Spin–orbit ab initio study of two low-lying states of chloroiodomethane cation

Spin–orbit coupling plays a crucial role in the determination of molecular structure and calculation of vibrational frequencies of CH 2 ClI + . We performed the geometry optimizations and vibrational frequency calculations of both the lower and upper spin–orbit (SO) states using an ab initio SO method based on multiconfigurational wave function. The multistate complete active space perturbation theory second-order SO (MS-CASPT2-SO) method reasonably describes the structures of the lower SO state, yielding the C–I distance and the I–C–Cl angle close to the experimental values. The geometrical parameters of the upper SO state is quite similar to that of the lower SO state, whereas structures of two states differ substantially in calculations prior to the introduction of SO coupling. The MS-CASPT2-SO method reproduces the difference between the lower and upper SO states for the I–C–Cl bending frequency. The vibrational frequencies calculated by MS-CASPT2-SO generally overestimate in comparison with the experiments. The energy gap between the two SO states calculated by MS-CASPT2-SO is reasonably close to the experimental value. To the best of our knowledge, this is the first attempt to calculate vibrational frequencies of two SO states of CH 2 ClI + , and the first time to apply MS-CASPT2-SO method to the geometry optimization and vibrational frequency calculation of polyatomic molecule.