Analysis of reaction kinetics by Fourier transform infrared spectrometry

Photokinetic studies of an intermolecular [4 + 2] photocycloaddition starting from 3,4,5,6-tetrachloro-1,2-benzoquinone ( o -chloranil) and of CO elimination from 3,5-di- tert -butyl-1,2-benzoquinone (DBQ) are reported. Both reactions were initiated by exciting the lowest energetic 1 (π-π) * [and also the 1 (n-π) * state in the case of DBQ] with argon ion laser emission lines. The kinetic analyses were mostly carried out with the use of time-dependent IR intensity alterations in the range of C=O stretching modes. The reactions have a non-uniform character which reveals the existence of intermediates. The intermediate of the dimerization could be established by the wavelength-dependent quantum yield related to the initial compound. The laser-induced reaction can be described by a two-photon process in which the first photon excites the starting compound while the second photon interacts with the intermediate. In the case of the CO elimination, the bis-ketene intermediate could be identified by recording the asymmetric stretching vibration at 2173 cm −1 in the vicinity of the vibration of free carbon monoxide at 2134 cm −1 .