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

Abstract

Integral cross sections and product recoil velocity distributions were measured for reaction of C(2)H(2)(+) with NO(2), in which the C(2)H(2)(+) reactant was prepared in its ground state, and with mode-selective excitation in the cis-bend (2ν(5)) and CC stretch (n · ν(2), n = 1, 2). Because both reactants have one unpaired electron, collisions can occur with either singlet or triplet coupling of these unpaired electrons, and the contributions are separated based on distinct recoil dynamics. For singlet coupling, reaction efficiency is near unity, with significant branching to charge transfer (NO(2)(+)), O(-) transfer (NO(+)), and O transfer (C(2)H(2)O(+)) products. For triplet coupling, reaction efficiency varies between 13% and 19%, depending on collision energy. The only significant triplet channel is NO(+) + triplet ketene, generated predominantly by O(-) transfer, with a possible contribution from dissociative charge transfer at high collision energies. NO(2)(+) formation (charge transfer) can only occur on the singlet surface, and appears to be mediated by a weakly bound complex at low energies. O transfer (C(2)H(2)O(+)) also appears to be dominated by reaction on the singlet surface, but is quite inefficient, suggesting a bottleneck limiting coupling to this product from the singlet reaction coordinate. The dominant channel is O(-) transfer, producing NO(+), with roughly equal contributions from reaction on singlet and triplet surfaces. The effects of C(2)H(2)(+) vibration are modest, but mode specific. For all three product channels (i.e., charge, O(-), and O transfer), excitation of the CC stretch fundamental (ν(2)) has little effect, 2 · ν(2) excitation results in ∼50% reduction in reactivity, and excitation of the cis-bend overtone (2 · ν(5)) results in ∼50% enhancement. The fact that all channels have similar mode dependence suggests that the rate-limiting step, where vibrational excitation has its effect, is early on the reaction coordinate, and branching to the individual product channels occurs later.

State-Selected Reactivity of Carbon Dioxide Cations (CO2+) With Methane

“…For ion-molecule reactions, some state-selected experiments have shown that for endothermic charge-transfer (CT) processes, vibrational excitation is more effective than translational energy in driving the reactions (Viggiano and Morris, 1996). However, in other cases, the effect of vibrational excitation is more varied (see for example Candori et al, 2003; Boyle et al, 2011; Chang et al, 2012; Bell and Anderson, 2013a,b; Bell et al, 2014 and reference therein).…”

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

“…There is considerable experimental interest in the chemical dynamics of ion-molecule reactions with the reactants in specific quantum states. [1][2][3][4][5][6][7] Viggiano and co-workers 1 have investigated the influence of vibrational and rotational energies on the rates of ion-molecule reactions. For exothermic reactions, they found that rotational energy has a negligible influence on the reaction efficiency, while rotational energy increases the efficiency for endothermic reactions.…”

“…More detailed information regarding the role of different types of energy on the reaction dynamics is obtained by studying stateselected molecular ions. 2 Anderson and co-workers [3][4][5] have been particularly interested in the effects of reactant vibrational excitation on the dynamics of ion-molecule reactions.…”