Reaction of C_2H_2 (n_2, m_5) with NO_2: Reaction on the singlet and triplet surfaces

摘要

Integral cross sections and product recoil velocity distributions were measured for reaction of C_2H_2 with NO_2, in which the C_2H_2 reactant was prepared in its ground state, and with mode-selective excitation in the cis-bend (2_5) and CC stretch (n_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_2H_2O) 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 _2H_2O) 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_2H_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 (_2) has little effect, 2_2 excitation results in ～50 reduction in reactivity, and excitation of the cis-bend overtone (2_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.