Ab‐initio sto‐3G energy of tautomerism for uracil

Ab‐initio sto‐3G energies for uracil (U) and the tautomer 4‐hydroxyuracil (U*) are reported. The four highest occupied molecular orbital energies for uracil correlate with experimental photoelectron ionization potentials with a least‐squares linear correlation coefficient of 0.999. The energy ΔE_t for U ⇋ U* using the best available geometries is 25.5 kcal/mole. The reaction field continuum model for the solvent effect indicates that ΔE_t may be lowered by approximately 3 kcal/mole. Therefore the diketo form of uracil is theoretically determined to be the most stable form in solution as well as in the gas phase, in complete agreement with experiment.