A classical molecular dynamics study of the intramolecular energy transfer of model trans-stilbene Original Research Article

An approximate anharmonic potential energy surface for the S1 state of trans-stilbene has been constructed. This model surface, which is based on the potential of toluene and ethene, was optimised to the available trans-stilbene S1 state experimental frequencies and the trans → cis isomerisation barrier height. The surface was employed in classical trajectory studies where the main goal was to investigate the internal energy transfer of the isolated trans-stilbene molecule. Although the applicability of classical dynamics for large molecules at low vibrational energies is uncertain, we find that the beating patterns that have been observed for the isolated photoexcited molecule are qualitatively reproduced by the classical trajectories. The simulated quasiperiodic beating patterns persist at elevated molecular thermal energies where zero point energy effects are approximately taken into account. The simulation data indicate the presence of a bottleneck to intramolecular energy redistribution that is also robust with respect to increased molecular thermal energies. Similarly, the RRKM rate coefficient that is based on our surface overestimates the isomerisation rates that are observed experimentally. The experimental data that have been obtained for the isolated molecule and for the molecule under low pressure thermal conditions are discussed in the light of these results.