Selective Tautomer Production and Cryogenic Ion Spectroscopy of Radical Cations: The Uracil and Thymine Cases

Abstract

The vibrational and electronic spectroscopy of the radical cations of two nucleobases(NB) (uracil and thymine) was studied by cryogenic ion photodissociation spectroscopy. The radicalcations have been generated from the photodissociation of NB−Ag+ complexes. A charge transferprocess from the NB to Ag+ governs the deactivation mechanism, leading to the formation of theradical cation without further tautomerization. Single- and double-resonance spectroscopy allows forstructural assignments of both the silver complexes and the radical cations by comparison with DFTbased calculations. Interestingly, a tautomer-dependent fragmentation is observed in the thymine enol form that involves the loss of NCO, a fragment which was never reported before for this NB. Thisselective photodissociation of silver complexes containing aromatic chromophore greatly expands thecurrent technique to produce isomer-selected radical cations in the gas phase providing benchmarkexperimental data to assess calculations of open-shell species.