Analysis of photochemical reaction kinetics by FTIR spectra intermolecular photocycloaddition of o-chloranil

The photochemical reaction of o—chloranil (3 ,4,5,6 —tetrachioro— 1 ,2 —benzoquinone) was initiated by the excitation of the lowest energetic l(fl_fl*) state using emission lines of an argon ion laser1 . The data of liquid chromatography establish that only one final compound is generated. The photoproduct has dimeric structure identified by mass spectroscopy. The isotopic distribution of the molecular peaks closely corresponds to the isotopic pattern of octachloro—substituted molecules. Moreover, the mass spectrum exhibits the stepwise loss of two carbon monoxide groups. This feature is characteristic for quinones. The dimeric photoproduct reveals that the reaction can be described by an intermolecular [4+21 photocycloaddltion. In contrast to the reaction of 3,5—di--tert.-butyl—1,2—benzoquinone (investigated under same conditions) in which an absorption band (at 2134 cm') of CO dissolved in the reaction mixture was visible, a parallel CO abstraction can be excluded.