CASPT2 study on electronic states of the dichlorodifluoromethane cation

Abstract

We studied the eight lowest-lying states of the CF 2 Cl 2 + 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 ( T 0) and vertical ( T v) excitation energies, relative energies ( T v ′ ) at the geometry of the molecule, and the CASSCF calculations predict vibrational frequencies. On the basis of the CASPT2 T 0 calculations, we assign the X, A, B, C, D, E, F, and G states of the CF 2 Cl 2 + ion to 1 2B 2, 1 2A 2, 1 2B 1, 1 2A 1, 2 2B 2, 2 2B 1, 2 2A 1, and 2 2A 2, respectively. Our A 2A 2 and B 2B 1 assignments are in line with the assignments of Pradeep and Shirley, but our F(2) 2A 1 and G(2) 2A 2 assignments are different from the F(2) 2A 2 and G(2) 2A 1 assignments presented in the experimental papers. The CASPT2 T 0 values for 1 2B 1 and 2 2B 2 are in good agreement with the available experimental values, and the CASPT2 T v ′ values are in very good (for 1 2B 2, 1 2A 2, 1 2B 1, 1 2A 1, and 2 2B 2) or reasonable (for 2 2B 1, 2 2A 1, and 2 2A 2) agreement with the experimental values. The CASSCF wavefunction of the 2 2A 1 state at the geometry of the molecule and the CASSCF wavefunctions of the 2 2B 2, 2 2B 1, and 2 2A 2 states at the CASPT2 geometries are found to have multi-configurational character. The CASSCF frequency values of the total symmetric modes in the 1 2B 2, 1 2A 2, 1 2B 1, and 2 2B 2 states are comparable with available experimental data.