Tautomerism in uracil, cytosine and guanine: a comparison of electron correlation predicted by ab initio and density functional theory methods

Abstract

The structures and relative energies of the tautomers of uracil, cytosine and guanine are predicted using high level ab initio methods and a Kohn-Sham density functional method employing a B-LYP functional. The structures calculated at the MP2 level are in better accord with available microwave data than are those at either the SCF or the B-LYP level. For both uracil and guanine highly correlated ab initio wavefunctions (MP4) and the density functional treatment (B-LYP) give relative energies of the tautomers to within 4 kJ mol −1, whilst for cytosine the corresponding value is somewhat larger. In the case of uracil and guanine, this difference is not sufficient to alter the ordering of the tautomers. For cytosine, where three tautomers are within about 5 kJ mol −1, the ordering given by the two approaches is different, with that from the ab initio method being in agreement with experiment. These results indicate that density functional theory methods, as presently used, may not be as accurate as traditional ab initio methods in describing tautomeric equilibria.