title: Photodimerization of the alpha'-polymorph of ortho-ethoxy-trans-cinnamic acid in the solid state. I. Monitoring the reaction at 293 K.
abstract: Structural changes that occur during the [2 + 2] photodimerization of the metastable alpha'-polymorph of ortho-ethoxy-trans-cinnamic acid at 293 K are presented here. Crystals of the alpha'-polymorph were first stabilized by exposing the alpha-polymorph to UV light for a short period of time at 343 K. The photodimerization reaction was then carried out at 293 K and observed in situ by single-crystal X-ray diffraction. The alpha'-polymorph contains three molecules in the asymmetric unit, labelled A, B and C, which are arranged to form two potential reaction sites. The intermolecular distance between the C=C bonds of molecules A and B (making up the AB site) is 3.6 A, and these were observed to undergo photodimerization at 293 K. The corresponding distance between centrosymmetrically related C=C bonds in the CC site (made up of C molecules) is 4.6 A, and these remain unreacted even after 60 h irradiation at 293 K. The crystal of the final product, which corresponds to a 66.7% conversion (only two out of three molecules in the asymmetric unit take part in the photodimerization reaction at 293 K), contains an ordered arrangement of the photodimer and unreacted monomer. The crystal retains many structural features of the original monomer crystal, including carboxylic acid hydrogen bonds and C-H.O interactions. Single-crystal X-ray diffraction was used to monitor changes in the unit-cell parameters, reacting molecules and molecular conformations as the reaction progressed. The conformation of the photodimer obtained from the solid-state reaction differs from that of the photodimer obtained by recrystallization from solution.
authors.affiliation: Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, PO WITS, 2050 Johannesburg, South Africa. manuel@hobbes.gh.its.ac.za
authors.forename: M A
authors.initials: MA
authors.lastname: Fernandes
authors.forename: D C
authors.initials: DC
authors.lastname: Levendis
mesh_terms: D002934:Cinnamates / Q000737:chemistry* / Q000528:radiation effects*; D003460:Crystallization; D018360:Crystallography, X-Ray; D019281:Dimerization; D006860:Hydrogen Bonding; D008958:Models, Molecular; D008968:Molecular Conformation / Q000528:radiation effects; D015394:Molecular Structure; D044367:Phase Transition / Q000528:radiation effects; D010777:Photochemistry; D013696:Temperature; D014466:Ultraviolet Rays
chemical_list: C488180:2-ethoxycinnamic acid; D002934:Cinnamates; C029010:cinnamic acid