Data in the Cambridge Structural Database (i.e., the CSD) for ca. 750 well determined (R &lt; 0.075) organic crystal structures having translational symmetry only and ca. 300 more with more than four independent formula units (i.e., Z’ &gt; 4) have been investigated in detail. Obvious approximate symmetry has been found in well over 50% of them. Approximate 21 screw axes, pseudoinversion centers, and pseudotranslations all occur frequently; approximate glides are less common. A program to find pseudotranslations (i.e., modulations) has been written in collaboration with Robin Taylor of the CCDC. The program has been used to identify relationships between phases archived in the CSD and also to identify the probable basic cell in modulated structures. It seems likely that the crystal nucleus is often more symmetric than the macroscopic crystal. The number of structures having obvious layers has been a surprise; those layers often have good approximate symmetry but are stacked such that the overall 3-D symmetry is lower than the near 2-D symmetry. If interlayer forces are weaker than intralayer forces then deformations during the early stages of crystal growth are more likely between layers than within them.

Pervasive approximate symmetry in P1 and high-Z’ organic crystals: Implications for crystal nucleation

2019 Fall National Meeting

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