Photoreaction kinetics of cinnamate salts - Influence of cation and halogen substitution as studied by solid-state NMR

The photodimerization kinetics of potassium, rubidium and cesium cinnamate, with aromatic substitution by chlorine or bromine in the meta position, was studied by solid -state NMR. All salts photoreact to the corresponding truxinates with a non -planar cyclobutane ring. Analysis of the kinetics by the JMAK equation shows that the reaction rate is faster with smaller cation and in general chlorine salts react faster than the corresponding bromine salts. Data are analyzed based on the crystal structure, e. g., distance and orientation of reacting double bonds and halogen-halogen contacts. Furthermore, determination of the chemical shift anisotropy (CSA) tensor for reactant and product can provide information for necessary atomic reorientations in consequence of photoreaction. Results are explained based on these necessary movements of atoms between reactant and product.