Hydrogen bond strength and bond geometry in cyclic dimers of crystalline carboxylic acids

In a study of 101 crystal structures of carboxylic acids we have observed a clear trend in the difference between the formally single and formally double C—O bond distances, as observed by X-ray diffraction, with a clear-cut distinction between aromatic acids, where the two distances are similar, and non-aromatic acids, where the two distances distinctly differ by 0.06–0.12 Å. A tentative energy classification – within the limits of the many assumptions – and a correlation with the O...O separation over the hydrogen bond indicate that the stability of the carboxylic acid dimer increases as the difference between the two apparent C—O distances becomes smaller, owing to an increasing Coulombic contribution to the dimerization energy. No simple hypothesis is adequate for a complete explanation of the origin, of the details and of the variations of this phenomenon. As often happens in crystal chemistry problems, one is presumably confronted with a balance of several subtle intra- and intermolecular factors.