Chemical Dynamics Simulations of Intermolecular Energy Transfer: Azulene + N₂ Collisions

Chemical dynamics simulations were performed to investigate collisional energy transfer from highly vibrationally excited azulene in a N₂ bath. The intermolecular potential between Az and N₂, used for the simulations, was determined from MP2/6-31+G∗ ab initio calculations. Az∗ is prepared with an 87.5 kcal/mol excitation energy by using quantum microcanonical sampling, including its 95.7 kcal/mol zero-point energy. The average energy of Az∗ versus time, obtained from the simulations, shows different rates of Az∗ deactivation depending on the N₂ bath density. Using the N₂ bath density and Lennard-Jones collision number, the average energy transfer per collision 〈δE_c〉 was obtained for Az∗ as it is collisionally relaxed. By comparing 〈δE_c〉 versus the bath density, the single collision limiting density was found for energy transfer. The resulting 〈δE_c〉, for an 87.5 kcal/mol excitation energy, is 0.30 ± 0.01 and 0.32 ± 0.01 kcal/mol for harmonic and anharmonic Az potentials, respectively. For comparison, the experimental value is 0.57 ± 0.11 kcal/mol. During Az∗ relaxation there is no appreciable energy transfer to Az translation and rotation, and the energy transfer is to the N₂ bath.