Analysis of Cooperativity and Group Additivity in the Hydration of 1,2-Dimethoxyethane

We calculate hydration free energies of 1,2-dimethoxyethane (DME) conformations in water at 298 K and 1 bar. We find that the preference for the two most abundant tgt and tgg conformations derives from favorable nonspecific (i.e., long-range) solute-water interactions that are partially offset by unfavorable free energies of forming cavities in water to accommodate these conformations. The much lower population of the third most abundant tg⁺g^- conformation, the most abundant conformation in the ideal gas at 298 K, is attributed to less favorable long-range solute-water interactions. We also find that long-range methyl/methylene group-water and ether oxygen-water interactions make significant nonadditive contributions to the free energy of DME hydration and propose a method based on quasichemical theory for reducing these nonadditive contributions by identifying constituent groups of DME that minimize the covariance in the long-range methyl/methylene group-water and ether oxygen-water interactions. We apply this method to show that the decomposition of DME into its constituent dimethyl ether groups is a better approximation of group additivity than decompositions based on distinguishing hydrophobic/hydrophilic constituent groups.