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-C-6 H4F2 (+) 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 1 B-2(3), state of p-C6H4F2 divided by the D-2h geometry was found not to correspond to an energy minimum, and one should consider 1(2)B(3) as the second excited state of the ion which has a nonplanar D, minimum-energy structure. Based on our CASPT2 adiabatic excitation energy (TO) calculations, we assign the X, A B, C, and D states Of p-C6H4F2 + to 1 B-2(2g) 1 B-2(3) ,1(2)B(3)u, 1(2) B-3u and 1(2) B-2u, respectively, and the X, A, B, C, and D states Of p-C6H4Cl2 to 1(2)B(2g), 1(2)B(2g), 1(2) B-3u 1(2) B-2u and 1(2)B(3)g, respectively. The CASPT2 To values and CASPT2 T-v (t)(relative energy at the molecular gaeometries) 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 1(2)B(2g) 1 (2) B-1g and 1(2) B-3u States of the p- C6H4F2+ and P-C6H4Cl2+ ions correlate with the X(1)A(1), 1(3)B(1), and 1(1)B(1) states of the C6H4F+ and C6H4Cl (+) ions, respectively. In the case Of P-C6H4F2 (+) the 1 B-2(3g) state correlates to the 1(1)A(1) state of the C6H4F ion and the 1(2) B-2u state to 1(3) A(2) while in the case Of p-C6H4Cl2+the 1(2) B-2u state correlates to the 1(1)A(1) state of the C6H4CI+ ion and the 1 (2)B(3)g state to 1(3)A(2). There are energy barriers along the 1 B-2(1) (1(2) B(2)g()), 1(2) A(2) (1(2) B-1g), 2 B-2(1) (1(2) B-3u), and 2 (2) B-2 (1(2) B-2u) PECS of the P-C6H4F,(+) ion and the 1(2) A(2) (1(2) B-1g) and 2 (2) B-2 (1(2) B-3g) PECS of the P-C6H4Cl2+ ion. (C) 2007 Elsevier B.V. All rights reserved.