CASPT2 study on the low-lying electronic states of the p-C6H4F2+ and p-C6H4Cl2+ ions

Geometries and energy levels for the five lowest-lying states of the p-C6H4F2+ ion and of the p-C6H4Cl2+ion were calculated by using the CASPT2 and CASSCF methods in conjunction with a contracted atomic natural orbital (ANO-L) basis set. For the 12B3g state of p-C6H4F2+ the D2h geometry was found not to correspond to an energy minimum, and one should consider 12B3 as the second excited state of the ion which has a nonplanar D2 minimum-energy structure. Based on our CASPT2 adiabatic excitation energy (T0) calculations, we assign the X, A, B, C, and D states of p-C6H4F2+ to 12B2g, 12B1g, 12B3, 12B3u, and 12B2u, respectively, and the X, A, B, C, and D states of p-C6H4Cl2+ to 12B2g, 12B1g, 12B3u, 12B2u, and 12B3g, respectively. The CASPT2 T0 values and CASPT2 Tv′(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-C6H4F2+ ion and for Cl-loss dissociation from the five states of the p-C6H4Cl2+ ion were calculated at the CASPT2//CASSCF level. The CASPT2//CASSCF PECs indicate that the 12B2g, 12B1g, and 12B3u states of the p-C6H4F2+ and p-C6H4Cl2+ ions correlate with the X1A1, 13B1, and 11B1 states of the C6H4F+ and C6H4Cl+ ions, respectively. In the case of p-C6H4F2+ the 12B3g state correlates to the 11A1 state of the C6H4F+ ion and the 12B2u state to 13A2, while in the case of p-C6H4Cl2+the 12B2u state correlates to the 11A1 state of the C6H4Cl+ ion and the 12B3g state to 13A2. There are energy barriers along the 12B1 (12B2g), 12A2 (12B1g), 22B1 (12B3u), and 22B2 (12B2u) PECs of the p-C6H4F2+ ion and the 12A2 (12B1g) and 22B2 (12B3g) PECs of the p-C6H4Cl2+ ion.