CASPT2 study on electronic states of the dichlorodifluoromethane cation

We studied the eight lowest-lying states of the CF2Cl2+ ion, which were observed in the PE spectra by Pradeep and Shirley, using the complete active space self-consistent field (CASSCF) and multiconfiguration second-order perturbation theory (CASPT2) methods. The CASPT2 calculations predict the equilibrium geometries, adiabatic (T0) and vertical (Tv) excitation energies, relative energies (Tv′) at the geometry of the molecule, and the CASSCF calculations predict vibrational frequencies. On the basis of the CASPT2 T0 calculations, we assign the X, A, B, C, D, E, F, and G states of the CF2Cl2+ ion to 12B2, 12A2, 12B1, 12A1, 22B2, 22B1, 22A1, and 22A2, respectively. Our A2A2 and B2B1 assignments are in line with the assignments of Pradeep and Shirley, but our F(2)2A1 and G(2)2A2 assignments are different from the F(2)2A2 and G(2)2A1 assignments presented in the experimental papers. The CASPT2 T0 values for 12B1 and 22B2 are in good agreement with the available experimental values, and the CASPT2 Tv′ values are in very good (for 12B2, 12A2, 12B1, 12A1, and 22B2) or reasonable (for 22B1, 22A1, and 22A2) agreement with the experimental values. The CASSCF wavefunction of the 22A1 state at the geometry of the molecule and the CASSCF wavefunctions of the 22B2, 22B1, and 22A2 states at the CASPT2 geometries are found to have multi-configurational character. The CASSCF frequency values of the total symmetric modes in the 12B2, 12A2, 12B1, and 22B2 states are comparable with available experimental data.