Theoretical Study of the Jahn-Teller Effect in CH3CN+((X)over-tilde2E) and CD3CN+((X)over-tilde2E): Multimode Spin-Vibronic Energy Level Calculations

The spin-vibronic energy levels for CH3CN+((X) over tilde E-2) and CD3CN+((X) over tilde E-2) have been calculated using a diabatic model including multimode vibronic couplings and spin-orbit interaction without adjusting any parameter. The diabatic potential energy surfaces are represented by the Taylor expansions including linear, quadratic and bilinear vibronic coupling terms. The normal coordinates used in the Taylor expansion were expressed by the mass-weighted Cartesian coordinates. The adiabatic potential energy surfaces for CH3CN+ and CD3CN+ were calculated at the level of CASPT2/cc-pvtz, and the spin-orbit coupling constant was calculated at the level of MRCI/CAS/cc-pvtz. The spin-orbit energy splittings for the ground vibrational states of CH3CN+((X) over tilde E-2) and CD3CN+((X) over tilde E-2) are 20 and 16 cm(-1), respectively, which are resulted from the quenching of the spin-orbit coupling strength of 51 cm(-1). The calculated spin-vibronic levels are in good agreement with the experimental data. The calculation results show that the Jahn-Teller effects in CH3CN+((X) over tilde E-2) and CD3CN+((X) over tilde E-2) are essential to understand their spin-vibronic energy structure.