Effects of Bending and Bending Angular Momentum on Reaction of NO₂⁺ with C₂H₂: A Quasi-Classical Trajectory Study

Abstract

A large set of quasi-classical trajectories were calculated at the PBE1PBE/6-311G** level of theory, in an attempt to understand the mechanistic origins of the large, mode-specific enhancement of the O-transfer reaction by NO2+ bending vibration and the surprisingly large suppressing effect of bending angular momentum. The trajectories reproduce the magnitude of the absolute reaction cross section, and also get the dependence of reactivity on NO2+ vibrational state, and the vibrational state dependent scattering behavior qualitatively correct. Analysis of the trajectories shows that the bending effect is not simply a consequence of enhanced reactivity in bent geometries but, rather, that excitation of bending motion allows reaction in a wider range of orientation angles, even if the NO2+ is not bent at the onset of the collisional interaction. There is a strong interplay between NO2+ bending and transient charge transfer during the collisions. Such charge transfer enhances reactivity, but only if the reactants are oriented correctly.

Potential energy surfaces for the HBr+ + CO2 → Br + HOCO+ reaction in the HBr+ 2Π3/2 and 2Π1/2 spin-orbit states

“…Of particular interest is the manner in which vibrational excitation of the reactants may enhance the reaction rate, which has been investigated for the reaction of H 2 CO + with D 2 and CD 4 , 13,14 and the NO + 2 + C 2 H 2 , and C 2 H + 2 + CH 4 reactions. 15,16 The possibility of establishing of "Polanyi Rules" for polyatomic reactions has been investigated. 13 Trajectories have also been used to investigate the role of state-specific vibrational excitation on collision-induced dissociation.…”

The origin of the large bending enhancement of the reaction of C2H2+ with methane: the effects of bending momentum, ruling out the precursor mechanism, and steps toward “Polanyi rules” for polyatomic reactions

“…Note that the trajectory calculated enhancement factors are smaller compared to the experimental values. This ''damping'' of the vibrational effects is typical in our experience with QCT studies of vibrational effects 23,28,[30][31][32] and we tentatively associate it with the failure to quantize vibrational motion. Nonetheless, the results suggest that the trajectories capture the physics of the mode-specific vibrational enhancement, at least qualitatively.…”

“…We also examined the 2-D correlation between the bending angle, a bend , the orientation of the bending plane with respect to the approaching H atom, F plane , and the reactivity. In previous systems, this correlation has been illuminating, 23 but here there is no obvious correlation.…”

“…Such reactions are ideal for quasi-classical trajectory study because error tends to accumulate during a trajectory. 23 The experimental integral cross-sections relevant to this study are summarized in Fig. 2 as a function of the center-of-mass collision energy E col , for C 2 H 2 1 in its ground state, and with excitation in the CC stretch (n 2 1 ), and cis-bending overtone (2n 5 1 ).…”

“…Because precursor lifetime should increase rapidly with decreasing energy, it was felt that collisions at lower E col might have collision times too long for practical trajectory simulations. Because the number of trajectories was necessarily limited, we adopted a previously successful strategy 23,28 of the reactant impact parameter rather than randomly sampling the b distribution. For both the ground and 2n 5 1 states, trajectories were run at ten discrete values of the impact parameter, ranging from 0.1 to 5.0 A ˚, allowing us to examine the b dependence of the scattering dynamics in detail.…”