ANALYSIS OF REACTION-KINETICS BY FOURIER-TRANSFORM INFRARED SPECTROMETRY - LASER-INDUCED PHOTOCHEMISTRY OF 1,2-BENZOQUINONES

Photokinetic studies of an intermolecular [4 + 2] photocycloaddition s tarting 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 repo rted. Both reactions were initiated by exciting the lowest energetic 1 (pi-pi) [and also the 1(n-pi)* state in the case of DBQ] with argon i on laser emission lines. The kinetic analyses were mostly carried out with the use of time-dependent IR intensity alterations in the range o f C=O stretching modes. The reactions have a non-uniform character whi ch reveals the existence of intermediates. The intermediate of the dim erization could be established by the wavelength-dependent quantum yie ld related to the initial compound. The laser-induced reaction can be described by a two-photon process in which the first photon ''cites th e starting compound while the second photon interacts with the interme diate. In the case of the CO elimination, the bis-ketene intermediate could be identified by recording the asymmetric stretching vibration a t 2173 cm-1 in the vicinity of the vibration of free carbon monoxide a t 2134 cm-1.