Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

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

Valerio Lattanzi, Miguel Sanz-Novo, Víctor M. Rivilla, Izaskun Jiménez-Serra, Paola Caselli

TL;DR

This study delivers the first laboratory detection and high-resolution rotational characterization of trans-HNSO, a mixed N–S–O isomer with promising astrochemical relevance due to its strong dipole moments. Using CASAC frequency-modulated absorption spectroscopy in a hollow-cathode discharge, 104 transitions between 200–530 GHz were assigned and analyzed with a Watson S-reduced Hamiltonian, achieving an rms of 40 kHz and excellent agreement with CCSD(T) predictions. The resulting accurate rest frequencies and dipole moments enhance the viability of astronomical searches for trans-HNSO and enable experiments to probe cis–trans isomerization and potential quantum tunneling effects in the interstellar medium. By coupling these measurements with cis-HNSO data and public databasing, the work provides a concrete framework for testing interstellar sulfur–nitrogen–oxygen chemistry and refining models of sulfur chemistry in space.

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.

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

TL;DR

This study delivers the first laboratory detection and high-resolution rotational characterization of trans-HNSO, a mixed N–S–O isomer with promising astrochemical relevance due to its strong dipole moments. Using CASAC frequency-modulated absorption spectroscopy in a hollow-cathode discharge, 104 transitions between 200–530 GHz were assigned and analyzed with a Watson S-reduced Hamiltonian, achieving an rms of 40 kHz and excellent agreement with CCSD(T) predictions. The resulting accurate rest frequencies and dipole moments enhance the viability of astronomical searches for trans-HNSO and enable experiments to probe cis–trans isomerization and potential quantum tunneling effects in the interstellar medium. By coupling these measurements with cis-HNSO data and public databasing, the work provides a concrete framework for testing interstellar sulfur–nitrogen–oxygen chemistry and refining models of sulfur chemistry in space.

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 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 (-HNSO) had been available until now. We report the first laboratory detection and rotational spectroscopic characterization of -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 -HNSO lies only a few kcal/mol above the 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 -to- isomerization at cryogenic temperatures, these results open the way to test directly whether quantum tunneling governs the interstellar distribution of HNSO isomers.

Paper Structure

This paper contains 6 sections, 3 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Experimental methods
  3. Theoretical calculations
  4. Analysis
  5. Discussion and Conclusions
  6. Supplementary Tables and Figures

Figures (3)

  • Figure 1: Raw experimental spectra of cis- (left) and trans-HNSO (right); both spectra were acquired with a 3 ms time constants and an integration time of 20 and 180 seconds for cis and trans respectively. Although the y-axis labels are arbitrary, they have been retained to highlight the differences in signal intensity between the two isomers when measured under identical experimental conditions.
  • Figure 2: Simulated spectra of the two HNSO conformers generated using the MADCUBA-SLIM tool Martin2019 and assuming (left) the same physical parameters derived for $cis$-HNSO toward G+0.693 (i.e., $N= 8\times10^{13}$cm$^{-2}$, $T_{ex}$ = 11 K and FWHM = 21.5 km/s; SanzNovo2024), (right) a 10:1 abundance ratio of $cis/trans$.
  • Figure 3: Structures and relative energy levels of $cis$-HNSO (left, as the ground state), $trans$-HNSO (right, at 3.7 kcal/mol), and the transition state (middle, at 14.5 kcal/mol); energy values were derived by Kumar2017. Red atom corresponds to oxygen, yellow to sulfur, blue to nitrogen, and grey to hydrogen.