IR SPECTROSCOPIC STUDY OF THIOACROLEIN AND ITS PHOTOISOMERIZATION TO METHYLTHIOKETENE

The molecules of thioacrolein (1) and metylthioketene (3), which are l abile under normal conditions, have been studied by the matrix Fourier transform infrared spectroscopy method. Compound 1 was obtained by va cuum pyrolysis (1120 K, 10(-4) Torr) of diallyl sulfide. The analysis of its IR spectrum shows that 1 exists as a mixture of s-trans and s-c is conformers. UV irradiation (lambda > 248 nm) of matrix isolated 1 r esults in a 1,3 hydrogen shift and the formation of 3, which is charac terized by a strong band of antisymmetric stretching vibrations of the thioketene fragment vC=C=S at 1777.2 cm(-1). The experimental IR band s of molecules 1 and 3 were assigned to fundamental modes. The relativ ely low frequencies of the stretching vibrations of the double bonds ( vC=C at 1598.0 cm(-1) and vC=S at 1071.8 cm(-1) and the heightened fre quency of the C--C single bond stretching (vC-C 1173.7 cm(-1)) in the spectrum of 1 indicate appreciable delocalization of the electron dens ity in the conjugated pi-orbital system.