A&A 601, A50 (2017) Astronomy DOI: 10.1051/0004-6361/201628551 & c ESO 2017 Astrophysics Submillimeter spectra of 2-hydroxyacetonitrile (glycolonitrile; HOCH2CN) and its searches in GBT PRIMOS observations of Sgr B2(N)? L. Margulès1, B. A. McGuire2, M. L. Senent3, R. A. Motiyenko1, A. Remijan2, and J. C. Guillemin4 1 Laboratoire de Physique des Lasers, Atomes, et Molécules, UMR CNRS 8523, Université de Lille I, 59655 Villeneuve d’Ascq Cedex, France e-mail: [email protected] 2 National Radio Astronomy Observatory, Charlottesville, VA 22903, USA 3 Departamento de Química y Física Teóricas, Instituto de Estructura de la Materia, IEM-C.S.I.C., Serrano 121, 28006 Madrid, Spain 4 Institut des Sciences Chimiques de Rennes, École Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, 11 allée de Beaulieu, CS 50837, 35708 Rennes Cedex 7, France Received 18 March 2016 / Accepted 10 October 2016 ABSTRACT Context. Recent experimental works have studied the possible formation of hydroxyacetonitrile on astrophysical grains. It was formed from hydrogen cyanide (HCN) and formaldehyde (H2CO) in the presence of water under interstellar medium conditions. Because these precursor molecules are abundant, hydroxyacetonitrile is an excellent target for interstellar detection. Aims. Previous studies of the rotational spectra were limited to 40 GHz, resulting in an inaccurate line list when predicted up to the millimeter-wave range. We measured and analyzed its spectra up to 600 GHz to enable is searches using cutting-edge millimeter and submillimeter observatories. Methods. The molecule 2-hydroxyacetonitrile exhibits large amplitude motion that is due to the torsion of the hydroxyl group. The analysis of the spectra was made using the RAS formalism available in the SPFIT program with Watson’s S-reduction Hamiltonians. Results. The submillimeter spectra of hydroxyacetonitrile, an astrophysically interesting molecule, were analyzed. More than 5000 lines were fitted with quantum number values reaching 75 and 25 for J and Ka, respectively. An accurate line list and partition function were provided. A search for hydroxyacetonitrile in publicly available GBT PRIMOS project, IRAM 30 m, and Herschel HEXOS observations of the Sgr B2(N) high-mass star-forming region resulted in a non-detection; upper limits to the column density were determined. Key words. ISM: molecules – methods: laboratory: molecular – submillimeter: ISM – molecular data – line: identification 1. Introduction models, to understand formation processes and observed inter- More than 30 nitriles (organic cyanides) have been observed stellar abundances (or upper limits). to date in the interstellar medium (ISM). Among these are the The simplest cyanohydrine, 2-hydroxyacetonitrile (gly- well-known family of cyanopolyynes, and isopropyl cyanide, the colonitrile; HOCH CN), is readily formed in the lab by the ad- first branched hydrocarbon detected in the ISM (Belloche et al. 2 dition of hydrogen cyanide (HCN) to formaldehyde (H2CO) in 2014). This large dataset is useful in constraining models of ni- water (Gaudry 1955). It is thought to play an important role in trile formation in the ISM, and in predicting which other nitrile prebiotic chemistry, and could lead to the formation of a number species might be present and detectable. However, many simple of simpler organic molecules (Arrhenius et al. 1994). The for- nitriles have yet to be detected. These non-detections could be mation of 2-hydroxyacetonitrile in astrophysical ices was first due to a weak or null dipolar moment (for example cyanogen studied from a theoretical point of view (Woon 2001). Recently, NCCN or 2-butyne-1,4-dinitrile NC–C≡C–CN) or to the lack its formation and photolytic decomposition on interstellar grains of recorded microwave spectra caused by unknown or difficult have been experimentally evaluated. These studies show that un- syntheses of the target products. With the advent of state-of- der these conditions, HCN reacts with H CO in the presence of the-art millimeter and submillimeter facilities like the Strato- 2 H2O to form HOCH2CN (Danger et al. 2012, 2014). Although spheric Observatory for Infrared Astronomy (SOFIA) and the not studied, the hydrogen-atom bombardment of formyl cyanide Atacama Large Millimeter/submillimeter Array (ALMA), labo- (HC(=O)CN) could also lead to the formation of HOCH CN, ratory studies of rotational spectra in this frequency range are 2 as could do the reaction of CN or OH with H2COH or CH2CN, critical for advancing the field. Such work will have to be fol- respectively. Photochemistry of pure HOCH CN leads to the for- lowed by attempts, both in the laboratory and through theoretical 2 mation of HC(=O)CN, HCN, ketenimine (H2C=C=NH), H2CO, ? Hydroxyacetonitrile fit is only available at the CDS via anonymous CO, and CO2 (Danger et al. 2013). It should be noted that most ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via of the compounds cited as possible precursors or as photoprod- http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/601/A50 ucts have been detected in the ISM. Article published by EDP Sciences A50, page 1 of6 A&A 601, A50 (2017) Table 1. Calculated ground-state rotational constants (MHz), energies The gas-phase, infrared spectrum of HOCH2CN has been −1 recently reinvestigated (Chrostowska et al. 2015), while the mi- and conformational barriers (cm ), and dipole moments (Debye) for HOCH2CN. crowave spectrum of HOCH2CN has not been treated since it was first observed by Cazzoli et al.