Vibrational effects on the reaction of NO2+ with C2H2: Effects of bending and bending angular momentum

Abstract

NO(2)(+) in six different vibrational states was reacted with C(2)H(2) over the center-of-mass energy range from 0.03 to 3.3 eV. The reaction, forming NO(+)+C(2)H(2)O and NO+C(2)H(2)O(+), shows a bimodal dependence on collision energy (E(col)). At low E(col), the reaction is quite inefficient (<2%) despite this being a barrierless, exoergic reaction, and is strongly inhibited by E(col). For E(col)> approximately 0.5 eV, a second mechanism turns on, with an efficiency reaching approximately 27% for E(col)>3 eV. The two reaction channels have nearly identical dependence on E(col) and NO(2)(+) vibrational state, and identical recoil dynamics, leading to the conclusion that they represent a single reaction path throughout most of the collision. All modes of NO(2)(+) vibrational excitation enhance both channels at all E(col), however, the effects of bend (010) and bend overtone (02(0)0) excitation are particularly strong (factor of 4). In contrast, the asymmetric stretch (001), which intuition suggests should be coupled to the reaction coordinate, leads to only a factor of approximately 2 enhancement, as does the symmetric stretch (100). Perhaps the most surprising effect is that of the bending angular momentum, which strongly suppress reaction, even though both the energy and angular momentum involved are tiny compared to the collision energy and angular momentum. The results are interpreted in light of ab initio and Rice-Ramsperger-Kassel-Marcus calculations.

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

“…This conclusion was verified with isotope labeling in the experimental study. 12 Properties such as vibrational state or collision energy effects on reactivity can only depend on dynamics leading up to the rate-limiting step on the reaction coordinate, [19][20][21][22] and because of the strong mode dependence it is clear that the rate-limiting step occurs early in C 2 H 2 1 þ CH 4 collisions, before the initial mode of vibrational excitation has been randomized. Product recoil behavior, of course, depends on dynamics throughout the course of the collisions, however, the gross features of the scattering (stripping vs. rebound dynamics, energy disposal) may still reflect events that occur early in the collisions.…”

Reaction of C2H2+ (n·ν2, m·ν5) with NO2: Reaction on the singlet and triplet surfaces

“…1 At E col ≥ 2 eV, the cross section for the NO + channel increases slightly, which may indicate a small contribution from dissociative CT, generating NO + + O ( 3 P) in this energy range. Because this state predissociates (see below), singlet collisions must actually generate most, if not all, of the observed NO 2 + signal even at high energies.…”

“…More quantitative interpretation relies on simulating the distributions, 1,25 in this case using a recoil velocity model combining angular distributions based on the osculating complex model, 26 with Gaussian recoil energy distributions defined in terms of the available energy. We do not attempt to fit the distributions below ∼500 m/sec, because this velocity range is strongly perturbed by stray fields, and also by contributions from ions that are back-scattered in the lab frame, and detected after reflection from the ion lens at the entrance to the ion guide.…”

“…1 The HCCOH isomer is considerably higher in energy than ketene, with correspondingly high tran-FIG. It should also be noted that in addition to the ketene isomer of the C 2 H 2 O (+) products, there are HCCOH and three-membered ring (oxirene) isomers, which we discussed in studies of the NO 2 + + C 2 H 2 reaction.…”

Study of gas‐phase reactions of NO2+ with aromatic compounds using proton transfer reaction time‐of‐flight mass spectrometry

“…It may play some roles in the heterogeneous chemistry of polar stratospheric clouds . While the study of ion chemistry involving the NO 2 + is currently the focus of considerable fundamental interest and is relevant in diverse fields ranging from mechanistic organic chemistry to atmospheric chemistry, clustering, and nucleation phenomena . Aromatic compounds (ArXs) comprise 20% to 50% of the nonmethane hydrocarbon mass in urban air and are regarded as some of the main precursors to secondary organic aerosols (SOA), which constitute up to 80% of the total organic aerosol in the atmosphere .…”