Theoretical investigations of the reaction dynamics of gas-phase HBr plus acetylene collisions

Rotationally averaged cross sections for nine channels in the bimolecular gas-phase reaction of HBr(upsilon) with acetylene in its vibrational ground state are computed using classical trajectory methods on a previously developed global potential energy surface for the electronic ground state. Translational energies in the range 1.0-6.4 eV are considered at HBr vibrational energies that correspond to the harmonic upsilon = 0, 2, 4, 6, and 11 vibrational states. Hydrogen exchange is found to be the dominant reaction pathway when both acetylene and HBr are in their vibrational ground states. At translational energies above 4.8 eV, hydrogen atom addition becomes competitive with hydrogen exchange. The hydrogen exchange and hydrogen atom addition cross sections are found to exhibit a minimum with respect to HBr vibrational energy. At higher HBr vibrational energies, the atomic addition cross sections are greatly enhanced. Near the HBr dissociation threshold, collisional dissociation of HBr and bromine atom addition are the dominant reaction channels. The large differences between the dependence of the reaction cross sections for the various channels on HBr vibrational energy suggest the possibility of effecting a high degree of product selectivity by HBr vibrational state selection. The reactions are found to proceed either by a complex mechanism involving the initial formation of vibrationally excited vinyl bromide followed by unimolecular decomposition or by a direct interaction that bypasses vinyl bromide formation. Analysis of individual trajectories shows that the unusual dependence of the hydrogen exchange and hydrogen atom addition cross sections upon initial HBr vibrational energy is a reflection of the relative importance of the two mechanisms at different HBr vibrational energies. H-2 and HBr dissociation from the vibrationally excited vinyl bromide formed by four-center addition of HBr to acetylene is found to occur predominately via a three-center alpha,alpha mechanism.