Diffuse-bound and valence-bound anions of cytosine

Diffuse-bound and valence-bound anions of cytosine have been examined with correlated, ab initio calculations employing large basis sets. Five structures of the cytosine anion have been considered for both kinds of anions. Adiabatic electron affinities of neutrals and vertical electron detachment energies were determined with perturbative, coupled-cluster and electron propagator methods. Basis sets with many diffuse, atom-centered functions were used. Among the five tautomeric cytosine structures, only the 1H-amino-oxo isomer is capable of forming both diffuse and valence-bound anions. Two imino-oxo isomers produce valence-bound anions with positive vertical electron detachment energies. The most stable neutrals, the amino-oxy tautomers. have adiabatic electron affinities that are negative with respect to both dipole-bound and valence-bound anionic forms. Valence-bound. amino-oxy anions are not even vertically bound with respect to electron loss. Diffuse-bound anions of the amino-oxy and imino-oxo isomers have negative adiabatic electron detachment energies. Close agreement with the lowest peak in an anion photoelectrum spectrum obtains for the vertical electron detachment energy of the diffuse-bound anion of 1H-amino-oxo cytosine. A peak of higher energy is assigned to the valence-bound anions of 1H-amino-oxo and imino-oxo tautomers.