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Issue
A&A
Volume 601, May 2017
Article Number A2
Number of page(s) 5
Section Atomic, molecular, and nuclear data
DOI https://doi.org/10.1051/0004-6361/201630231
Published online 19 April 2017

© ESO, 2017

1. Introduction

Millimeter and submillimeter observations (Belloche et al. 2013, 2016) revealed that a remarkably rich chemistry occurs in the dense molecular clouds. Much of the molecular data gathered over the last 30 yr from laboratory experiments and quantum-chemical calculations have been summarized by Schoier et al. (2005). Complex organic molecules such as HCOOCH3 (methyl formate), CH3OCH3 (dimethyl ether), CH3CH2OH (ethyl alcohol), CH2CHCN (vinyl cyanide), and CH3CH2CN (ethyl cyanide) have been recognized in hot cores (Gibb et al. 2000a; Johnson et al. 1977) of massive star-forming regions such as Orion KL (Friedel & Widicus Weaver 2012), and Sgr B2 (Nummelin et al. 1998, 2000; Hollis et al. 2003; Müller et al. 2008).

Ethyl cyanide is an abundant molecule observed in hot molecular clouds. The core in Sgr B2, designated the Sgr B2(N) Large Molecule Heimat source or Sgr B2(N-LMH) by Snyder (2006), has the highest column densities of CH3CH2CN of all sources studied so far (~9.6 × 1016 cm-2) (Miao et al. 1995; Kuan et al. 1996; Miao & Snyder 1997). Formation mechanisms for CH3CH2CN and other COMs were discussed by Laas et al. (2011), Garrod et al. (2008), Belloche et al. (2009, 2014).

The first spectroscopic detection of ethyl cyanide in Titan’s atmosphere, obtained with the Atacama Large Millimeter Array (ALMA), was recently reported by Cordiner et al. (2015). CH3CH2CN has also been observed in the coma of comet Halley (Altwegg et al. 1999), in regions of low-mass star formation (Bottinelli et al. 2007; Cazaux et al. 2003) and in high mass young stellar objects (YSOs; Bisschop et al. 2007). Observation of this molecule and its isotopologues is necessary for the comprehension of molecular cloud chemistry.

The relative natural abundance ratio of 13C is about 1% of 12C and is constant on the Earth. The dense clouds of the ISM contain many gas-phase organic molecules that are characterized by large deuterium enrichment compared to the terrestrial standart. Carbon fractionation is also observed and measured in ISM molecules but is less pronounced due to the smaller differential between 13C and 12C atomic masses. Deriving 13C isotopic fractionation of the molecules by observations is one of the promising methods to constrain their main formation mechanism. The derived abundance ratios of mono-, double and triple- 13C isotopologues can give suggestions about fractionation mechanisms and the origin of the fractionated carbon.

The rotational spectrum of ethyl cyanide has been measured many times in the microwave, millimeter and sub-millimeter ranges (Lovas 1982; Pearson et al. 1994) and (Fukuyama et al. 1996; Brauer et al. 2009; Fortman et al. 2010a,b). The rotational spectra of deuterated, mono-13C, and -15N containing isotopologs of ethyl cyanide (Margulès et al. 2009, 2016; Richard 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. 2013, 2016) have also been investigated and successfully detected in the ISM.

Table 1

An example calculation of the offset value (the difference between calculated and observed rotational constants for 13CH3CH213CN).

Table 2

Determination of the rotational constants of 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN using the offset method which is based on the DFT calculations and previous experimental data for doubly 13C-substituted isotopic species of ethyl cyanide.

A recent laboratory spectroscopic investigation (Margulès et al. 2016) deals with the spectra and space identification of three double 13C 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 13C atoms.

