PROBING POLYMORPHISM AND REACTIVITY IN THE ORGANIC-SOLID STATE USING C-13 NMR-SPECTROSCOPY - STUDIES OF PARA-FORMYL-TRANS-CINNAMIC ACID

p-Formyl-trans-cinnamic acid (p-FCA) is known to exist in two different crystal phases (denoted β and γ). When crystals of the β phase of p-FCA are exposed to UV radiation, a solid state dimerization reaction occurs to produce 4,4′-diformyl-β-truxinic acid. In contrast, crystals of the γ phase of p-FCA are photostable. It is shown in this paper that high resolution solid state 13C NMR spectroscopy is a sensitive technique for distinguishing the β and γ phases of p-FCA, and can be used to investigate, in detail, the chemical transformation that occurs upon UV irradiation of the β phase. Specifically, the 13C NMR spectra presented here were recorded using the TOSS (total suppression of sidebands) pulse sequence; this is based upon the standard 13C CPMAS (cross polarization/magic angle sample spinning/high power 1H decoupling) method, but has the additional feature that all orders of spinning sidebands are eliminated from the spectrum. The photoproduct obtained from UV irradiation of β-p-FCA contains a significant noncrystalline component (assessed via powder X-ray diffraction), and our NMR studies suggest that this noncrystalline component of the photoproduct contains some amount of the γ phase of the monomer p-FCA. A mechanism is proposed to explain the fact that UV irradiation of β-p-FCA can generate, in addition to the expected photodimer, an impurity amount of the γ phase of p-FCA. © 1991.