Rotational spectroscopy of imidazole: improved rest frequencies for astrophysical searches^⋆

¹ Center for Astrochemical Studies, Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
e-mail: giuliano@mpe.mpg.de, bizzocchi@mpe.mpg.de, caselli@mpe.mpg.de
² Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca’ Foscari Venezia, via Torino 155, 30172 Mestre, Italy
³ Deutsches Elektronen-Synchrotron (DESY), Notkestraße 85, 22607 Hamburg, Germany
⁴ The Hamburg Centre for Ultrafast Imaging (CUI) at the Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany
⁵ Department of Chemistry, University of Virginia, McCormick Road, Charlottesville, VA 22903, USA
⁶ Univ. Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, ISCR – UMR6226, 35000 Rennes, France
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 25 April 2019
Accepted: 25 June 2019

Abstract

Context. Organic ring compounds play a key role in terrestrial biochemistry, and they were also most likely pivotal ingredients in Earth’s prebiotic chemistry. The five-membered ring imidazole, c-C₃N₂H₄, is a substructure of fundamental biological molecules such as the purine nucleobases and the amino acid histidine. An unsuccessful search for imidazole in a sample of cold-core clouds and massive star-forming regions was performed almost 40 years ago. At that time, the spectroscopic knowledge of this species was scarce: the existing laboratory study was limited to the centimetre-wave region, and the precision of the rest frequencies in the millimetre regime was not adequate.

Aims. The goal of the present work is to perform a comprehensive investigation of the rotational spectrum of imidazole in its ground vibrational state from the microwave region to the 1 mm wavelength regime.

Methods. The rotational spectrum of imidazole was recorded in selected frequency regions from 2 to 295 GHz. These intervals were covered using various broadband spectrometers developed at DESY (Hamburg) and at the University of Virginia. High-level ab initio calculations were performed to obtain reliable estimates of the quartic and sextic centrifugal distortion constants. We used the EMoCA imaging spectral line survey to search for imidazole towards the hot molecular core Sgr B2(N2).

Results. About 700 rotational transitions spanning a J interval from 0 to 59 and K_c interval from 0 to 30 were analysed using the Watson S-reduced Hamiltonian. These new data allowed the determination of a much extended set of spectroscopic parameters for imidazole in its vibrational ground state. The improved spectral data allow us to set an upper limit to the column density of imidazole in Sgr B2(N2). Its non-detection implies that it is at least 3400 times less abundant than ethyl cyanide in this source.

Conclusions. With the new set of spectroscopic constants, it has been possible to compute reliable rest frequencies at millimetre wavelengths. We suggest a search for imidazole towards TMC-1, where the aromatic molecule benzonitrile was recently detected.