Knowledge Bank

An Infrared spectroscopic investigation has been performed on the trans and cis isomers of thionitrous acid (HSNO) and their D- and $\mathrm{<mrow\; data-mjx-texclass="ORD">15</mrow>}NO$-isotopic modifications in argon matrices at 12 K. The substances were prepared photolytically from thionylimid (HNSO) isotopes in the matrix With UV (250 nm), VIS (585 nm), and IR irradiation the cis $\to$ trans or the trans $\to$ cis isomerization of HSNO was induced, allowing an unequivocal distinction between the closely resembling IR spectra of the trans and cis isomers. Complete set 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. These results and those of a parallel study on methylthionitrite $(CH3SNO)$ are discussed and compared to the behavior of the nitrite molecules HONO and $CH3ONO$.