The photodimerisation of trans-cinnamic acid and its derivatives: a study by vibrational microspectroscopy

Infrared and Raman spectroscopies have been used to monitor the [2+2] photo dimerisation reactions of alpha-tr cms-cinnamic acid and of a number of its derivatives. The principal changes observed in the spectra upon dimerisati on are decay of a band around 1637 cm(-1), which is assigned to v(C=C) of t he ethene bond of the monomer, and growth of bands just above 3000 cm(-1), which result from v(C-H) of saturated carbon atoms of the dimer. The use of microscope attachments has allowed us to follow the reactions of single cr ystals and we conclude that the reactions are topotactic in nature. We have carried out preliminary kinetic experiments in which spectra of one single crystal an recorded after sequential periods of photolysis. We find that t he rates of dimerisation of differently substituted cinnamic acids are simi lar, with physical effects, such as the thickness of an individual crystal, outweighing any observed electronic effects (inductive or mesomeric). (C) 2000 Elsevier Science B.V. All rights reserved.