Multireference calculations of the fluorescence, phosphorescence and photodissociation of p-chlorotoluene.

Equilibrium geometries and vibrational frequencies of the ground and some excited states of p-chlorotoluene were calculated by the complete active space self-consistent field (CASSCF) method. Multi-reference CASSCF second order perturbation theory (MSCASPT2) calculations were performed on the vertical excitation energies of six singlet and triplet excited states. The potential energy curves along the Cl-C6H4CH3 bond distance of a number of low-lying singlet and triplet excited states were calculated by the CASPT2 method based on CASSCF partially optimized geometries. The fluorescence and one component of the dual phosphorescence observed experimentally were clearly explained by the CASPT2 calculated transition energies. According to those CASPT2 potential energy curves, the photodissociation of p-chlorotoluene at 266 nm was attributed to the predissociation of the first triplet excited state after its intersystem crossing with the first singlet excited state. The internal rotation and substitution effects of methyl on the photodissociation were discussed in detail.