The topochemical [2+2] photocycloaddition reaction of alpha-trans-cinnamic acid: A perspective from solid-state NMR measurements

Solid-state nuclear magnetic resonance (NMR) spectroscopy experiments have been employed in order to investigate the [2+2] photocycloaddition reaction of alpha-trans-cinnamic acid. Scientists have known for over one hundred years that this particular photoreaction occurs in the solid state, and they have generally understood it within the framework of Cohen and Schmidt's "topochemical postulate." However, the detailed reaction mechanism is not understood from first principles, despite the prevalence of cinnamates in both nature and the laboratory. Cinnamate photoreactions occur in coca leaves and Bermuda grass in which they are linked to the degradation of plant cell walls. In addition, cinnamates have been utilized in synthetic systems for applications in photo-shape-memory and cracking healing polymers.;In order to understand this photoreaction, NMR experiments were performed on powders and a single crystal of alpha-trans-cinnamic acid before and after photoirradiation. 13C {1H} Cross polarization magic angle spinning (CPMAS) NMR experiments demonstrate that alpha-trans-cinnamic acid forms different polymorphs (P21/n and C2/c) of the photoproduct, alpha-truxillic acid, depending on the photon energy of the incident light. Furthermore, crystals of alpha- trans-cinnamic acid undergo a polymorphic phase transition during the course of the photoreaction when the incident photons are in the tail of the absorption band (lambda = 350 rim).;A single crystal double resonance {1H-13C} NMR probe has been designed and engineered in order to perform the first NMR experiment on a single crystal of alpha-trans-cinnamic acid. Both crystal angle and contact time were varied in 13C {1H} cross polarization (CP) NMR experiments, and the results were modeled with the known crystal structure. Transient oscillations were observed in variable contact time experiments, and the frequency of oscillation was found to be orientation dependent. Separated local field spectroscopy experiments were carried out as a function of crystal orientation using the MREV-8 pulse sequence in order to accurately measure the 1H- 13C heteronuclear dipolar coupling constant. The 1H- 13C bond length for the beta-carbon of alpha-trans-cinnamic acid was found to be 0.962 or 1.013 A depending on the scaling factor.;13C {1H} CP NMR experiments were performed on the photoirradiated single crystal of alpha-trans-cinnamic acid. Resonances of the alpha-truxillic acid photoproduct were observed, demonstrating that NMR can potentially be used to follow single crystal-to-single crystal photoreactions.