Cytosine anions: ab initio study Original Research Article

Theoretical ab initio calculations have been performed to determine the stability of covalent and dipole-bound anions of cytosine. The work is related to the recent anion spectroscopy experiments performed on some nucleic acid bases including cytosine by Schiedt et al. [Chem. Phys. 239 (1998) 511], where two anion states attributed to dipole-bound anions of the amino–hydroxy (A–H) and amino–oxo (A–O) tautomers were detected. The present calculations reveal that (i) the A–O isomer and two rotamers of the A–H isomer have sufficient dipole moments to bind σ excess electrons in stationary dipole-bound states. The calculated adiabatic electron affinities are 58, 22, and 6 meV for the A–O cytosine and the two rotamers of the A–H cytosine, respectively. These values are considerably smaller than the two experimentally determined values of 85±8 and 230±8 meV; (ii) our calculations also describe covalent anions of the A–O and A–H tautomers. In both systems, the six-member ring is noticeably distorted from planarity. According to the calculations only, the A–O covalent anion is vertically stable with respect to the electron attachment, and the corresponding vertical electron detachment energy is 102 meV. However, both A–O and A–H cytosine anions are predicted to be unstable with respect to the adiabatic electron detachment. This finding is consistent with the lack of a covalent anion feature in the experimental spectrum.