Protonation and deprotonation energies of uracil Implications for the uracil–water complex

Abstract

The proton affinities PA(B) of the four oxygen lone pairs and of the two nitrogen atoms and the deprotonation energies PA(A^-) of the two NH bonds of uracil are calculated by density functional theory (DFT) using the 6-31G(d,p) and 6-31G++(d,p) basis set. The PAs are also calculated by abinitio MO theory (MP4) using 6-31G(d,p) for comparison. The DFT/B3LYP energies and the frequency shifts of the ν(OH) stretching vibrations are calculated with the 6-31++G(d,p) basis for the three cyclic structures of the uracil–water complexes. The usual correlations between energies of frequency shifts and the PAs do not hold in the case of closed structures. The most stable cyclic complex is formed at the oxygen lone pair characterized by the lowest basicity and at the NH bond characterized by the highest acidity. The energy of the complex decreases with increasing value of the difference PA(A^-)-PA(B).