title: Dipole-induced polymorphs of trans-2-hydroxycycloheptanecarboxylic acid with virtually the same unit cell.
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.
authors.affiliation: Institute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, Budapest 114, H-1525, Hungary. akalman@chemres.hu
authors.forename: Alajos
authors.initials: A
authors.lastname: Kálmán
authors.forename: László
authors.initials: L
authors.lastname: Fábián
authors.forename: Gyula
authors.initials: G
authors.lastname: Argay
authors.forename: Gábor
authors.initials: G
authors.lastname: Bernáth
authors.forename: Zsuzsanna
authors.initials: Z
authors.lastname: Gyarmati