The unimolecular dissociation of vinylcyanide: A theoretical investigation of a complex multichannel reaction

摘要

Ab initio molecular orbital calculations were performed toward the determination of the potential energy surface for the unimolecular ground-state dissociation of vinylcyanide. Reaction pathways for the three- and four-center elimination reactions of HCN and H_2, as well as migration and radical elimination channels of H and CN, were examined. MP2 gradient geometry optimizations and QCISD(T) single point energy calculations were performed for all the relevant product species and transition states. The results are compared to the analogous unimolecular dissociation of vinylchloride which has been theoretically investigated by Morokuma and co-workers [J. Chem. Phys. 100, 8976 (1994)]. The unimolecular rates for all reaction channels have been calculated using Rice-Ramsperger-Kassel-Marcus (RRKM) theory employing ab initio transition state energies and MP2 vibrational frequencies. Our calculations indicate that the elimination of H_2 and HCN preferentially proceed via three-center transition states. We also find that H-migration reactions are rapid, suggesting that substantial H-atom scrambling precedes dissociation. This result is important for the interpretation of several isotopic substitution measurements which are all consistent with exclusive three-center elimination of H_2 and HCN.