The Jahn-Teller effect in ${\rm CH}_{\rm 3} {\rm Cl}^{\rm + } ( {{\rm \tilde X}{}^{\rm 2}{\rm E}} )$ CH 3 Cl +(X̃E2): A combined high-resolution experimental measurement and ab initio theoretical study

The energy levels of ${\rm CH}_{\rm 3} {\rm Cl}^{\rm + } \left( {{\rm \tilde X}{}^{\rm 2}{\rm E}} \right)$ CH 3 Cl +X̃E2 showing strong spin-vibronic coupling effect (Jahn-Teller effect) have been measured up to 3500 cm−1 above the ground vibrational state using one-photon zero-kinetic energy photoelectron and mass-analyzed threshold ionization spectroscopic method. Theoretical calculations have been also performed to calculate the spin-vibronic energy levels using a diabatic model and ab initio adiabatic potential energy surfaces (PESs). In the theoretical calculations the diabatic potential energy surfaces are expanded by the Taylor expansions up to the fourth-order including the multimode vibronic interactions. The calculated spin-orbit energy splitting (224.6 cm−1) for the ground vibrational state is in good agreement with the experimental data (219 ± 3 cm−1), which indicates that the Jahn-Teller and the spin-orbit coupling have been properly described in the theoretical model near the zero-point energy level. Based on the assignments predicted by the theoretical calculations, the experimentally measured energy levels were fitted to those from the diabatic model by optimizing the main spectroscopic parameters. The PESs from the ab initio calculations at the level of CASPT2/vq(t)z were thus compared with those calculated from the experimentally determined spectroscopic parameters. The theoretical diagonal elements in the diabatic potential matrix are in good agreement with those determined using the experimental data, however, the theoretical off-diagonal elements appreciably deviate from those determined using the experimental data for geometric points far away from the conical intersections. It is also concluded that the JT effect in CH3Cl+ mainly arises from the linear coupling and the mode coupling between the CH3 deform (υ5) and CH3 rock (υ6) vibrations. The mode couplings between the symmetric C-Cl stretching vibration υ3 with υ5 and υ6 are also important to understand the spin-vibronic structure of the molecule.