Addition of hydrogen atom/hydride anion to the double bonds of cytosine tautomers: radical and anion structures and energetics

Abstract

The structures, energetics, and electron affinities of 18 H-addition cytosine radical isomers (C + H)· are predicted using carefully calibrated density functional methods. Radical rO1 in which H is attached at the N3 site of amino-oxo cytosine, has the lowest energy. The lowest energy anion is aO2, in which, qualitatively, H− is attached to the C6 site of amino-oxo cytosine. The theoretical adiabatic electron affinities (AEAs) for the 18 radicals range from −0.20 to 2.59 eV. Radical rO4, where the additional H atom is at the C4 site has the largest AEA. In contrast, when H is attached to the N3 site of the trans amino-hydroxy form, the resulting radical rA1 has a negative AEA, −0.20 eV. The radical rB1, in which H is appended on the N1 atom of the cis amino-hydroxy form, also has a negative AEA value, −0.16 eV. Generally, the AEA values for the cytosine H-addition radicals are rather smaller than those of the H-abstraction cytosine radicals studied earlier, which range from 2.22 to 3.00 eV.