Deciphering the Photochemistry of Thionylimide (HNSO): Isomers and Tunneling-Controlled Reactions

Thionylimide (HNSO) and its isomer thionitrous acid (HSNO) are important intermediates in atmospheric chemistry and biology, respectively. However, the chemical network linking HNSO and HSNO is not fully understood due to the absence of some high-energy isomers. Herein, we report the identification of <i>trans</i>-HNSO and <i>cis</i>-HONS during the photochemistry of <i>cis</i>-HNSO in a solid Ne matrix at 3 K. The characterization of both species with matrix-isolation IR spectroscopy is supported by D- and ¹⁵N-isotope labeling experiments and quantum chemical calculations at the CCSD(T)-F12B/VTZ-F12 level of theory using configuration-selective vibrational configuration interaction theory (VCI). Interestingly, spontaneous reactions via quantum mechanical tunneling (QMT) have been observed at 3 K for the <i>trans</i> → <i>cis</i> conformational rotation in HNSO and HOSN, as well as for the 1,3-hydrogen migration in <i>cis</i>-HONS (→ <i>cis</i>-HSNO). The QMT mechanism by the hydrogen atom is consistent with the kinetic isotope effects (KIEs) and the multidimensional instanton theory calculations.