Cytosine anions: Ab initio study

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. (C) 2000 Elsevier Science B.V.