A CASPT2 study of the excited states of adenine tautomers with silver ions

The multiconfigurational complete active space (CAS) SCF method and multi-state second order perturbation theory (MS-CASPT2) are used to study the electronic spectra of adenine and the seven low-lying energy tautomers of adenine with the metal cation Ag(I). In all metal complexes a weak Rydberg 4d → 5s transition has been found below the two valence 1π → π∗ states, also identified in the isolated adenine. In most cases, silver metalation leads to a red-shift of the first and a blue-shift of the second valence 1π → π∗ bands, with respect to the isolated adenine. Energetically the most favourable Ag–adenine species in gas phase are bidentated, however, the experimental spectrum in solution is explained with the presence of monodentated tautomers, suggesting that bidentation is destabilized in aqueous phase.