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Spectral Lines
Published in Ronald L. Snell, Stanley E. Kurtz, Jonathan M. Marr, Fundamentals of Radio Astronomy, 2019
Ronald L. Snell, Stanley E. Kurtz, Jonathan M. Marr
Molecules!electronic angular momentum The second effect that alters the molecular rotational energy levels occurs when the molecule has a net (non-zero) electronic angular momentum in the ground electronic state. Examples of such molecules include the ethynyl radical (CCH) and the cyano radical (CN). Both of these molecules have an unpaired electron, giving rise to a net electronic spin angular momentum in the ground electronic state of the molecule. The interaction between the net electronic spin and the molecular rotation produces splitting of the rotational energy levels. The details are somewhat complicated; the interested reader should consult a book on molecular spectroscopy. Some molecules, such as CN, have both quadrupole hyperfine structure and structure produced by the non-zero electronic angular momentum.
The 130–360 GHz rotational spectrum of syn-2-cyano-1,3-butadiene (C5H5N) – a molecule of astrochemical relevance
Published in Molecular Physics, 2021
Maria A. Zdanovskaia, Brian J. Esselman, Samuel M. Kougias, Aatmik R. Patel, R. Claude Woods, Robert J. McMahon
More than 200 molecules have been detected in the interstellar medium (ISM) or in circumstellar shells – the majority of these detections via radioastronomy [1,2]. Approximately ten percent of the detected species are organic nitriles, including recent detections of benzonitrile [3], 1- and 2-cyanonaphthalene [4], hydroxyacetonitrile [5], and silyl cyanide [6]. Due to their characteristically strong dipole moments and composition of relatively abundant elements, nitriles (R–CN) represent inviting targets for additional radioastronomical detections. A series of highly unsaturated nitrile-containing carbon chains (HC2n+1N with n = 0–5) have been detected [7–12], as well as several partially unsaturated [13–16] and fully saturated [17–19] organic nitriles. Specifically, the known interstellar molecules vinyl cyanide (C3H3N) [14], cyanoallene (C4H3N) [16], and cyanodiacetylene (C5HN) [7] exhibit structural similarities to the molecule of interest in the current work: 2-cyano-1,3-butadiene (C5H5N) (Figure 1). Our group synthesised and characterised several astrochemically relevant nitriles and isonitriles in order to analyse their rotational spectra [20–23]. Although neither McCarthy et al., who examined the electrical discharge of benzene with molecular nitrogen [24], nor Zwier and coauthors, who examined the gas-phase pyrolysis of 3-pentenenitrile [25], detected 2-cyano-1,3-butadiene among their products, it may nevertheless be considered a likely interstellar molecule. Irradiation of cyanoacetylene with ethylene with 254 nm light, for example, produces 2-cyano-1,3-butadiene [26]. A theoretical study suggested that 2-cyano-1,3-butadiene is a predominant product in the reactions of cyano radical with 1,2-butadiene and cyano radical with 1-butyne [27]. There are, however, no published spectroscopic data available to enable a radioastronomical search for this nitrile.