(1973) from 8–40 GHz. The gauche rotamer has been observed. Doublets of the rotational Parameter gauche−HOCH2CN trans−HOCH2CN transitions were observed arising from tunneling between the A0 33 621.46 35 519.53 two equivalent gauche rotamers. The rotational constants indi- B0 4828.88 4833.28 cate that the angle between the HOC and OCC planes at the C0 4370.77 4369.98 potential minima is approximately 123◦ from the trans posi- ∆E 506.3 tion, while the rotational constants calculated from a molecu- V(gauche-gauche) 425.8 lar model, and the potential functions, indicate that this angle is V(gauche-trans) 645.4 likely closer to 116◦. New spectra were measured in Lille in the range 150– µ 2.94 600 GHz, and combined with high-level ab initio calculations µa 2.32 µb 1.31 for an accurate analysis. A search of the publicly available GBT µ 1.23 PRIMOS project, IRAM 30 m, and Herschel HEXOS observa- c tions of Sgr B2(N) for the molecule found no evidence for a cold population of the species, and upper limits were established. The ground vibrational state rotational constants of the two conformers were determined from the equilibrium parameters using Eq. (1) (cf. Senent et al. 2015) 2. Experiments core 2.1. Synthesis B0 = Be + ∆Be + ∆Bvib; (1) HOCH2CN has been prepared as previously reported (Gaudry where ∆Bcore represents the core-valence correlation correction 1947, 1955). e and ∆Bvib is the vibrational contribution to the rotational con- stants derived from the vibration-rotation interaction parameters. 2.2. Lille – submillimeter spectra The results are shown in Table1. The low torsional energy levels were calculated using a vari- The measurements from 50–660 GHz were performed using the ational solution to the Hamiltonian given in Eq. (2)(Senent Lille spectrometer (Zakharenko et al. 2015). A quasi-optic di- 1998a,b): electric hollow waveguide, 3 m in length, containing HOCH2CN ! ! was used as the sample cell in the spectrometer. The measure- @ @ Hˆ (α) = − B + Veff(α): (2) ments were typically performed at pressures of 10 Pa and at ∂α α ∂α room temperature. The frequency ranges of 50–315 and 400– 550 GHz were covered with various active and passive frequency Here, α is the OH torsional coordinate, Bα are the kinetic energy multipliers where an Agilent synthesizer (12.5–18.25 GHz) was parameters, and Veff(α) is the vibrationally corrected 1D-PES used as the source of radiation. Estimated uncertainties for mea- (Császár et al. 2004) shown in Fig.1. sured line frequencies are 30 kHz and 50 kHz depending on the The ground vibrational state splits into two components observed signal-to-noise ratio and the frequency range. E(0−) and E(0+) because of tunneling in the V(gauche- gauche) barrier. Ab initio calculations predict a splitting of 96 563.148 MHz (3.2210 cm−1). In addition, the first excited vi- 3. Theoretical calculations brational state shows two components lying at 223.05 cm−1 (1+) High-level ab initio calculations were used to determine the ge- and 261.27 cm−1 (1−) over the ground state. The first trans level ometries of the two conformers, gauche and trans, and to com- lies at 459 cm−1 (0). pute low-energy torsional levels. Electronic structure calcula- tions were performed using both the MOLPRO (Werner et al. 4. Analysis of the spectra 2012) and GAUSSIAN packages (Frisch et al. 2009). The tor- sional energy levels were determined by variational calculations We have analysed the spectra of the gauche conformer, which using the procedure previously employed for other non-rigid is the more stable of the two. This analysis was not trivial be- species (Senent 2004). cause the gauche rotamer exhibits one large amplitude motion The equilibrium structures, gauche and trans, of HOCH2CN, (LAM) with a symmetric two minima potential due to the two as well as the most relevant spectroscopic parameters (equi- equivalent gauche rotamers. As a result of tunneling through librium rotational constants and the one-dimensional potential the barrier to OH-group torsion, the ground state is split into energy surface, 1D-PES, for the hydroxyl torsion) were com- two substates: 0− and 0+. This rotamer is very close to a sym- puted using explicitly correlated coupled cluster theory with metric prolate rotor with an asymmetry parameter κ = −0:968.