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

Intermolecular energy transfer for the vibrationally excited propylbenzene cation (C ₉ H ₁₂ ⁺ ) 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 C ₉ H ₁₂ ⁺ . Spin component scaled MP2/6-311++G** calculations were used to develop an intermolecular potential for He + C ₉ H ₁₂ ⁺ . The He + He intermolecular potential was determined from a previous explicitly correlated Gaussian electronic structure calculation. For the simulations, C ₉ H ₁₂ ⁺ was prepared with a 100.1 kcal/mol excitation energy to compare with experiment. The average energy transfer from C ₉ H ₁₂ ⁺ , ⟨ΔE _c ⟩, decreased as C ₉ H ₁₂ ⁺ 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 + C ₉ H ₁₂ ⁺ collisions is compared with reported values of ⟨ΔE _c ⟩ for He colliding with other molecules.