Laboratory detection and rotational spectroscopy of trans-HNSO: implications for astronomical observations

Sulfur-bearing molecules are central to interstellar chemistry, yet their observed abundances in the gas phase remain far below cosmic expectations in dense interstellar regions. Mixed N-S-O species such as thionylimide (HNSO) are particularly relevant, as they incorporate three key biogenic elements. The cis conformer of HNSO has recently been detected in the Galactic Center cloud G+0.693-0.027, but no high-resolution data for the higher energy conformer (trans-HNSO) had been available until now. We report the first laboratory detection and rotational spectroscopic characterization of trans-HNSO. Spectra were recorded with the Center for Astrochemical Studies Absorption Cell (CASAC) free-space spectrometer employing a hollow-cathode discharge source, yielding 104 assigned transitions between 200 and 530 GHz. A Watson S-reduced Hamiltonian fit reproduced the data with an rms of 40 kHz, providing accurate rotational and centrifugal distortion constants in excellent agreement with CCSD(T) predictions. Although trans-HNSO lies only a few kcal/mol above the cis form, it has larger dipole components, making its lines particularly intense (more than 5 times brighter, assuming equal abundances) and a very promising candidate for future astronomical detection. The new measurements enable reliable frequency predictions for astronomical searches and will be added to public databases. Combined with recent evidence for tunneling-driven trans-to-cis isomerization at cryogenic temperatures, these results open the way to test directly whether quantum tunneling governs the interstellar distribution of HNSO isomers.