The heavy-atom effect in intramolecular vibrational energy transfer

Quasiclassical trajectory calculations were performed to understand intramolecular vibrational energy redistribution in tetraallyl tin and carbon. In these calculations a C=C bond initially contains 50, 75, 100, or 125 kcal/mol excess energy, with the remaining modes containing zero-point energy. For tetraallyl tin the trajectory calculations show rapid intramolecular vibrational energy redistribution within the allyl group containing the excited C=C bond. In contrast, energy transfer beyond the central tin atom is negligible for the initial C=C excitation of 50 kcal/mol. As the excitation is increased, energy transfer past the tin atom is enhanced. However, it is still incomplete within 4 ps for the calculation with a 125 kcal/mol excitation. A "pure" heavy-atom effect for intramolecular vibrational energy transfer is not observed. Simply substituting carbon for tin in the tetraallyl tin Hamiltonian has no detectable effect on the rate of the energy redistribution. However, energy redistribution is nearly complete within a picosecond for a Hamiltonian with a potential energy surface chosen to more accurately model that of tetraallyl carbon. The results of the trajectory calculations are compared with recent chemical activation studies of molecules containing heavy atoms, and other trajectory studies of intramolecular vibrational energy redistribution.