Dipole-Induced Polymorphs of trans-2-Hydroxycycloheptanecarboxylic Acid with Virtually the Same Unit Cell
- 10 December 2002
- journal article
- research article
- Published by American Chemical Society (ACS) in Journal of the American Chemical Society
- Vol. 125 (1) , 34-35
- https://doi.org/10.1021/ja0289447
Abstract
The polymorphs of trans-2-hydroxycycloheptanecarboxylic acid have exactly the same lattice parameters and thus mimic isomorphism. They differ only in their space group: Pna21 versus Pn21a. In form II, the screw axes turn the 18-membered rings of hydrogen-bonded tetramers around the b axis. In this way, the stacking of the layers becomes antiparallel, which cancels out the dipoles within the unit cell. In form I, the same turn around the c axis leaves the stacking of the layers parallel. Thus, the dipoles are canceled out by antiparallel domains in the crystals. Between the antiparallel domains of I, each frontier is a double layer of II. This implies that (a) a pure form of I cannot be isolated and (b) the percentage of II in I may alter from crystal to crystal.Keywords
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