Spectroscopic study and astronomical detection of doubly¹³C-substituted ethyl cyanide

Abstract

Context. We have performed a spectral line survey called Exploring Molecular Complexity with ALMA (EMoCA) toward Sagittarius B2(N) between 84.1 and 114.4 GHz with the Atacama Large Millimeter/submillimeter Array (ALMA) in its Cycles 0 and 1. Line intensities of the main isotopic species of ethyl cyanide and its singly 13 C-substituted isotopomers observed toward the hot molecular core Sagittarius B2(N2) suggest that the doubly 13 C-substituted isotopomers should also be detectable. Aims. We want to determine the spectroscopic parameters of all three doubly 13 C-substituted isotopologues of ethyl cyanide to search for them in our ALMA data. Methods. We investigated the laboratory rotational spectra of the three species between 150 GHz and 990 GHz. We searched for emission lines produced by these species in the ALMA spectrum of Sagittarius B2(N2). We modeled their emission and the emission of the 12 C and singly 13 C-substituted isotopologues assuming local thermodynamic equilibrium. Results. We identified more than 5000 rotational transitions, pertaining to more than 3500 different transition frequencies, in the laboratory for each of the three doubly 13 C-substituted isotopomers. The quantum numbers reach J ≈ 115 and K a ≈ 35, resulting in accurate spectroscopic parameters and accurate rest frequency calculations beyond 1000 GHz for strong to moderately weak transitions of either isotopomer. All three species are unambiguously detected in our ALMA data. The 12 C/ 13 C column density ratio of the isotopomers with one 13 C atom to those with two 13 C atoms is about 25. Conclusions. Ethyl cyanide is the second molecule after methyl cyanide for which isotopologues containing two 13 C atoms have been securely detected in the interstellar medium. The model of our ethyl cyanide data suggests that we should be able to detect vibrational satellites of the main species up to at least 19 = 1 at ∼1130 K and up to 13 + 21 = 2 at ∼600 K for the isotopologues with one 13 C atom in our present ALMA data. Such satellites may be too weak to be identified unambiguously for isotopologues with two 13 C atoms.

The millimeter and sub-millimeter rotational spectrum of triple 13C-substituted ethyl cyanide

“…Using the spectroscopic results of this work we searched for 13 CH 13 3 CH 13 2 CN in the EMoCA survey (Margulès et al 2016) toward Sgr B2(N). We were not able to identify any isolated rotational transition of 13 CH 13 3 CH 13 2 CN above the detection limit in this survey.…”

“…In this work, in order to obtain more accurate spectroscopic parameters for the prediction of the 13 CH 13 3 CH 13 2 CN rotational spectra, we used scaling factors based on the experimental data taken from the previous study of doubly 13 C-substituted isotopic species of ethyl cyanide (Margulès et al 2016). We performed the following procedure.…”

“…We performed the following procedure. In a first step we calculated the "offsets", that is, the differences between the theoretical and experimental values for the rotational constants A, B, C and centrifugal distortion constants for 13 CH 3 CH 13 2 CN, 13 CH 13 3 CH 2 CN and CH 13 3 CH 13 2 CN from Margulès et al (2016) (Table 1). The experimental errors for all three isotopologues are close to each other, with an average of 1.925% for rotational constant A and less than 0.5% for B and C. Then from our theoretical rotational constants A, B, and Table 2).…”

“…The rotational spectra of deuterated, mono-13 C, and -15 N containing isotopologs of ethyl cyanide (Margulès et al , 2016Richard et al 2012;Heise et al 1974), as well as vibrationally excited ethyl cyanide (Gibb et al 2000b;Heise et al 1981;Mehringer et al 2004;Daly et al 2013;Belloche et al 2013Belloche et al , 2016 have also been investigated and successfully detected in the ISM.…”

“…A recent laboratory spectroscopic investigation (Margulès et al 2016) deals with the spectra and space identification of three double 13 C isotopologs studied in their ground vibrational states. However, no experimental data are available and there has been no study of the ethyl cyanide containing three 13 C atoms.…”