The tautomeric equilibria of thio analogues of nucleic acid bases. Part 2. AM1 and ab initio calculations of 2-thiouracil and its methyl derivatives

Abstract

Relative stabilities predicted by the AM1 method, and relative energies from 3-21 G* and 6-31 G ** calculations agree with qualitative and quantitative experimental vapour phase data for 2-thiouracil and for its four tautomeric monomethyl derivatives. The AM1 proton affinities for the more stable tautomeric forms are usually significantly lower than, but almost within experimental values of, the measured PA values. A plot of experimental vs. calculated PA gives a straight line of almost unit slope.