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 Kc 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.
Highlights
Heterocyclic aromatic compounds are a class of molecules that is characterised by a carbon ring structure in which one or more atoms are substituted by heteroatoms such as N, O, or S
The rotational spectrum of imidazole was recorded across the 2–295 GHz frequency range employing several broadband chirped-pulse Fourier transform rotational spectrometers located at the Deutsches Elektronen-Synchroton (DESY; Hamburg) and at the University of Virginia
We used the best-fit model of the rotational spectrum of imidazole obtained in this work to search for this molecule in the hot molecular core Sgr B2(N2)
Summary
Heterocyclic aromatic compounds are a class of molecules that is characterised by a carbon ring structure in which one or more atoms are substituted by heteroatoms such as N, O, or S. N-heterocycles are of great biological significance as they form the backbone of nucleobases, that is, subunits of DNA and RNA polymers, which carry the genetic information of living systems. Based on their structure, the five nucleobases can be considered as derivatives of pyrimidine (c-C4H4N2) or purine (c-C5H4N4). The five nucleobases can be considered as derivatives of pyrimidine (c-C4H4N2) or purine (c-C5H4N4) The latter is a two-ring aromatic compound obtained by the fusion of two structural moieties: the sixmembered pyrimidine (c-C4H4N2) and the five-membered. Grain-mantle reactions of NH3 in H2O ices doped with small quantities of polycyclic aromatic hydrocarbons produce traces of N- and O-containing heterocycles and nucleobases under UV irradiation (Nuevo et al 2014; Materese et al 2015; Bera et al 2017)
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