Formation of C4H4(•+) from the pyridine radical cation: a theoretical mechanistic and kinetic study.

The potential energy surface (PES) for the formation of C(4)H(4)(•+) from the pyridine radical cation by loss of HCN was determined from quantum chemical calculations using the G3//B3LYP method. The complete reaction pathways for the formation of the low-energy C(4)H(4)(•+) isomers, radical cations of methylenecyclopropene (MCP(•+)), vinylacteylene (VA(•+)), cyclobutadiene, and butatriene were obtained. Based on the PESs, a Rice-Ramsperger-Kassel-Marcus model calculation was performed to investigate the dissociation kinetics. The calculated dissociation rate constants agreed with the previous experimental data. It was predicted that a mixture of MCP(•+) and VA(•+) was formed by loss of HCN. The formation of MCP(•+) was more favored near the dissociation threshold and at high energies, whereas the formation of VA(•+) was more favored at the low energies corresponding to the ion lifetime of microseconds.