Rotational spectra of isotopic species of methyl cyanide, CH₃CN, in theirv₈= 1 excited vibrational states

Abstract

Context. Methyl cyanide is an important trace molecule in space, especially in star-forming regions where it is one of the more common molecules used to derive kinetic temperatures. Aims. We want to obtain accurate spectroscopic parameters of minor isotopologs of methyl cyanide in their lowest excited 8 = 1 vibrational states to support astronomical observations, in particular, with interferometers such as ALMA. Methods. The laboratory rotational spectrum of methyl cyanide in natural isotopic composition has been recorded from the millimeter to the terahertz regions. Results. Transitions with good signal-to-noise ratios could be identified for the three isotopic species CH 13 3 CN, 13 CH 3 CN, and CH 3 C 15 N up to about 1.2 THz (J ≤ 66). Accurate spectroscopic parameters were obtained for all three species. Conclusions. The present data were already instrumental in identifying 8 = 1 lines of methyl cyanide with one 13 C in IRAM 30 m and ALMA data toward Sagittarius B2(N).

Exploring molecular complexity with ALMA (EMoCA): Deuterated complex organic molecules in Sagittarius B2(N2)

“…They are based on Müller et al (2009) with transition frequencies in the range of our survey from Demaison et al (1979). Predictions for 13 C isotopologues in their 8 = 1 states are based on preliminary data from Müller et al (2016a). Vibrational corrections to the partition function were included in the private entries for the main isotopic species.…”

Laboratory spectroscopic study of isotopic thioformaldehyde, H2CS, and determination of its equilibrium structure

“…Predictions were generated from the published data for the isotopic species H 2 CS, H 2 C 34 S, H 2 C 33 S, H 13 2 CS, and HDCS (Johnson et al 1971;Beers et al 1972;Cox et al 1982;Brown et al 1987;McNaughton & Bruget 1993;Minowa et al 1997). Higher order spectroscopic parameters of isotopic species with heavy atom substition were estimated from those of H 2 CS by scaling the parameters with appropriate powers of the ratios of 2A − B − C, B + C, and B − C. Even though these estimates do not hold strictly, they are almost always better than constraining the parameters to zero and also mostly better than contraining the parameters to values directly taken from the main isotopic species, see, e.g., below or the examples of isotopic CH 3 CN (Müller et al 2016) or H 2 CO (Müller & Lewen 2017). This scaling procedure is not recommended for H to D We carried out the rotational assignment for the FASSST spectra of thioformaldehyde with the Computer Aided Assignment of Asymmetric Rotor Spectra (CAAARS) program applying the Loomis-Wood procedure, with which the observed spectrum is visually compared with predicted line positions and intensities to make new assignments (Medvedev et al 2005).…”

Rotational spectroscopy of singly 13C substituted isotopomers of propyne and determination of a semi-empirical equilibrium structure

“…Trends in the small deviations observed for quartic and some sextic distortion parameters were taken into account in a second step. Such empirically scaled parameters are usually much better than fixing these higher order parameters to zero and often also better than fixing the values to those of the main isotopic species, as also shown for isotopic species of methyl cyanide [41]. The relations hold strictly for the Dunham parameters of a diatomic molecule.…”

“…Such rest frequencies are usually based on laboratory spectroscopic investigations and are frequently provided in databases such as the Cologne Database for Molecular Spectroscopy, CDMS 1 [35]. Recent investigations of 13 C-containing isotopologs of molecules detected in space include for example dimethyl ether with one and two 13 C [36], the acetaldehyde isotopomers with one 13 C [37], 13 CH 3 NH 2 [38], isotopic acetone with emphasis on CH 3 13 C(O)CH 3 [39], isotopic c-C 3 H 2 with several doubly substituted species [40], and isotopic methyl cyanide in the 8 = 1 excited state [41].…”