화학공학소재연구정보센터
Journal of Physical Chemistry B, Vol.112, No.41, 12920-12926, 2008
An Unexpected Phase Transition during the [2+2] Photocycloaddition Reaction of Cinnamic Acid to Truxillic Acid: Changes in Polymorphism Monitored by Solid-State NMR
We have detected a phase transition during the progress of the solid-state [2 + 2] photocycloaddition reaction of alpha-trans-cinnamic acid. The reaction was monitored using C-13 CPMAS experiments as a function of irradiation time of the parent (alpha-trans-cinnamic acid, which forms the product dimer, alpha-truxillic acid. UV light centered at 350 nm was used for photoirradiation, which is in the "tail" of the absorption band of cinnamic acid. Two different crystal polymorphs of alpha-truxillic acid are observed (P2(1)/n and C2/c) at different stages of conversion of the parent crystal, assigned through C-13 NMR and powder X-ray diffraction. The two polymorphs showed clear, distinguishable patterns in the C-13 NMR spectra: a 2-peak versus 3-peak pattern corresponding to sites on the 4-membered Sp(3) hybridized ring in the photoproduct. A phase transition is observed midway through the reaction, which we have assigned to the conversion of the P2(1)/n polymorph to the C2/c polymorph of a-truxillic acid. The packing energy of the resultant mixed crystal of cinnamic acid and truxillic acid changes during the course of the photoreaction, which allows for the C2/c polymorph of truxillic acid to appear. Both phases have been confirmed via X-ray powder diffraction. Two techniques-differential scanning calorimetry and solid-state CPMAS NMR using increasingly fast rotational frequencies-demonstrate that the P2(1)/n phase is metastable.