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

Abstract

Excitation functions from quasiclassical trajectory calculations on the H + H2O --> OH + H2, H + HF --> F + H2, and H + H'F --> H' + HF reactions indicate a different behavior at low and high vibrational excitation of the breaking bond. When the reactant tri- or diatomic molecule is in vibrational ground state or in a low vibrationally excited state, all these reactions are activated; i.e., there is a nonzero threshold energy below which there is no reaction. In contrast, at high-stretch excited-states capture-type behavior is observed; i.e., with decreasing translational energy the reactive cross-section diverges. The latter induces extreme vibrational enhancement of the thermal rate consistent with the experiments. The results indicate that the speed-up observed at high vibrational excitation is beyond the applicability of Polanyi's rules in their common form; instead, it can be interpreted in terms of an attractive potential acting on the attacking H atom when it approaches the reactant with a stretched X-H bond.

Insights into the bond-selective reaction of Cl + HOD(nOH) → HCl + OD

“…Interestingly, the corresponding ICS behaves much like that of a barrierless capture reaction often seen in complex-forming reactions. 58,59 As pointed out by Lendvay and coworkers, 25 such reactions with highly vibrationally excited reactants do not follow the conventional minimum energy pathway via the transition state and the effective interaction potential becomes attractive for the stretched HOD. This is borne out in Fig.…”

“…[9][10][11][12][13][14] Both reactions have ''late'' barriers, underscoring the potential enhancement effects of reactant vibrational excitation, according to Polanyi's rules. 15 Theoretical studies of this [16][17][18][19][20] and the analogous H + HOD reactions [21][22][23][24][25] have been reported before, many with approximations.…”

Influence of collision energy and vibrational excitation on the dynamics for the H+HBr→H2+Br reaction

“…The dynamic behavior of vibrationally excited HBr discussed above is beyond the applicability of Polanyi's rules. 50,51 Actually, extreme vibrational excitation of reagent has been amply discussed for H+H 2 O 52 and H+HF 53 abstraction reactions. For the title reaction, as soon as the vibrational energy in the HBr molecule exceeds the barrier (1.53 kcal/mol) at v =1, the excitation function could easily switch from activated at v =0 to capture-type beginning with v =1 shown in figure 4.…”

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

“…These values are similar to those observed for the H + H 2 O( v stretch = 4) reaction. 36 − 38 Trajectories were integrated with the velocity–Verlet and the Runge–Kutta–Gill method for reactions R1 and R2 , 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.…”

“…The explanation, the appearance of capture-type excitation functions at vibrational excitation by more than 2 quanta, was provided by the related theoretical studies. 35−38 The phenomenon was also observed for the very endothermic H + HF 37,39 and for the almost thermoneutral H + HCl 40 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.…”