IR spectroscopic study of thioacrolein and its photoisomerization to methylthioketene

Abstract

The molecules of thioacrolein (1) and metylthioketene (3), which are labile under normal conditions, have been studied by the matrix Fourier transform infrared spectroscopy method. Compound1 was obtained by vacuum pyrolysis (1120 K, 10⁻⁴ Torr) of diallyl sulfide. The analysis of its IR spectrum shows that1 exists as a mixture ofs-trans ands-cis conformers. UV irradiation (λ > 248 nm) of matrix isolated1 results in a 1,3 hydrogen shift and the formation of3, which is characterized by a strong band of antisymmetric stretching vibrations of the thioketene fragmentvC=C=S at 1777.2 cm⁻¹. The experimental IR bands of molecules1 and3 were assigned to fundamental modes. The relatively low frequencies of the stretching vibrations of the double bonds (vC=C at 1598.0 cm⁻¹ andvC=S at 1071.8 cm⁻¹) and the heightened frequency of the C-C single bond stretching (vC-C 1173.7 cm⁻¹) in the spectrum of1 indicate appreciable delocalization of the electron density in the conjugated π-orbital system.