The present work is motivated by the relatively high abundance of ethyl cyanide in star-forming regions such as OMC-1 and Sgr B2 where other isotopologs have been observed. Our microwave studies of ethyl cyanide were extended to further isotopic species of this molecule. Since all three double-13C ethyl cyanide isotopologs have already been detected in Sgr B2, we expect that 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN will also be observable. The characterization of isotopologs is also important from an astronomical point of view for determining isotope abundance ratios and the observations of the denser parts of the molecular clouds. Although many isotopic varieties containing 13C have been spectroscopically characterized to allow their astronomical detection (i.e., for methyl formate Carvajal et al. 2009), the 13C isotopologs of many other species cannot be identified in the ISM due to their lack of spectral recordings.

Several spectroscopic investigations were performed in support of this study, for example on iso-propyl cyanide (Müller et al. 2011), anti-ethanol with one 13C (Bouchez et al. 2012), 2-aminopropionitrile (Møllendal et al. 2012), and n-butyl cyanide (Ordu et al. 2012). In addition, a number of spectroscopic studies have been prompted by the results of these line surveys, for example, on methyleneaminoacetonitrile (Motiyenko et al. 2013), aminoacetonitrile (Motoki et al. 2013), and 1,2-propanediol (Bossa et al. 2014).

This paper is organized as follows. The 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN synthesis and spectrometer description are reported in Sect. 2. The assignment of measured transitions and obtained spectroscopic parameters are presented in Sect. 3 and conclusions are given in Sect. 4.

2. Experimental details

2.1. Synthesis

Triethylene glycol (20 mL), potassium cyanide-13C (K13CN, 0.71 g, 10.7 mmol), and iodoethane-13C2 (1 g, 6.4 mmol) were introduced into into a three-necked flask equipped with a stirring bar, a reflux condenser, and a nitrogen inlet. The mixture was heated to 110 °C and stirred at this temperature for one hour. After cooling to room temperature, the flask was fitted on a vacuum line equipped with two U-tubes. The high boiling compounds were condensed in the first trap cooled at −30 °C and propanenitrile-13C3 (13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN) was selectively condensed in the second trap cooled at −90 °C. Yield: 90% based on iodoethane. The nuclear magnetic resonance data (NMR) are given in Appendix A.

2.2. Measurements with Lille’s fast scan DDS spectrometer

The spectra of triple 13C ethyl cyanide were recorded in the ranges 150–330, 400–660, 780–990 GHz using a fully solid-state spectrometer. The spectrometer based on the direct digital synthesizers (DDS), described in detail by Alekseev et al. (2012), was used to measure millimeter and submillimeter wave spectra of the 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN which is thought to be present in detectable abundances in the ISM. The Lille spectrometer provides high-precision and fast-scan measurements. This system covers the 150–990 GHz range with small gaps and uses frequency multiplier chains from VDI Inc. to multiply the frequency of the radiation generated by an Agilent synthesizer operating around 12–20 GHz. A stainless absorption cell of approximately 2 m was used. The spectrometer was equipped with a 4.2 K He cooled QMC bolometer. The measurement accuracy is estimated at 30 kHz or 50 kHz depending on the frequency range.

3. Results

Ethyl cyanide and its isotopologs are asymmetric tops. Watson’s A-reduced Hamiltonian in the Ir representation has been used. The rotational spectra were predicted and fit employing the SPCAT and SPFIT programs. 13C isotopic substitution changes the reduced mass leading to a change in the moment of inertia and thus affecting the rotational spectrum. The 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN dipole moment was assumed to be unchanged upon isotopic substitution, and the values determined for ethyl cyanide given in (Kraśnicki & Kisiel 2011) of μa = 3.816 D and μb = 1.235 D were used.

