Temperature stability and photodimerization kinetics of β-cinnamic acid and comparison to its α-polymorph as studied by solid-state NMR spectroscopy techniques and DFT calculations

Photoreactions of the α- and β-polymorphs of trans-cinnamic acid were studied by ¹³C CPMAS solid-state nuclear magnetic resonance spectroscopy, and the reactants and products were spectroscopically characterized in detail. Chemical shifts and chemical shift anisotropy tensors calculated using density functional theory (DFT) were found to be in good agreement with the experimental results and helped to identify the polymorphs and the individual assignments of reactant and photoproduct carbon atoms. The β-polymorph is metastable. Its transformation into the α-cinnamic acid polymorph is monitored by temperature-dependent ¹³C NMR spectroscopy. The transformation occurs at a very slow rate at room temperature but is highly accelerated at elevated temperatures. Analysis of the kinetics of the photoreaction shows that the β-polymorph progresses at a slower rate compared to that of α-cinnamic acid. Based on chemical shift tensor values of reactants and products as obtained from 2D PASS spectra, the difference in reaction rates is suggested to be due to the higher amount of molecular reorientation of functional groups upon photoreaction and the larger distance between the reacting double bonds.