UV Photofragmentation of Cold Cytosine-M⁺ Complexes (M⁺: Na⁺, K⁺, Ag⁺)

The UV photofragmentation spectra of cold cytosine-M⁺ complexes (M⁺: Na⁺, K⁺, Ag⁺) were recorded and analyzed through comparison with geometry optimizations and frequency calculations of the ground and excited states at the SCS-CC2/Def2-SVPD level of theory. While in all complexes, the ground state minimum geometry is planar (C symmetry), the ππ* state minimum geometry has the NH₂ group slightly twisted and an out-of-plane metal cation. This was confirmed by comparing the simulated ππ* Franck-Condon spectra with the vibrationally resolved photofragmentation spectra of CytNa⁺ and CytK⁺. Vertical excitation transitions were also calculated to evaluate the energies of the CT states involving the transfer of an electron from the Cyt moiety to M⁺. For both CytK⁺ and CytNa⁺ complexes, the first CT state corresponds to an electron transfer from the cytosine aromatic π ring to the antibonding σ* orbital centered on the alkali cation. This πσ* state is predicted to lie much higher in energy (>6 eV) than the band origin of the π-π* electronic transition (around 4.3 eV) unlike what is observed for the CytAg⁺ complex for which the first excited state has a n_Oσ* electronic configuration. This is the reason for the absence of the Cyt⁺ + M charge transfer fragmentation channel for CytK⁺ and CytNa⁺ complexes.