Effects of aromatic substitution on the photodimerization kinetics of β-trans cinnamic acid derivatives studied with ¹³C solid-state NMR

In our efforts to study photodimerizations in the solid state, we present data on the influence of the position of aromatic substitution by bromine on the photodimerization rate in cinnamic acid derivatives. Results were obtained by ¹³C CPMAS NMR spectroscopy together with chemical shift tensor analysis, DFT calculations using the NMR-CASTEP program, and crystal structure data. Reaction rates are highest for para bromo substitution, whose parent crystal structure was solved in this work. To explain the differences in photoreaction rate, several factors such as distance between double bonds, best π-orbital overlap of the reacting C=C double bonds, and CSA tensor analysis (using 2D PASS) were taken into account. Calculations of ¹³C chemical shifts and chemical shift anisotropy tensor parameters show very good agreement with experimental data, including the carboxylic carbon that is hydrogen bonded to the neighboring cinnamic acid molecule. For the cinnamic acid photodimerization, the best angle between reacting double bonds and the smallest degree of molecular reorientation favor faster photoreaction.