Comparative Study of the Gas-phase Cyclodimer Formations of Uracil and 6-azauracil in Excited State and through Conical Intersections S0/S1

The photocyclodimerization mechanisms of two oxo tautomers of uracil and 6-azauracil were studied at the CC2 level of theory and cc-pVDZ basis functions. Uracil was explored in three orientations of the monomers - cis-anti, trans-syn and trans-anti, while 6-azauracil - in all four orientations. Conical intersections S0/S1 were found at the CASSCF(2,2)/6-31G* theoretical level. The study propose a photophysical mechanisms of photoformation and photodestruction of cyclodimers (CDs), which occur through the 1∆∆* electron excited states (spectroscopically bright states). The photophysical relaxations (internal conversions) to the ground states are mediated by conical intersections S0/S1, which we found and discussed in the current paper.