Chemical Dynamics Simulations of Energy Transfer for Propylbenzene Cation and He Collisions

Intermolecular energy transfer for the vibrationally excited propylbenzene cation (C₉H₁₂⁺) in a helium bath was studied with chemical dynamics simulations. The bond energy bond order relationship and electronic structure calculations were used to develop an intramolecular potential for C₉H₁₂⁺. Spin component scaled MP2/6-311++G** calculations were used to develop an intermolecular potential for He + C₉H₁₂⁺. The He + He intermolecular potential was determined from a previous explicitly correlated Gaussian electronic structure calculation. For the simulations, C₉H₁₂⁺ was prepared with a 100.1 kcal/mol excitation energy to compare with experiment. The average energy transfer from C₉H₁₂⁺, ⟨ΔE_c⟩, decreased as C₉H₁₂⁺ was vibrationally relaxed and for the initial excitation energy ⟨ΔE_c⟩ = 0.64 kcal/mol. This result agrees well with the experimental ⟨ΔE_c⟩ value of 0.51 ± 0.26 kcal/mol for collisions of He with the ethylbenzene cation. The ⟨ΔE_c⟩ value found for He + C₉H₁₂⁺ collisions is compared with reported values of ⟨ΔE_c⟩ for He colliding with other molecules.