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 A. A tentative energy classification - within the limits of the many assumptions - and a correlation with the O center dot center dot center dot 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.