Theoretical study of the interaction between uracil and hydrogen peroxide

The optimized geometries, energies, harmonic vibrational frequencies and natural charges of the uracil-hydrogen peroxide (U-HP) complexes are computed using density functional theory (B3LYP) combined with the 6-31++G(d,p) basis set. Four stable structures are found on the potential energy surface. In three of these structures labeled A, B, and C, one of the OH bonds of HP accepts the NH acidic proton while donating a proton to the carbonyl oxygen of U, forming a six-membered ring. In structure D, the CH bond of U acts as a proton donor, forming with the two O atoms of HP, a seven-membered ring. For all the structures, complex formation results in an elongation of the NH, CH, and OH bonds, and a red-shift of the corresponding stretching vibrations. For complexes A, B, and C, the binding energies span a range of -28 to -37 M mol(-1), the most stable complex being formed at the carbonyl site of U characterized by the lowest proton affinity and at the NH bond having the largest acidity. In these complexes, the charge transfer taking place from U to HP is moderate and ranges between 0.013 and 0.019 e. In complex D, the binding energy (-29.7 kJ mol(-1)) is larger than that expected from the acidity of the CH bond and the charge transfer of 0.030 e is larger than in the three other complexes. These features can be accounted for by a more favorable linear arrangement of the hydrogen bonds in the seven-membered ring. Comparison of the geometrical and vibrational data for the four stable U-H2O complexes demonstrates that H2O is a better proton acceptor and HP a better proton donor, in agreement with the experimental gas-phase proton affinities and deprotonation enthapies of both molecules. These conclusions are consistent with the relations between the elongations of the NH and OH bonds and the shifts of the corresponding stretching vibrations.