Proton transference in hydrogen bonded systems. I. Uracil tautomerism

We have made a detailed analysis of the adequacy of different theoretical approaches to the study of uracil tautomerism. We have considered the effect of molecular relaxation and the ability of several methods to reproduce it. We have also considered their performance in predicting the tautomerization energy barrier. We found that semiempirical methods are not adequate for predicting the energy value but that they can be used for obtaining geometry optimizations. We also found a strong discrepancy between the energy values predicted by a minimal basis set and a 2ζ basis set, the last one being 24.9 kcal/mol. The implications of these findings are discussed and the results compared with experimental evidence.