Normal‐mode vibrational frequencies and thermodynamic properties of uracil and 4‐hydroxyuracil

Normal‐mode vibrational frequencies were obtained for uracil and 4‐hydroxyuracil with the MINDO /3 SCF method. MINDO /3 vibrational frequencies and the moments of inertia for STO ‐3G optimized geometries yielded a Δ S   T 0for U ⇄ U* of 1.06 e.u./mol by standard statistical thermodynamic equations. Δ G   T 0is then 6.29 kcal/mol in the gas phase for U ⇄ U*. The reaction field continuum model of solvent effect lowers Δ G   T 0to 5.3 kcal/mol in aqueous solution. The tautomeric equilibrium constant in solution is then 1.3 × 10 −4 , in good agreement with experimental values.