Hydrogen bonding of phosphodiesters to water, methanol, and methylamine as studied by the CNDO/2 method

Abstract

Hydrogen bonding of dimethylphosphate anion (DMP) and dimethyl phosphoric acid (DMPH) with water, methanol, and methylamine was studied using the CNDO/2 molecular orbital method. The computed lengths of O–H⋅⋅⋅O hydrogen bonds involving the phosphate group were significantly shorter than were the computed values for nonphosphate O–H⋅⋅⋅O bonds, in agreement with experiment. The lowest energy for the monohydration of DMP or DMPH was obtained for the linear hydrogen bond in which the atoms of water were coplanar with the anionic oxygens of the phosphate group. The hydrogen bond strengths of methanol and water with DMP or DMPH were of comparable magnitude, but the N–H⋅⋅⋅O bond involving methylamine and DMP was about 25% weaker. The C–H⋅⋅⋅O interaction between the methyl group of methanol or methylamine and the phosphoryl oxygen was also examined and found to have all the characteristics (bond strength, charge transfer, and atomic polarization) of a hydrogen bond. The computed bond strength for HOCH3⋅⋅⋅DMP was −7.1 kcal/mole, as compared to −21.8 kcal/mole for a conventional CH3OH⋅⋅⋅DMP hydrogen bondinginteraction.