CONSISTENT FORCE-FIELD STUDIES OF INTER-MOLECULAR FORCES IN HYDROGEN-BONDED CRYSTALS .1. CARBOXYLIC-ACIDS, AMIDES, AND THE C=O...H- HYDROGEN-BONDS

Energy functions for intermolecular atom-atom interactions in carboxylic acids are derived. The energy parameters are determined by the consistent force field method of least-squares fitting of calculated to experimental properties. Crystal structure parameters, sublimation energies, and dipole moments are used, taken from a data base of 14 mono-and dicarboxylic acids. Two force fields are examined, denoted 9-6-1 and 12-6-1, having for each type of atoms a repulsive term, proportional to either 9th or 12th inverse power of interatomic distance, respectively, a dispersion attractive term, and a partial atomic charge. The transferability of energy parameters from a similar force field for amides derived previously is examined in detail. It is found that the 110 observables may be fit, with only a single adjustable parameter, the charge on the hydroxyl hydrogen. Optimizing additional force constants rather than transferring them does not lead to a significant improvement in fit. The hydrogen bond of carboxylic acids is found to resemble that in amides in (1) requiring no special function, thus being well represented as a balance between van der Waals and Coulomb interactions between two polar bonds; (2) having negligibly small van der Waals parameters for the hydrogen. The difference between NH…O and OH…O hydrogen bond lengths and energies falls out naturally as a result of the different van der Waals radii of N and O. The concept of the hydrogen bond derived from our analysis is found to be in qualitative agreement with recent quantum-mechanical calculations of the hydrogen bond energy and its partitioning. © 1979, American Chemical Society. All rights reserved.