Microsolvation of 1,4-Butanediol: The Competition between Intra- and Intermolecular Hydrogen Bonding

The conformational space of 1,4-butanediol was examined at omega B97x-D/6-311+G(d,p). Of the 65 conformers examined, the seven lowest energy conformations have an internal hydrogen bond. The strength of this hydrogen bond is estimated to be 4 kcal mol(-1). A broad variety of microsolvated configurations of both the open form So and hydrogen-bonded form 5r of 1,4-butanediol involving one to four water molecules were located at omega B97X-D/6-311+G(d,p). When one to three water molecules are included in the clusters, the lowest energy configurations involve the hydrogen-bonded form 5r. With four water molecules, configurations involving the open form So are favored enthalpically, but configurations with the hydrogen bonded form 5r are the lowest in free energy. These calculations suggest that both 5r and So will coexist in aqueous solution.