Ab initio characterization of the conical intersections involved in the photochemistry of phenol

The nature of the vibronic interactions between the (1)pi pi(*) (A(')), the (1)pisigma(*) (A(")), and the S(0) (A(')) states at the CI(pi pi(*)/pi sigma(*)) and CI(pi sigma(*)/pi pi) conical intersections has been investigated by accurate ab initio calculations. Potential energy surfaces have been constructed at the complete-active-space self-consistent-field and multireference configuration-interaction (MRCI) levels of theory along each of the ten normal coordinates of A(") symmetry that potentially can be coupling modes at these conical intersections. The OH torsion was found to be by far the strongest coupling mode in each case. As for benzene, a "channel three" radiationless decay mechanism associated with a prefulvenic conical intersection, CI(pref), was found to exist in phenol. The reaction path connecting the prefulvenic form of phenol with the minimum-energy structure of the S(1) state was computed at different levels of theory. The barrier to be overcome for the opening of the prefulvenic decay channel is estimated as 6370 cm(-1) at the MRCI level, that is, about 2300 cm(-1) above the energy of CI(pi pi(*)/pi sigma(*)). With sufficient excess energy in the S(1) state, the prefulvenic decay mechanism can be in competition with the hydrogen detachment process.