CONSISTENT FORCE-FIELD STUDIES OF INTER-MOLECULAR FORCES IN HYDROGEN-BONDED CRYSTALS .3. C=O...H-O HYDROGEN-BOND AND THE ANALYSIS OF THE ENERGETICS AND PACKING OF CARBOXYLIC-ACIDS

The detailed structures of 14 crystals of mono-and dicarboxylic acids and 2 gas-phase dimers are studied by the 126-1 and 9-6-1 force fields of the preceding papers. The energy of packing of the crystal lattice is partitioned in various ways and the different contributions are compared, with special emphasis on the energetics of the “secondary structure”, i.e., the hydrogen-bonded arrays. The electrostatic energy in a homologous series of acids is almost constant, arising from interactions of neighboring carboxylic groups, while the van der Waals energy contribution increases with alkyl chain length. The interaction between the hydrogen-bonded molecules is a balance between an electrostatic attraction and a van der Waals repulsion between the carbonyl and hydroxyl oxygens. A comparison of the alternative secondary structures reveals that for small mono-carboxylic molecules the electrostatic interactions stabilize the catamer motif relative to the cyclic dimer motif. For the dicarboxylic acids the analysis shows that the shortening of the alkyl chain reduces the intermolecular electrostatic interaction. This interaction is even smaller in the a form of oxalic acid (catamer motif) where stability of the secondary structure is due to favorable dispersion interactions. The analysis of the secondary structure in terms of group interactions also indicates the reasons for the larger deviations from experiment in some of the crystals. For example, the larger a axis of acetic acid emerges from the short ̿O…HCH distance which is too repulsive in this potential. © 1979, American Chemical Society. All rights reserved.