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Vibrational relaxation and bond dissociation of excited methylpyrazine in the collision with HF

Vibrational relaxation and competitive C-H methyl and C-H ring bond dissociations in vibrationally excited methylpyrazine in the collision with HF have been studied by use of classical trajectory procedures. The energy lost by the vibrationally excited methylpyrazine upon collision is not large and it increases slowly with increasing total vibrational energy content between 20,000 and 45,000 cm -1 . Above the energy content of 45,000 cm -1 , however, energy loss decreases. The temperature dependence of energy loss is negligible between 200 and 400 K, but above 45,000 cm -1 the energy loss increases as the temperature is raised. Energy transfer to or from the excited methyl C-H bond occurs in strong collisions with HF, that is, relatively large amount of translational energy is transferred in a single step. On the other hand, energy transfer to the ring C-H bond occurs in a series of small steps. When the total energy content E T of methylpyrazine is sufficiently high, either or both C-H bonds can dissociate. The C-H methyl dissociation probability is higher than the C-H ring dissociation probability. The dissociation of the ring C-H bond is not the result of the direct intermolecular energy flow from the direct collision between the ring C-H and HF but the result of the intramolecular flow of energy from the methyl group to the ring C-H stretch.