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 s symmetry), the ππ* state minimum geometry has the NH 2 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 O σ* electronic configuration. This is the reason for the absence of the Cy + + M charge transfer fragmentation channel for CytK + and CytNa + complexes.