Collision-induced Intramolecular Energy Flow and C-H Bond Dissociation in Excited Toluene

초록

The collision-induced intramolecular energy flow and C-H bond dissociation in toluene have been studied using classical dynamics procedures. The molecule initially contains high amounts of vibrational excitation in the methyl C-H stretch and the nearby benzene ring C-H stretch and it is to interaction with Ar. The two excited C-H stretch are coupled to each other through teo C-C stretching, two H-C-C bending and one C-C-C bending modes, all of which are initially in the ground state. At 300K, the energy lost by the excited molecule upon collision is not large and it increases slowly with increasing total vibrational energy content between 10000 and 20000cm-1. Above the energy content of 40000cm-1, energy loss increases rapidly. Near 65000cm-1 energy loss takes a maximum value of about 1000cm-1. The temperature dependence of energy loss is weak between 200 and 400K. When the energy contentis sufficienly high, either or both C-H bonds can dissociate, producing free radicals. C5H5CH2, C6H4CH3 or C5H4CH2. The ring C-H dissociation occurs almost entirely in a direct-mode mechanism on a subpicosecond time scale and the rest through a complex-mode collision in which bond dissociation occurs several picoseconds after the initial impact. In the complex-mode collision, Ar binds to the radical forming a weakly bound benzyl-Ar complex. In both dissociationative and nondissociative events. Intramolecular energy flow is efficient, taking place upon the initial impact on a subpicosecond time scale.