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.