title: A new polymorph of ortho-ethoxy-trans-cinnamic acid: single-to-single-crystal phase transformation and mechanism.
abstract: The alpha-polymorph of ortho-ethoxy-trans-cinnamic acid (OETCA) undergoes a reversible single-crystal-to-single-crystal phase transformation at 333 K. The new high-temperature polymorph (alpha'-OETCA) is stable between 333 and 393 K with three molecules in the asymmetric unit (Z' = 3), space group P1;. Unlike the other polymorphs (and solvate) of OETCA recently reported, two of the molecules in alpha'-OETCA deviate significantly from planarity. This conformational change results in the corrugated sheet-type structure of alpha'-OETCA. The sheets are made up of ribbons, each composed of R(2)(2)(8) hydrogen-bonded pairs (via the -COOH groups), which are further connected by CH.O interactions. When exposed to UV radiation the alpha'-OETCA polymorph can be stabilized below 333 K with ca 8% of the monomer converted into the photodimer. The crystal structures of alpha'-OETCA are reported at two temperatures above the phase transition point (at 345 and 375 K) as well as the stabilized forms at 173 and 293 K. A mechanism for the phase transition involving a cooperative conformational transformation coupled with a shift of layers of OETCA molecules is proposed. The alpha'-OETCA polymorph is also an example of a cinnamic acid derivative where two different potentially photoreactive environments exist in one crystal in which each unit cell has two non-centrosymmetric predimer sites and one centrosymmetric predimer site.
authors.affiliation: Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, PO WITS, 2050 Johannesburg, South Africa.
authors.forename: Manuel A
authors.initials: MA
authors.lastname: Fernandes
authors.forename: Demetrius C
authors.initials: DC
authors.lastname: Levendis
authors.forename: F R Ludwig
authors.initials: FR
authors.lastname: Schoening
mesh_terms: D002152:Calorimetry, Differential Scanning; D002934:Cinnamates / Q000737:chemistry* / Q000528:radiation effects; D003460:Crystallization; D018360:Crystallography, X-Ray; D019281:Dimerization; D006860:Hydrogen Bonding; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation / Q000528:radiation effects; D015394:Molecular Structure; D044367:Phase Transition / Q000528:radiation effects; D010777:Photochemistry; D013696:Temperature; D044366:Transition Temperature; D014466:Ultraviolet Rays
chemical_list: C488180:2-ethoxycinnamic acid; D002934:Cinnamates; C029010:cinnamic acid