A theoretical approach to the hydration of phosphodiesters in the presence of protons

Abstract

Hydration of the dimethylphosphate anion (DMP^–) and dimethylphosphoric acid (DMPH) and of DMP^– in the presence of a proton has been studied by the CNDO/2 molecular-orbital method. The hydration energies of the (DMP^–·H₂O) and (DMPH·H₂O) complexes and the stabilization energies of (DMP^–·H⁺·H₂O) systems of different structures have been found, and additionally Wiberg indices and bond energies (together with their three components) have been computed. Regardless of the type of structure considered, for the trihydrate of dimethylphosphate in the presence of a proton only one water molecule in the first hydration shell is characterized by the lowest bond energy, which is about twice as low as energies of the two remaining water molecules. For the trihydrate of DMP^– two water molecules at two equivalent positions, E₁₃ and E₃₁, are characterized by the lowest hydrogen-bond energy and are ca. 20 kcal mol^–1 lower in energy than the most stable sites in acid structures. The results of theoretical studies support the experimental data obtained in investigations of the hydration of synthetic poly(dialkylphosphate) by the pulsed n.m.r. method.