3.1. Computational details: Theoretical prediction of the rotational spectra

Initially, 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN spectroscopic parameters (the ground-state rotational and the centrifugal distortion constants) have been theoretically evaluated with the Density functional theory (DFT) calculations at the B3LYP level of theory with a 6-311++g(3df, 2pd) basis set. The calculations were performed using Gaussian 09 suite of programs (Frisch et al. 2009). The theoretically optimized and experimental geometries (the bond angles, lengths and dihedrals), as well as the calculated and experimental frequencies are slightly different principally due to the use of finite basis sets, incomplete incorporation of electron correlation and the neglect of anharmonicity effects (Scott & Radom 1996). However, good agreement between the calculated and experimental frequencies can be obtained by applying scale factors.

The results of the performed DFT calculations may be sufficient to provide initial assignment of the recorded spectra. In this work, in order to obtain more accurate spectroscopic parameters for the prediction of the 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN rotational spectra, we used scaling factors based on the experimental data taken from the previous study of doubly 13C-substituted isotopic species of ethyl cyanide (Margulès et al. 2016). 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 13CH3CH132\hbox{$_2^{13}$}CN, 13CH133\hbox{$_3^{13}$}CH2CN and CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}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 C for 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN isotopologue we subtracted corresponding offset values of 13CH3CH132\hbox{$_2^{13}$}CN (Table 2). The rotational constants resulting from this computational technique are compared with the experimental values in Table 2. The agreement between offset rotational constants and experimental ones is excellent (experimental error for A, B, C is less than 0.05%). It is interesting to note that offsets from 13CH133\hbox{$_3^{13}$}CH2CN proved to give a more precise determination of rotational constants and the centrifugal distortion constants among other double 13C-substituted species (with an experimental error less than 0.001% for A, B, C). This approach provided significantly more accurate parameters and is simple and straightforward.

3.2. Assignment and analysis

Given that all three sets of rotational constants have similar accuracy, one may use any of them for initial spectral assignment. In our case we used the set of constants scaled from 13CH133\hbox{$_3^{13}$}CH2CN isotopic species. The scaled constants allowed a very accurate calculation of rotational transition frequencies. The lines involving low Ka and J ≤ 30 transitions were predicted to be better than 5 MHz. Thus, the offset method is very efficient for derivation of rotational constants of isotopologues when the parent species have been previously investigated.

By adding new identified transitions to the fit we cyclically improved the predictions. The analysis was strict and straightforward even given the fact that the triple 13C isotopologue shows a very dense and intense spectrum. Transitions with high Ka values sometimes were difficult to assign because of weakness or line blending. Over 4000 lines have been assigned and fitted with Watson’s type A-and S-reduced Hamiltonian in Ir representation (Watson 1977) to a set of spectroscopic parameters. Table 4 lists all of the experimental rotational constants determined in the A- and S-reduction, and these are compared to the values from the theoretical calculations and to the experimental values of the main isotopologue from (Brauer et al. 2009). Both A- and S-reduction analyses from the present work demonstrate good similar results, with the S-reduction giving a slightly better rms than A-reduction (0.686 vs. 0.699 and 28.2 vs. 28.9) but S-reduction allowed to determine two additional parameters (l4 and PJJK). While all spectroscopic parameters of the 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN have been firmly determined with small relative uncertainties, some parameters were just barely determined, most notably lJK and lKJ. However, the values of these parameters appear to be reasonable in comparison with the corresponding values of the parent isotopic species. Figure 1 demonstrates generally good agreement between the 13CH313CH213CN experimental spectra and the spectra predicted by the SPCAT program (the simulation is based upon the spectroscopic constants determined in this work). The complete list of measured rotational transitions of the ground state, presented in Table 3, is available at the CDS. Here, only a part of Table 3 is shown as an example.

thumbnail Fig. 1

Predicted (in blue) and observed (in black) rotational spectrum of 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN between 820 and 850 GHz. A slight inconsistency between predicted and observed spectrum, which may be visible for some strong lines, is due to source power and detector sensitivity variations.

Table 3

Assigned rotational transitions of the ground state of 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN.

Table 4

Spectroscopic parameters of triple 13C isotopologue 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN of ethyl cyanide compared to normal species CH3CH2CN and calculated values.

