Isocyano compounds newly recognized in photochemical reaction of thiazole: matrix-isolation FT-IR and theoretical studies

Abstract

UV-induced photoreactions of thiazole isolated in low-temperature argon matrices have been investigated by a joint use of infrared spectroscopy and density-functional-theory calculations. Photoproducts have been identified by comparison of the observed infrared spectra with the corresponding calculated spectral patterns, leading to the conclusion that undetected open-chain molecules, syn-2-isocyanoethenethiol (CN–CHCH–SH) and 2-isocyanothiirane, are initially produced by cleavage of the CS–CN bond with hydrogen-atom migration, when the matrix samples are exposed to UV radiation coming from a super high-pressure mercury lamp for 3 min. In the secondary photolysis, syn-2-isocyanoethenethiol and 2-isocyanothiirane change to another unknown molecule, 2-isocyanoethanethial (CN–CH₂–CHS), by hydrogen-atom migration with generation of the CS double bond. These photoreaction pathways are supported by kinetic analysis of the absorbance changes of IR bands against irradiation time. We have also found that HCN and the ˙CHCH–S˙ biradical are photodecomposed from thiazole by cleavage of the CN–CC bond following the cleavage of the CS–CN bond, where the hydrogen atom on the center carbon atom of ˙CHCH–S˙ immediately migrates to the end carbon atom to form CH₂CS or to the sulfur atom to form HCC–SH. In addition, weak bands of the species of interest in astrophysics and astrochemistry such as HCCH, NC–SH, HNCS, HCNS, and the ˙CN radical are detected, but the photoconversion from thiazole to isothiazole or Dewar thiazole is not found. The ring-opening photoreaction, photoisomerization and photodecomposition pathways of thiazole isolated in low-temperature argon matrices are discussed comprehensively.