“Coulombic Compression”, a Pervasive Force in Ionic Solids. A Study of Anion Stacking in Croconate Salts

We describe coulombic compression as the driving factor in the tight packing of crystals formed under ionic interactions. In the crystal structures of croconic acid salts, the disk-shaped dianions form stacks with interlayer distances as short as 3.1–3.4 Å. Crystal packing energies have been estimated using atom–atom potentials (AA–CLP) and semi-classical density sums (PIXEL) with evaluation of coulombic, polarization, dispersion, and repulsion terms; the simpler model yields reliable energy estimates even for the multiatomic molecular anions. The structure of the potassium salt is discussed in detail. Calculations show that although the repulsion energy between adjacent anionic disks is enormous, as result of coulombic compression, the overall structure is stable because the cation–anion interaction energy exceeds the combined cation–cation and anion–anion interaction energies. Ion–water coulombic terms are much smaller, and dispersion energies are even smaller but not negligible. Even for crystals with packing energies of several hundred kilojoules per mole, energy differences of a few kilojoules per mole determine structural details, such as the preference of neighboring stacked anions to be staggered rather than eclipsed or the relative stability of two polymorphs of calcium croconate trihydrate.