Photodimerization of the alpha'-polymorph of ortho-ethoxy-trans-cinnamic acid in the solid state. I. Monitoring the reaction at 293 K.

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.

The low-temperature study of D- and DL-camphoric anhydride.

The low-temperature crystal stuctures of D- and DL-camphoric anhydride, C(10)H(14)O(3), have been determined by X-ray diffraction methods. Although the two enantiomers crystallize in different space groups, the cell volumes and densities are essentially the same. The six-membered rings deviate significantly from planarity, both exhibiting half-boat conformations. The dihedral angle between the six- and five-membered rings is 80.3 (1) degrees in both cases. The main difference in the molecular stuctures can be described by two torsion angles associated with the H atoms of the methyl substituents. The packing of the racemic and chiral structures are essentially the same.