Vibrational Relaxation and Bond Dissociation in Methylpyrazine on Collision with N2 and O2

. The present study uses quasi-classical trajectory procedures to examine the vibrational relaxation and dissociation of the methyl and ring C-H bonds in excited methylpyrazine (MP) during collision with either N2 or O2. The energy-loss (-Delta E) of the excited MP is calculated as the total vibrational energy (ET) of MP is increased in the range of 5,000 to 40,000cm -1. The results indicate that the collision-induced vibrational relaxation of MP is not large, increasing gradually with increasing ET between 5,000 and 30,000 cm-1, but then decreasing with the further increase in ET. In both N2 and O2 collisions, the vibrational relaxation of MP occurs mainly via the vibration-to-translation (V -> T) and vibration-to-vibration (V -> V) energy transfer pathways, while the vibration-to-rotation (V -> R) energy transfer pathway is negligible. In both collision systems, the V -> T transfer shows a similar pattern and amount of energy loss in the ET range of 5,000 to 40,000cm-1, whereas the pattern and amount of energy transfer via the V -> V pathway differs significantly between two collision systems. The collision-induced dissociation of the C-Hmethyl or C-Hring bond occurs when highly excited MP (65,000-72,000 cm-1) interacts with the ground-state N2 or O2. Here, the dissociation probability is low (10-4-10-1), but increases exponentially with increasing vibrational excitation. This can be interpreted as the intermolecular interaction below ET = 71,000 cm-1. By contrast, the bond dissociation above ET = 71,000 cm-1 is due to the intramolecular energy flow between the excited C-H bonds. The probability of C-Hmethyl dissociation is higher than that of C-Hring dissociation.