CASPT2 study on the low-lying electronic states of the [formula omitted] and [formula omitted] ions

Abstract

Geometries and energy levels for the five lowest-lying states of the p - C 6 H 4 F 2 + ion and of the p - C 6 H 4 Cl 2 + ion were calculated by using the CASPT2 and CASSCF methods in conjunction with a contracted atomic natural orbital (ANO-L) basis set. For the 1 2B 3g state of p - C 6 H 4 F 2 + the D 2h geometry was found not to correspond to an energy minimum, and one should consider 1 2B 3 as the second excited state of the ion which has a nonplanar D 2 minimum-energy structure. Based on our CASPT2 adiabatic excitation energy ( T 0) calculations, we assign the X, A, B, C, and D states of p - C 6 H 4 F 2 + to 1 2B 2g, 1 2B 1g, 1 2B 3, 1 2B 3u, and 1 2B 2u, respectively, and the X, A, B, C, and D states of p - C 6 H 4 Cl 2 + to 1 2B 2g, 1 2B 1g, 1 2B 3u, 1 2B 2u, and 1 2B 3g, respectively. The CASPT2 T 0 values and CASPT2 T v ′ (relative energy at the molecular geometries) values are in reasonable agreement with the available experimental data evaluated using adiabatic and vertical ionization potentials. The potential energy curves (PECs) for F-loss dissociation from the five states of the p - C 6 H 4 F 2 + ion and for Cl-loss dissociation from the five states of the p - C 6 H 4 Cl 2 + ion were calculated at the CASPT2//CASSCF level. The CASPT2//CASSCF PECs indicate that the 1 2B 2g, 1 2B 1g, and 1 2B 3u states of the p - C 6 H 4 F 2 + and p - C 6 H 4 Cl 2 + ions correlate with the X 1A 1, 1 3B 1, and 1 1B 1 states of the C 6H 4F + and C 6H 4Cl + ions, respectively. In the case of p - C 6 H 4 F 2 + the 1 2B 3g state correlates to the 1 1A 1 state of the C 6H 4F + ion and the 1 2B 2u state to 1 3A 2, while in the case of p - C 6 H 4 Cl 2 + the 1 2B 2u state correlates to the 1 1A 1 state of the C 6H 4Cl + ion and the 1 2B 3g state to 1 3A 2. There are energy barriers along the 1 2B 1 (1 2B 2g), 1 2A 2 (1 2B 1g), 2 2B 1 (1 2B 3u), and 2 2B 2 (1 2B 2u) PECs of the p - C 6 H 4 F 2 + ion and the 1 2A 2 (1 2B 1g) and 2 2B 2 (1 2B 3g) PECs of the p - C 6 H 4 Cl 2 + ion.