Oxo-hydroxy tautomerism of uracil and 5-fluorouracil

The three lowest energy tautomeric forms of uracil and 5-fluorouracil have been studied by using the second-order many-body perturbation theory (MBPT(2)) with Gaussian DZP basis sets. The zero-point nuclear energy has been estimated by means of the harmonic approximation using analytical derivatives of the SCF 3-21G energy. The uracil and 5-fluorouracil molecules have been predicted to exist in the gas phase in the dioxo forms in agreement with the majority of experimental data. The 2-hydroxy-4-oxo and 2-oxo-4-hydroxy tautomeric forms of uracil have been found to be less stable than the main dioxo form by 44 and 50 kJ mol^-1, respectively. The corresponding values for 5-fluorouracil are 29 and 52 kJ mol^-1. The relevance of the present results to recent IR and fluorescence spectroscopical experimental works is discussed. Some aspects of the point mutation theory in view of the present work are also considered.