CASPT2 Study on the Low-Lying Electronic States of $m$-$C_6H_4F^+_2$ Ion

Abstract

Electronic states of the $m$-$C_6H_4F^+_2$ ion were studied by using the CASPT2 and CASSCF methods in conjunction with a contracted atomic natural orbital (ANOL) basis set. For the five lowest-lying states, geometries and adiabatic excitation energies $(T_0)$ were calculated at the CASPT2 level. The CASPT2 $T_0$ values and CASPT2 $T'_v$ values are in reasonable agreement with the available experimental data. The assignments of the $B,$ $C,$ and $D$ states of the $m$-$C_6H_4F^+_2$ ion were difficult since the three states are closely lying. Based on the CASPT2 $T_0$ calculations, the $X,$ $A,$ $B,$ $C,$ and $D$ states of the $m$-$C_6H_4F^+_2$ ion were assigned to $1^2A_2,$ $1^2B_1,$ $2^2B_1,$ $1^2A_1,$ and $1^2B_2,$ respectively, which supports the suggested assignment of the lowest excited state to $^2B_1$ by Tsuchiya et al. based on their experiments.