Excited-state deactivation of the monohydrated complexes of cytosine, uracil, and thymine through S/S conical intersections.

The nucleobases cytosine, uracil, and thymine and their H-bonded complexes with one molecule of water were studied at the CC2/aug-cc-pVDZ level of theory. The excited-state deactivation mechanisms through the conical intersections S/S were investigated. It was found that the ππ excited states of cytosine, the hydroxy tautomer of uracil, and the hydroxy tautomer of thymine relax non-radiatively to the ground state S. However, only monohydrated and anhydrous thymine do not have energy barriers of the ππ excited-state reaction paths. The photophysical reaction of monohydrated thymine could proceed in two directions on the S surface. Graphical abstract: [Figure not available: see fulltext.]. [ABSTRACT FROM AUTHOR]