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sp3)C7-H7AC•••C3' 2.789(3) 2.726(2) 2.667(2) 2.668(6) x-1, y, z (sp2)C6-H6•••C5' 2.706(2) 2.661(2) 2.642(2) 2.632(5) x-0.5, y, 0.5-z

Structural changes proceeding in a crystal during the Yang photocyclization of the salt 6,6-diethyl-5-oxo-5,6,7,8-tetrahydronaphthalene-2-carboxylate with (1S)-1-(4-methylphenyl)ethylamine were monitored by means of X-ray structure analysis. The course of the photoreaction was evaluated on the basis of the geometrical parameters for the pure reactant crystal. Variations in the cell constants, the product content, the geometry of the reaction centre, the orientation of molecular fragments and the geometry of hydrogen bonds were described and analyzed. It was found that the cell volume increased until 56% product content and decreased thereafter. The distance between the directly reacting C atoms was constant, approximately 3.0 A, until approximately 75% reaction progress. Analysis of the distance between atoms that would participate in the formation of the second (unobserved) enantiomorph excluded the formation of such an isomer. Molecular fragments varied their orientation during the photoreaction, and the largest change was observed for the carboxylate group despite its participation in strong hydrogen bonds. The geometry of the hydrogen bonds changed during the photoreaction. The largest change was 0.17 A for the D...A distance and 13 degrees for the D-H...A angle. A comparison of the intra- and intermolecular parameters for the studied salt with data for other compounds undergoing the Yang photocyclization in crystals revealed a diversity of structural changes brought about by this type of photochemical reaction.

The [2 + 2] photodimerization of 2,6-difluorocinnamic acid was gradually induced in crystals by UV radiation at 0.5, 1.1 and 2.1 GPa. The crystal and molecular structures were determined before the photochemical reaction and for many of its steps. For the pure monomer structures, the following parameters were analyzed in order to gain knowledge of the influence of pressure on the reaction environment: (a) the volume of free space, (b) the geometry of close intermolecular interactions, including hydrogen bonds and (c) the geometry of intermolecular reactivity parameters. The following structural changes brought about by the [2 + 2] photodimerization at high pressure were monitored step-by-step: (a) the cell parameters and the cell volume, (b) the content of the monomer and the dimer, (c) the distance between the reactive carbon atoms in adjacent monomer molecules and (d) the geometry of mutual orientation of molecules in the crystals. The rate of the [2 + 2] photodimerization of 2,6-difluorocinnamic acid increased as follows: 0.1 MPa < 0.5 GPa < 1.1 GPa ≤ 2.1 GPa. This was rationalized by the decrease in the volume of free space columns protecting the intermolecular geometry suitable for the reaction and by the decrease of the distance between the reactive carbon atoms of adjacent monomer molecules.

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