Theoretical study on protonated and deprotonated 5-substituted uracil derivatives and their complexes with water

Abstract

The proton affinity (PA) of the oxygen atoms and the deprotonation enthalpies (DPE) of the NH bonds of 5-substituted uracil derivatives (X=CN, F, Cl, NH2) are computed using the density functional theory (B3LYP) at the 6-31+G(d,p) level. The PAs vary from 766 to 875kJ mol−1 and the DPEs from 1315 to 1442kJ mol−1. For all the studied uracils, the acidity of the N1H bond is larger than that of the N3H bond and the basicity of the O8 atom is larger than that of the O4 atom. The effect of the intramolecular hydrogen bond in 5-NH2 uracil is discussed. The three stable complexes with one water molecule are the ones in which the oxygen atom of water accepts the acidic NH proton while donating a proton to the carbonyl oxygen of uracils. The intermolecular distances and angles, and the binding energies with water are discussed in terms of the PA and DPE of the atoms or bonds involved in the interaction with one water molecule.