Theoretical insights on nonradiative deactivation mechanisms of protonated xanthine

The nonradiative deactivation mechanisms of the lowest (1)pi pi excited states in four protonated isomers of the 7H- and 9H-xanthine, based on the MP2, CC2, ADC(2) and CASSCF theoretical methods have been investigated. It has been predicted that out-of-plane driving coordinates are effectively responsible for photostability of these systems at the Franck-Condon region of the (1)pi pi* excited state, while the NH or OH stretching coordinates can be suggested to play the deactivation role in the higher energetic range based on the (1)pi sigma* state. The different deactivation mechanisms provide significant supports for photostability of protonated xanthine in the wide range of UV radiation as well as its neutral homologue. Also, from spectroscopy points of view, no significant alteration in the (1)pi pi*-S-0 electronic transition has been predicted to occur following protonation.