UNIMOLECULAR ISOMERIZATION OF CHEMICALLY ACTIVATED THIIRANE TO VINYLTHIOL

Excited S(1D2) atoms from the in situ UV photolysis of COS react with ethylene to yield thiirane and vinylthiol in 100% yield. The relative yields of these two products are pressure dependent and the proportion of vinylthiol decreases with increasing total pressure while the sum remains constant. From detailed kinetic studies it is concluded that thiirane forms via pressure stabilization of the hot ground state cycloadduct thiirane, which at sufficiently high pressure is completely stabilized, and vinylthiol forms via direct insertion into the C-H bonds and also by unimolecular isomerization of the chemically activated ground-state thiirane. The isomerization reaction is complete at zero pressure. This novel rearrangement reaction is the formal analogue of the cyclopropane → propylene rearrangement, and like this reaction it proceeds concertedly through a bicyclic activated complex. The isomerization rate constant and preexponential factor could be satisfactorily reproduced with theoretical rate computations. The distribution of products is influenced by the wavelength of light employed for the photolysis of COS: at short wavelength the relative yield of S(1D2) atom formation increases at the expense of S(3Pj) atom formation and the S(1D2) atom carries increasing amounts of translational energy. S(3Pj) atoms react with ethylene to yield only thiirane. Rateconstant values for the various elementary steps involved in the reaction sequence have been derived. © 1979, American Chemical Society. All rights reserved.