Conical intersections S0/S1 of thymine mediating the non-radiative photodestruction of cyclobutane dimers: a CASSCF level study

Three new conical intersections S0/S1 (cis-anti, trans-syn and trans-anti) were fond which mediate the processes of photo-induced formation of cyclobutane thymine dimers. Their structures were optimized at the CASSCF(2,2) (Complete Active Space Self Consistent Field method) level of theory with the minimal bases set STO-3G*. The geometries were explored with respect to their structural and electron features. The found conical intersections S0/S1 give a clear indication that they could be included in the internal conversions of the 1ππ* excited states of the stacked dimers to cyclobutane photodimers and vice versa.