The kinetics of the 2pC2p photodimerisation reactions of single-crystalline derivatives of trans-cinnamic acid: A study by infrared microspectroscopy Samantha L. Jenkins a , Matthew J. Almond a, * , Samantha D.M. Atkinson a , Michael G.B. Drew a , Peter Hollins a , Joanne L. Mortimore a , Mark J. Tobin b a School of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK b Daresbury Laboratory, Warrington WA4 4AD, UK Received 26 July 2005; received in revised form 4 October 2005; accepted 8 October 2005 Available online 21 November 2005 Abstract The kinetics of the photodimerisation reactions of the 2- and 4-b-halogeno-derivatives of trans-cinnamic acid (where the halogen is fluorine, chlorine or bromine) have been investigated by infrared microspectroscopy. It is found that none of the reactions proceed to 100% yield. This is in line with a reaction mechanism developed by Wernick and his co-workers that postulates the formation of isolated monomers within the solid, which cannot react. b-4-Bromo and b-4-chloro-trans-cinnamic acids show approximately first order kinetics, although in both cases the reaction accelerates somewhat as it proceeds. First order kinetics is explained in terms of a reaction between one excited- and one ground-state monomer molecule, while the acceleration of the reaction implies that it is promoted as defects are formed within the crystal. By contrast b-2-chloro-trans- cinnamic acid shows a strongly accelerating reaction which models closely to the contracting cube equation. b-2-Fluoro- and b-4-fluoro-trans- cinnamic acids show a close match to first order kinetics. The 4-fluoro-derivative, however, shows a reaction that proceeds via a structural intermediate. The difference in behaviour between the 2-fluoro- and 4-fluoro-derivative may be due to different C–H/F hydrogen bonds observed within these single-crystalline starting materials. q 2005 Elsevier B.V. All rights reserved. 1. Introduction The photodimerisation reactions of trans-cinnamic acid and its derivatives (Scheme 1) are well-known and widely studied processes [1–8]. It is of considerable interest to follow the kinetics of solid-state reactions such as these but this has proved to be difficult, partly because of their complexity and partly because of the lack of suitable methods for in situ monitoring [9]. Such lack of information regarding reaction pathways has held back the practical application of solid-state reactions in synthesis, because of the difficulty in accurately predicting products and yields. Recent theoretical studies by Wernick and his co-workers [10], building upon the earlier work of Schmidt and his co-workers, [11] backed up by preliminary experiments, proposed a model for the photo- dimerisation of trans-cinnamic acid derivatives. In single crystals of these derivatives monomers pack together in layers with the individual monomers within a layer held together by hydrogen-bonding. At the start of the reaction each monomer may react with a second monomer positioned parallel to either its upper or lower face (these are known as two-sided reactants). As the reaction progresses isolated pairs of monomers are formed, which may react on one face only (one-sided reactants). Eventually isolated monomers are left which have no available second reactant molecule with which to form a dimer. The results of this proposed mechanism are as follows: 1. That a 100% yield of dimer is not attained—the model predicts a yield of 86G5%; 2. That the reaction will deviate somewhat from the first order kinetics predicted on the basis that the reaction proceeds between one photo-excited and one ground state monomer molecule. In recent work, we have utilised the technique of infrared microspectroscopy to monitor the dimerisation reactions in single crystals of derivatives of trans-cinnamic acid [1–3]. In a Journal of Molecular Structure 786 (2006) 220–226 www.elsevier.com/locate/molstruc 0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2005.10.013 * Corresponding author. Fax: C44 1 1893 16332. E-mail address: m.j.almond@reading.ac.uk (S.L. Jenkins).