Microsolvation of 1,4-butanediol: the competition between intra- and intermolecular hydrogen bonding

The conformational space of 1,4-butanediol was examined at ωB97X-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 5o and hydrogen-bonded form 5r of 1,4-butanediol involving one to four water molecules were located at ωB97X-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 5o are favored enthalpically, but configurations with the hydrogen bonded form 5r are the lowest in free energy. These calculations suggest that both 5r and 5o will coexist in aqueous solution.