Electronic excitation and singlet-triplet coupling in uracil tautomers and uracil-water complexes

Electronic spectra of uracil in its diketo (lactam) form and five enol (lactim) tautomeric forms have been investigated by means of combined density functional and configuration interaction methods. We have simulated the effects of hydrogen bonding with a protic solvent by recomputing the spectrum of uracil in the presence of two, four, or six water molecules. Geometries of the electronic ground state and several low-lying excited states have been optimized. Spin-orbit coupling has been determined for correlated wavefunctions employing a non-empirical spin-orbit mean-field approach. In accord with experiment, we find the diketo tautomer to be the most stable one. The calculations confirm that the first absorption band arises from the 1($\pi\to\pi^\ast$) ${\rm S}_0 \to {\rm S}_2$ excitation. The experimentally observed vibrational structure in this band originates from a breathing mode of the six ring. Complexation with water molecules is seen to cause a significant blue shift of $n\to\pi^\ast$ excitations while leaving $\pi\to\pi^\ast$ excitations nearly uninfluenced. Computed radiative lifetimes are presented for the experimentally known weak phosphorescence from the $\pi\to\pi^\ast$ excited T1 state. Among the uracil lactim tautomers, one is particularly interesting from a spectroscopic point of view. In this tautomer, the $\pi\to\pi^\ast$ excitation gives rise to the S1 state