Investigation of the mechanisms of photo-induced formation of cyclobutane dimers of cytosine and 2,4-diaminopyrimidine

The mechanisms of the formation of cyclobutane dimers (CBD) of cytosine and 2,4-diaminopyrimidine were studied at the CC2 theoretical level and cc-pVDZ basis functions. Four orientations of the two monomers are explored: cys-syn, cis-anti, trans-syn, and trans-anti. The research revealed that in all cases the cyclobutane structures are formed along the ¹ππ* excited-state reaction paths of the stacked aggregates. We localized the S₁/S₀ conical intersections mediating those transformations. The results obtained agree well with the previously reported investigations on the cis-syn cyclodimer formations of other pyrimidines.