THEORETICAL-STUDY OF THE UNIMOLECULAR DISSOCIATION OF THE ACETONE CATION-RADICAL

Two parallel unimolecular dissociation channels of the acetone cation radical are known experimentally. Methyl loss is the dominant reaction channel for high internal energies of the reactant ion whereas methan e elimination is the major reaction channel for low internal energies. However, there exists a controversy about the mechanisms of dissociat ion of acetone cation radical. High level ab initio MP3/6-311G(d,p)//M P3/6-311G(d, p) calculations have been carried out to investigate the most relevant parts of the C3H6O+. ground state potential energy surfa ce. Reactant and transition state frequencies also have been obtained at the MP3/6-311G(d,p) level to include zero-point energy effects. The se energy results reinforce the idea that methane elimination at low i nternal energies may take place only via a tunnelling mechanism throug h the hydrogen transfer energy barrier. To analyse the influence of th e entropic contribution on the mechanism the Gibbs free energy was cal culated. Variational calculations at the hydrogen transfer region indi cate that conventional transition state theory correctly describes the main features of the system at that bottleneck.