A note on a non-empirical molecular orbital study of some cytosine and thymine tautomers

The relative stabilities of a series of cytosine and thymine tautomers have been investigated by means of non-empirical (ab initio) LCAO-MO-SCF Hartree-Fock-Roothaan calculations employing a small contracted Gaussian basis set (STO-3G) in all cases. The relative stabilities of the various tautomeric forms agree in general with the results of earlier empirical and semi-empirical calculations on these molecules. In particular, the “enol forms” of cytosine and thymine, within the isolated molecule approximation inherent in these MO calculations, are predicted to possess greater stability than the forms commonly assigned to these molecules in aqueous solution and found in the solid state by X-ray crystallographic studies.