Wavelength-dependent photodissociation of benzoic acid monomer in alpha C-O fission.

In concert with the latest laser-induced fluorescence (LIF) experiment [Wei et al. J. Phys. Chem. A 2008, 112, 4727], we investigated the photodissociation mechanics of the benzoic acid monomer (BAM) with alpha C-O fission by means of state-of-the-art ab initio calculations. Complete active space self-consistent-field (CASSCF) and multireference CASSCF second-order perturbation theory (MSCASPT2) calculations were performed on the ground and a number of low-lying excited states of BAM. Our calculations indicated that alpha C-O fission from the S(1) state is in competition with the fission from the T(2) state upon the 266-284 nm wavelength photon. This differs from the conclusion of the previous theoretical investigation and clarified the vague experimental conclusion made earlier. According to our calculations, alpha C-O fission mainly occurs at the T(2) state upon photoexcitation at 284-294 nm, and the photon with a wavelength longer than 294 nm is unable to present the alpha C-O fission. This conclusion agrees with the LIF experimental observation.