A MODE-SELECTIVE DIFFERENTIAL SCATTERING STUDY OF THE C2H2- INFLUENCEOF COLLISION INTERMEDIATES, COLLISION TIMES, AND TRANSITION-STATES(+METHANOL REACTION )

We report the vibrational and collision energy dependence of cross sec tions and product branching in the reaction of C2H2+ with CD3OD, CD3OH , and CH3OD. We also report axial recoil velocity distributions, along with modeling. At low collision energies, reaction is mediated by a p icosecond lifetime complex of the [C2H2:methanol](+) form. The bottlen eck that controls overall reaction efficiency appears to be formation of the complex, and reactivity is influenced by collision energy and C 2H2+ CC stretch excitation, but not by bending vibration. The most ene rgetically favorable exit channel from the complex is isomerization to covalently bound C3H6O+ complexes, but this does not occur. Instead t he [C2H2:methanol](+) decays by breakup to C2H2+CH4O+, C2H3+CH2OH+, an d C2H+CH3OH2+ channels. Changes in the branching with available energy provide some insight into the nature of the transition states that co ntrol decay of the complex. As collision energy is raised above simila r to 1 eV, the reaction gradually becomes direct, i.e., the collision time drops to well below the rotational period of the collision comple x (< similar to 0.5 ps). In this regime, the dominant charge transfer and hydride abstraction products mostly form in large impact parameter collisions. At high energies there is little dependence of either rea ction efficiency or product branching on collision energy or reactant vibrational state, suggesting that both are probably controlled largel y by collision geometry. (C) 1998 American Institute of Physics.