Quasiclassical trajectory calculations of collisional energy transfer: the methyl internal rotor in ethane

Quasiclassical trajectory calculations of collisional energy transfer from highly vibrationally excited ethane in neon bath gas are reported. The presence of the torsional mode about the C-C bond significantly enhances energy transfer. In the first collision, V --> R, torsion energy flow is observed. Evidence from this and other work indicates that V, torsion --> torsion, T energy flow will occur in subsequent collisions, in which the torsional mode acts as a "'gateway'' for collisional energy transfer. This torsional effect explains why toluene has much larger experimental collision energy transfer values than benzene. Future calculations involving alkanes and other flexible molecules must include torsional potential terms to obtain the correct qualitative and quantitative behaviour.