Theoretical study of tautomeric forms of uracil. 1. Relative order of stabilities and their relation to proton affinities and deprotonation enthalpies

The potential-energy surface of the uracil tautomerization is comprised of 12 different tautomeric forms, except uracil itself. Their optimized structures are determined at the B3LYP/6-31+G(d,p) computational level. Five structures are reported for the first time, although four of them fall into the relative-energy interval already investigated in the past. The relative order of stability of all uracil structures is established. The proton affinities (PAs) and the deprotonation enthalpies (DPEs) of the atoms or bonds involved in the tautomerization process are calculated at the same computational level. The PAs and DPEs values sensitively depend on the tautomeric form, with the PAs of the oxygen and nitrogen atoms ranging from 815 to 947 kJ mol(-1) and the DPEs of the OH and NH groups from 1295 to 1448 kJ mol(-1). The relative energies of the uracil structures with respect to uracil are then rationalized in terms of a second-order polynomial of the difference between their mean PAs and DPEs.