Monitoring structural transformations in crystals. 8. Monitoring molecules and a reaction center during a solid-state Yang photocyclization

Abstract

Structural changes taking place in a crystal during an intramolecular photochemical reaction [the Yang photocyclization of the alpha-methylbenzylamine salt with 1-(4-carboxybenzoyl)-1-methyladamantane] were monitored step-by-step using X-ray structure analysis. This is the first example of such a study carried out for an intramolecular photochemical reaction. During the photoreaction, both the reactant and product molecules change their orientation, but the reactant changes more rapidly after the reaction is about 80% complete. The distance between directly reacting atoms in the reactant molecule is almost constant until about 80% reaction progress and afterwards decreases. The torsion angle defined by the reactant atoms that form the cyclobutane ring also changes in the final stages of the photoreaction. These phenomena are explained in terms of the influence of many product molecules upon a small number of reacting molecules. The adamantane portion shifts more than the remaining part of the anionic reactant species during the reaction, which is explained in terms of hydrogen bonding. The structural changes are accompanied by changes in the cell constants. The results obtained in the present study are compared with analogous results published for intermolecular reactions.

Monitoring Structural Transformations in Crystals. Part 15. Structural Changes in Crystals Caused by UV Radiation Despite the Lack of a Photochemical Reaction

“…Nevertheless, many of them are statistically significant. In the case of chemical reactions changes are usually bigger, namely even ca 0.25 Å [4,5,11]. The increase of intermolecular distances was also seen in the case of disappearance of diffracting properties of 1,5-diphenyl-1,4-pentadiene-3-one [21].…”

“…In the case of intra-and intermolecular chemical processes, a cell volume can change by several percent [4,[7][8][9][10][11]. It is interesting that even for one type of a chemical reaction, cell constants and a cell volume for different compounds can change in a different manner [34,35].…”

“…Although, the changes are small, they are statistically significant at the 3r level. For chemical reactions the changes in orientation of molecular fragments are usually much bigger, namely even ca 20° [5][6][7][9][10][11].…”

“…In the recent decade the papers on monitoring photochemical reactions in crystals by means of X-ray structure analysis have started to appear in the scientific literature [1][2][3][4][5][6][7][8][9][10][11][12]. Those papers have provided the knowledge on structural changes brought about by photochemical reactions, in particular in a reaction center and positions of molecules in crystals.…”

Supramolecular solids and time-resolved diffraction

“…Since detectors are currently not fast enough to allow very short time intervals between measurements, the study of irreversible processes requires a large number of samples or, if possible, one large sample that can be moved rapidly to allow exposure of different areas of the same crystal. Though in recent studies several reactions have been monitored as the reaction proceeds in the crystal, [6][7][8] in only a few cases has the study of reaction mechanism on a timescale of picoseconds been attempted. 9 The latter class is clearly much more demanding experimentally and has not been achieved as yet for chemical reactions in solids at atomic resolution.…”