Oxidation of the nucleobases is of great concern for the stability of DNA strands and is considered as a source of mutagenesis and cancer. However, precise spectroscopy data, in particular in their electronic excited states are scarce if not missing. We here report an original way to produce isomer-selected radical cations of DNA bases, exemplified in the case of cytosine, through the photodissociation of cold cytosine-silver (C-Ag) complex. IR-UV dip spectroscopy of C-Ag features fingerprint bands for the two keto-amino cytosine tautomers. UV photodissociation (UVPD) of the isomer-selected C-Ag complexes produces the cytosine radical cation (C˙) without isomerization. IR-UV cryogenic ion spectroscopy of C˙ allows for the unambiguous structural assignment of the two keto-amino isomers of C˙. UVPD spectroscopy of the isomer-selected C˙ species is recorded at a unique spectral resolution. These benchmark spectroscopic data of the electronic excited states of C˙ are used to assess the quantum chemistry calculations performed at the TD-DFT, CASSCF/CASPT2 and CASSCF/MRCI-F12 levels.
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