Dimer or catemer? Low-energy crystal packings for small carboxylic acids

The ability of a search for minima in the lattice energy to predict the crystal structures of three carboxylic acids was examined, using an ab initio based distributed multipole model for the dominant electrostatic contribution to the intermolecular forces. This model was combined with an empirical atom-atom potential with the polar hydrogen atom parameters specifically adjusted to reproduce a range of carboxylic acid crystal structures. The catemer structure of formic acid, the dimer-based structure of benzoic acid, and both the catemer and dimer polymorphs of tetrolic acid were found reasonably accurately, close to the global minimum in the lattice energy. However, as in the recently highlighted case of acetic acid, there are a variety of energetically competitive structures, with both the catemer and dimer motifs, for formic and tetrolic acid. In contrast, all low-energy structures for benzoic acid were based on the dimer motif. These hypothetical structures demonstrate the role of energetics in determining the crystal packing of carboxylic acids.