Intramolecular energy flow and bond dissociation in iodoacetylene and iododiacetylene

Abstract

Intermolecular and intramolecular energy flow and subsequent bond dissociation in collinear collisions I–C≡C–H+Ar and I–C≡C–C≡C–H+Ar have been studied by classical trajectory techniques over the collision energy range of 0 to 10 eV. When the molecule is initially in the ground state, the overall energy transfer in I–C≡C–H+Ar is very small, but in I–C≡C–C≡C–H+Ar it is large. The collisionally perturbed C–H bond stores a large amount of energy from translation for a brief period during the early stage of collision and transfers most of it to the inner region of the molecule, specifically to the low frequency C–I vibration. Thus the high-frequency vibration of the perturbed C–H bond during the collision plays a crucial role in determining the extent of intramolecular energy transfer and, in turn, C–I dissociation. But in nondissociative collisions, there is another series of the C–H vibration at the latter stage of collision, transferring energy back to translation. This study also considers collision-induced intramolecular energy flow in the molecule with an initially excited C–H bond. The relaxation of the low-lying C–H excitation is very slow on a nanosecond time scale. However, when the excitation is high, the vibrational frequency of the C–H bond is significantly weakened, thus becoming comparable to that of the triple bond, in which case the isolating effect of the adjacent C≡C bond is no longer important and intramolecular energy flow becomes efficient.