4. Conclusions

Ethyl cyanide is a particularly abundant molecule in hot-core sources. In this study we have characterized the rotational spectrum of triply substituted 13C ethyl cyanide for the first time. We measured and assigned about 4000 rotational lines of the 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN isotopologue in the frequency range from 150 to 990 GHz. The assigned lines involve rotational transitions with the rotational quantum numbers J up to 115 and Ka up to 39. The frequency range covered by direct measurements in this work corresponds to the range where the most intense lines may be observed under the characteristic temperature of 150 K in dense molecular clouds (see Fig. 2). Both A- and S-reduction Hamiltonians allow fitting all the assigned lines within experimental accuracy. The set of determined spectroscopic parameters should provide reliable predictions of the rotational transition frequencies of 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN up to 1.5 THz and for J and Ka quantum numbers up to 130 and 45 correspondingly.

Using the spectroscopic results of this work we searched for 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN in the EMoCA survey (Margulès et al. 2016) toward Sgr B2(N). We were not able to identify any isolated rotational transition of 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN above the detection limit in this survey. The ALMA data from the EMoCA survey suggest a column density ratio of each of the three doubly C isotopologs to the triply substituted one of greater than eight (A. Belloche, priv. comm.). Given that we expect a 12C/13C ratio of ~25, the current sensitivity is too small for the detection of the triply substituted isotopolog by at least a factor of three. Nevertheless, the present laboratory data permit to search for triply C-substituted ethyl cyanide throughout the entire frequency region of ALMA.

thumbnail Fig. 2

Simulated spectrum of 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN at 150 K in the frequency range up to 1 THz.

Acknowledgments

This work was funded by the French ANR under the Contract No. ANR-13-BS05-0008-02 IMOLABS. This work was also supported by the French program “Physique et Chimie du Milieu Interstellaire” (PCMI) funded by the Conseil National de la Recherche Scientifique (CNRS) and Centre National d’Etudes Spatiales (CNES).

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Appendix A: Propanenitrile-13C3

1H NMR (CDCl3, 400 MHz) δ 1.29 (dddt, 1JCH = 126.1 Hz, 3JCH = 6.6 Hz, 2JCH = 4.6 Hz, 3JHH = 7.7 Hz, CH3), 2.35 (tdq, 2H, 1JCH = 135.0 Hz, 2JCH = 9.7 Hz, 3JHH = 7.7 Hz, CH2). 13C NMR (CDCl3, 100 MHz) δ 10.5 (ddq, 1JCC = 33.5 Hz, 2JCC = 4.4 Hz, 1JCH = 126.1 Hz, CH3), 11.1 (ddt, 1JCC = 48.7 Hz, 1JCC = 33.5 Hz, 1JCH = 135.0 Hz, CH2); 120.8 (dd, 1JCC = 48.7 Hz, 2JCC = 4.4 Hz, CN).

All Tables

Table 1

An example calculation of the offset value (the difference between calculated and observed rotational constants for 13CH3CH213CN).

In the text
Table 2

Determination of the rotational constants of 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN using the offset method which is based on the DFT calculations and previous experimental data for doubly 13C-substituted isotopic species of ethyl cyanide.

In the text
Table 3

Assigned rotational transitions of the ground state of 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN.

In the text
Table 4

Spectroscopic parameters of triple 13C isotopologue 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN of ethyl cyanide compared to normal species CH3CH2CN and calculated values.

In the text

All Figures

thumbnail Fig. 1

Predicted (in blue) and observed (in black) rotational spectrum of 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN between 820 and 850 GHz. A slight inconsistency between predicted and observed spectrum, which may be visible for some strong lines, is due to source power and detector sensitivity variations.
In the text
thumbnail Fig. 2

Simulated spectrum of 13CH133\hbox{$_3^{13}$}CH132\hbox{$_2^{13}$}CN at 150 K in the frequency range up to 1 THz.
In the text

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