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

Ab initio molecular orbital calculations were performed toward the determin ation of the potential energy surface for the unimolecular ground-state dis sociation of vinylcyanide. Reaction pathways for the three- and four-center elimination reactions of HCN and H-2, as well as migration and radical eli mination channels of H and CN, were examined. MP2 gradient geometry optimiz ations and QCISD(T) single point energy calculations were performed for all the relevant product species and transition states. The results are compar ed to the analogous unimolecular dissociation of vinylchloride which has be en theoretically investigated by Morokuma and co-workers [J. Chem. Phys. 10 0, 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 calcu lations indicate that the elimination of H-2 and HCN preferentially proceed via three-center transition states. We also find that H-migration reaction s are rapid, suggesting that substantial H-atom scrambling precedes dissoci ation. This result is important for the interpretation of several isotopic substitution measurements which are all consistent with exclusive three- ce nter elimination of H-2 and HCN. (C) 1999 American Institute of Physics. [S 0021-9606(99)00606-6].