Estimation of the Energy of Hydrogen Bonds Formed in Crystals. II. Phenols

The energies of hydrogen bonds formed in the crystalline state of twelve phenols have been estimated by a procedure proposed in a previous paper. The values of energy range from zero to 7 kcal./mol. depending upon the difference of molecular structures: the phenols with bulky substituents in the vicinity of OH groups are non- or weakly-hydrogen-bonded, while those with carbonyl or methoxy groups at para positions are strongly bonded presumably by head-to-tail association. The other phenols fall between the two extremes. Incidentally it is interesting that the highest values of OH···O bond are rather close to the energies of association of carboxylic acids in the gaseous state. With 1- and 2-naphthols, a parallel relation between the transitions of the above substances and the melting of phenol has been pointed out.