Laboratory Detection and Rotational Spectroscopy of $trans$-HNSO: Implications for Astronomical Observations

TL;DR

This study reports the first laboratory detection and detailed spectroscopic analysis of trans-HNSO, a sulfur-bearing molecule relevant to interstellar chemistry, providing data crucial for future astronomical observations and understanding isomerization processes.

Contribution

The paper presents the first laboratory rotational spectrum of trans-HNSO, including precise constants and predictions, facilitating its detection in space and advancing knowledge of interstellar sulfur chemistry.

Findings

First laboratory detection of trans-HNSO

Accurate rotational constants obtained for trans-HNSO

Trans-HNSO lines are over 5 times brighter than cis-HNSO assuming equal abundance

Abstract

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.