Studies of solvent effects. IV. Study of hydration of the dimethyl phosphate anion (DMP−) and of the solvent effect upon its conformation

Abstract

Three models of solute-solvent interaction (discrete, continuum, and combined model) were applied to the study of the hydration of DMP−. In agreement with previous ab-initio calculations, around the anionic and esteric oxygen atoms of DMP− a region of space was found where the water molecules are strongly attracted, which results in the formation of a first hydration shell rather strongly bound to these oxygen atoms. A “second shell” of water molecules preferably bound to those of the first shell (“radial” structure) was also found. The “third shell” of water molecules makes the transition with the isotropic structure of bulk water. On the other hand, around the nonpolar part of the molecule (methyl groups) a “first shell” of water molecules was observed which interact only very weakly with these methyl groups but give rise to a network of hydrogen bonds between themselves (“concentric” structure). This is an example of the so-called iceberg effect, describing the water structure around nonpolar solutes. These results were applied to the evaluation of the hydration energy of DMP− in several conformations (gauche-gauche, gauche-trans) in order to assess the solvent effect upon the relative stabilities of these conformations. This solvent effect is found to be quite small, and consequently unable to modify strongly the relative stabilities as they are deduced from conformational energy calculations in vacuo. This smallness of the solvent effect in the present case may be related to the fact that the conformational changes involved have no strong influence upon the formation of the first shell of water molecules strongly bound to the polar part of DMP−, as a consequence of the fact that the conformational changes only concern the nonpolar part (methyl groups) and do not result in any significant steric hindrance between these methyl groups and the water molecules of the first shell bound to the polar part of DMP−.