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+, 〈ΔEc〉, decreased as C9H12+ was vibrationally relaxed and for the initial excitation energy 〈ΔEc〉 = 0.64 kcal/mol. This result agrees well with the experimental 〈ΔEc〉 value of 0.51 ± 0.26 kcal/mol for collisions of He with the ethylbenzene cation. The 〈ΔEc〉 value found for He + C9H12+ collisions is compared with reported values of 〈ΔEc〉 for He colliding with other molecules.