Transient States in [2 + 2] Photodimerization of Cinnamic Acid: Correlation of Solid-State NMR and X-ray Analysis

13C-CPMAS and other solid-state NMR methods have been applied to monitor the solid-state
reactions of trans-cinnamic acid derivatives, which are the pioneer and model compounds in the field of
topochemistry previously studied by X-ray diffraction, AFM, and vibrational spectroscopy. Single-crystal
X-ray analyses of photoirradiated α-trans-cinnamic acid where the monomers are arranged in a head-to-tail manner have revealed the formation of a centrosymmetric α-truxillic acid photodimer. For a
centrosymmetric dimer, however, two cyclobutane carbon signals and one carbonyl carbon signal were
expected apart from other aromatic carbon signals. Instead, four cyclobutane and two carbonyl carbon
signals were observed suggesting the formation of a non-centrosymmetric photodimer. Removing hydrogen
bonds from the system by esterfication of α-truxillic acid yield a centrosymmetric photodimer. Careful analysis
of the obtained products via solid-state NMR clearly showed that the observed peak splittings in the 13C-CPMAS spectra did not originate from packing effects but rather result from asymmetric hydrogen bonds
distorting the local symmetry. Further evidence of this rather dynamic hydrogen-bonding stems from high-temperature X-ray data revealing that only the joint approach of both X-ray analysis and solid-state NMR
at similar temperatures allows for the successful characterization of dynamic processes occurring in
topochemical reactions, thus, providing detailed insight into the reaction mechanism of organic solid-state
transformations.