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Conformational variability of molecules in different crystal environments: a database study

Acta Cryst. (2008). B64, 348–362 (

[Ephedrine] Ephedrine in CSD entries EPHDHP and FIMVAY (a co-crystal) showing conformational differences due to rotation of the methylamino group.
Are molecular conformations systematically affected by crystal environments? This study identifies organic molecules having multiple occurrences in the Cambridge Structural Database (CSD): alone, as solvates, co-crystals, etc. and uses cluster analysis to reveal their conformational variability. About 40% of flexible molecules adopt more than one conformation, but diversity arises principally through, e.g., free rotation of functional group substituents, and in large-ring compounds, peptides, etc. – cases where different conformers are approximately equi-energetic. The study provides further reassurance that crystal conformations are good guides to conformational flexibility in vacuo and in solution.
Z. F. Weng, W. D. S. Motherwell, F. H. Allen and J. M. Cole