

Theoretical Study of the Reaction of H Atoms with Vibrationally Highly Excited HF Molecules

“…In our previous studies, , we reported that the very large rate coefficients are due to a characteristic change of the excitation function when the vibrational excitation of the stretch mode participating in the reaction is gradually increased. The reaction switches from activated at low vibrational excitation to nonactivated at high excitation.…”