Ideal Molecular Conformation versus Crystal Site Symmetry

The X-ray structures of hydrocarbons that crystallize on special positions, and thus possess one or more crystallographic symmetry elements, were compared to DFT-calculated structures of the same hydrocarbons in the gas phase. Of the roughly 400 structures examined, at least 9% crystallize with a site symmetry that is not a subgroup of the symmetry of the ideal, lowest-energy, gas-phase structure. Thus, the crystal conformations of these molecules are very different from their ground-state conformations in the absence of packing forces, not merely modest distortions of their ideal structures. Most of the anomalous structures in our sample are higher-energy, inversion-symmetric conformations of molecules that possess chiral or polar ground-state conformations, suggesting that the well-known propensity of crystals to form with centers of inversion frequently leads molecules to adopt anomalous conformations in the solid state.