Theoretical investigation of excimer and exciplex states of uracil and halogen derivatives: effect of nonparallelism of bases.

Theoretical modeling of initial steps of the photodimerization mechanism of uracil, 5-methyl-and 5-halogen derivatives was performed. The interaction energy of bases in stacked dimers in the ground and lowest excited states was calculated as a function of the distance between the base planes and of the rotation angles within the perturbation theory for the extended Hückel treatment. The existence of excimer and exciplex region on the potential surface of the excited state was revealed. The excimer (exciplex) geometry has the planes nonparallel with more close contact of the C5-C6 bonds as compared to the ground state of dimers. The results provide new information useful for understanding the photodimerization mechanism of bases and testifies that the singlet excimer state can be a precursor of the photodimerization reaction.