Ideal Molecular Conformation versus Crystal Site Symmetry

Abstract

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.

A Million Crystal Structures: The Whole Is Greater than the Sum of Its Parts

“…The final paper focused on CSD hydrocarbon molecules situated on crystallographic special positions (almost always inversion centers) . As in the previous study, there was a noticeable preference for some of these molecules to adopt strained planar geometries in the crystalline state, their gas-phase optimum geometries being very different and typically twisted (e.g., Figure ).…”

Conformational Dependence of Triplet Energies in Rotationally Hindered N‐ and S‐Heterocyclic Dimers: New Design and Measurement Rules for High Triplet Energy OLED Host Materials

“…The X‐ray structures proved more twisted in almost all cases. This is attributed to intermolecular packing interactions within the crystal structure which can result in significant differences between predicted and experimental geometries of organic molecules [55, 56] . Further analysis of the crystal packing effects are included in the Supplementary Information.…”

Conformation and Aromaticity Switching in a Curved Non‐Alternant sp2 Carbon Scaffold

“…The C 2 ‐symmetric cisoids of 1 , 1 .+ and 1 2+ are lower in energy than the corresponding C i ‐symmetric transoids by 12.1, 7.2, and 10.3 kJ mol −1 , respectively. This suggests that the cis ‐conformations are more energetically favorable in vacuum, and the trans ‐conformation of 1 2+ in the crystal state might arise from the effects of the two counterions at the opposite sides of π‐scaffold and the crystal packing forces [18] …”