Laboratory spectroscopic study and astronomical detection of vibrationally excitedn-propyl cyanide

Abstract

Context. We 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. We determined line intensities of n-propyl cyanide in the ground vibrational states of its gauche and anti conformers toward the hot molecular core Sagittarius B2(N2) which suggest that we should also be able to detect transitions pertaining to excited vibrational states. Aims. We wanted to determine spectroscopic parameters of low-lying vibrational states of both conformers of n-propyl cyanide to search for them in our ALMA data. Methods. We recorded laboratory rotational spectra of n-propyl cyanide in two spectral windows between 36 and 127 GHz. We searched for emission lines produced by these states in the ALMA spectrum of Sagittarius B2(N2). We modeled their emission and the emission of the ground vibrational states assuming local thermodynamic equilibrium (LTE). Results. We have made extensive assignments of a-and b-type transitions of the four lowest vibrational states of the gauche conformer which reach J and K a quantum numbers of 65 and 20, respectively. We assigned mostly a-type transitions for the anti conformer with J and K a quantum numbers up to 48 and 24, respectively. Rotational and Fermi perturbations between two anti states allowed us to determine their energy difference. The resulting spectroscopic parameters enabled us to identify transitions of all four vibrational states of each conformer in our ALMA data. The emission features of all states, including the ground vibrational state, are wellreproduced with the same LTE modeling parameters, which gives us confidence in the reliability of the identifications, even for the states with only one clearly detected line. Conclusions. Emission features pertaining to the highest excited vibrational states of n-propyl cyanide reported in this work have been identified just barely in our present ALMA data. Features of even higher excited vibrational states may become observable in future, more sensitive ALMA spectra to the extent that the confusion limit will not have been reached. The 13 C isotopomers of this molecule are expected to be near the noise floor of our present ALMA data. We estimate that transitions of vibrationally excited iso-propyl cyanide or aminoacetonitrile, for example, are near the noise floor of our current data as well.

Rotational rest frequencies of the low lying vibrational states of n-propyl cyanide from extensive laboratory measurements up to 506 GHz

“…The previous authors determined from intensity measurements that the anti conformer is lower in energy than the gauche by 1.1 ± 0.3 kJ mol −1 . Durig et al (2001) used infrared spectroscopy of n-PrCN dissolved in liquid Xenon to determine that the gauche conformer is lower than the anti by 0.48 ± 0.04 kJ mol −1 (or 58 ± 4 K) and Müller et al (2016) found this value to be fully consistent with their model of the ALMA spectra. Hirota (1962) also determined the rotational constants of the three lowest fundamental vibrational states of the anti and gauche conformers.…”

“…2. The experimental arrangement for measurements between 36−70 GHz and between 89.25−126.75 GHz have been described previously (Müller et al 2016). In the two new higher frequency ranges (171−251 and 310−506 GHz) we used a 5.1 m long double path (10.2 m total) absorption cell with inner diameter of 100 mm and equipped with Teflon windows.…”

“…This survey, between 84.1 and 114.4 GHz was taken with ALMA toward Sgr B2(N). Müller et al (2016), using the same survey, reported the identification of four vibrationally excited states for both anti-and gauche-n-PrCN following new spectroscopic work summarized in the last paragraph.…”

“…Firstly, the frequencies of these lines need to be known so as to make a complete spectroscopic model of an object, identify all lines due to known molecules and hence be able to focus on remaining lines as candidates for new unidentified species (see for example, Mehringer et al 2004;Daly et al 2013). Secondly, work on the vibrational states can be used to take the latter into account in the partition function and hence to better estimate the column density of the molecule observed (for example Müller et al 2016). Thirdly, lines of molecules in vibrationally excited states can be used to focus on hotter, or shocked regions of an object such as hot cores in star-forming regions and to model the physical and chemical properties of these regions (for example Goldsmith et al 1983;Ziurys & Turner 1986;Mehringer et al 2004).…”

“…As can be seen from the equivalent temperatures in Table 1, these vibrational states are predicted to be substantially populated in hot core regions of star-formation where temperatures can rise to around 100−300 K. For comparison the lowest vibrational states are around 525 K for methyl cyanide, 302 K for ethyl cyanide, as detailed above, and 266 K for i-PrCN. Recently, the laboratory spectroscopic study up to 127 GHz of these vibrationally excited states (Müller et al 2016) led to their detection in space as explained above. Following this identification we decided to carry out a more extensive analysis up to 506 GHz of these vibrational states with the aim of providing reliable predictions over the whole operating band of ALMA.…”