Theoretical study of low-lying triplet states of aniline

Multireference complete active space self-consistent-field CASSCF(10, 12)/ANO and second-order perturbation theory MS-CASPT2 calculations were performed to determine the vertical low-lying singlet and triplet states of aniline. The sequence of the seven lower lying triplet states is T-1(I(3)A'), T-2(I-3 A), T-3(2(3) A'), T-4(3(3)A'), T-5(2(3) A), T-6(4(3)A'), and T-7(3(3)A). The 3(3)A', 4(3)A', and 3(3)A states are assigned as 3s, 3p(y), and 3p(z) Rydberg states, respectively, while other states correspond to pi* <- pi excitations. Both the T-1 and T-2, states are found to be below at the lowest-lying singlet S-1(I(1)A) state. Geometry, vibrational modes, and electron distribution of the lowest lying T, state were determined using UB3LYP calculations. The vertical and adiabatic singlet-triplet energy gaps Delta E(S-0-T-1) amount to 3.7 and 3.5 +/- 0.2 eV, respectively. In clear contrast with the So state, the triplet aniline is no longer aromatic, and its protonation occurs preferentially at the ring metacarbon site, with a proton affinity PA = 243 +/- 3 kcal/mol.