Ionization energies and dyson orbitals of cytosine and 1-methylcytosine

Electron propagator methods are applied to the vertical ionization energies of the five most stable tautomers of cytosine and to two oxo forms of 1-methylcytosine. For both molecules, there are several isomers within a few kcal/mol of each other. Ionization energies for each isomer have been calculated, therefore. The lowest feature in gas phase photoelectron spectra corresponds to pi(1), holes, but broad, higher-energy peaks are produced by two or three cationic states in each isomer. The order of final states is pi(1), pi(2), n(1), n(2),pi(3), pi(4) for all isomers except the amino-oxy forms of cytosine, where the order is pi(1), pi(1), pi(2), pi(2), pi(3), pi(4).