UV-, VIS- and IR-light-induced isomerization of HSNO in a low-temperature matrix

An infrared spectroscopic investigation has been performed on the trans and cis isomers of thionitrous acid (HSNO) and their D- and ¹⁵NO-isotopic modifications in argon matrices at 12 K. The substances were prepared photolytically from thionylimide (HNSO) isotopes in the matrix. With UV (250 nm), VIS (585 nm), and IR irradiation the cis → trans or the trans → cis isomerization of HSNO was induced, allowing an unequivocal distinction between the closely resembling IR spectra of the trans and cis isomers. Complete sets of fundamental frequencies of both rotamers were obtained and assigned by normal coordinate analysis using the transferable valence force field (TVFF) approach. Parallel to this analysis ab initio calculations on the SCF- and CI-levels were performed to predict energy, geometry, and barrier of internal rotation for the two HSNO rotamers.