Conformational dimorphism of the spiropyran 1',3',3'-trimethylspiro[2H-1-benzopyran-2,2'-indoline]-6-carbaldehyde).
The title compound, C(20)H(19)NO(2), crystallizes from an acetone-heptane solution as two dimorphs in the space groups C2/c and Cc. Each dimorph has two molecules in the asymmetric unit. The four molecules adopt slightly different conformations expressed by the degree of bending around a vector connecting the O and C atoms attached to the bridgehead C atom of the pyran ring. Due to the fact that all four molecules are chemically identical, the difference in bending is attributed to packing forces. This is evident from the close contacts of neighbouring molecules perpendicular to the plane of the benzopyran moiety observed in the Cc structure and not in the C2/c structure. These observations provide a unique example that shows how packing forces can affect the conformation of a specific molecule.
Zouev
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Schulich Faculty of Chemistry, Technion - Israel Institute of Technology, Haifa 32000, Israel.
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