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Issue
A&A
Volume 544, August 2012
Article Number A19
Number of page(s) 6
Section Atomic, molecular, and nuclear data
DOI https://doi.org/10.1051/0004-6361/201219674
Published online 19 July 2012

© ESO, 2012

1. Introduction

The simplest molecule containing a carbon–nitrogen double bond is methanimine (CH2NH), which is also called methylenimine or formaldimine. It was detected for the first time in the molecular cloud Sgr B2 (Godfrey et al. 1973) towards the Galactic centre, although it is present there in low abundance (Turner 1991). CH2NH is a reactive species, therefore unstable in the terrestrial environment, which may be considered as a pre-biotic interstellar molecule. Danger et al. (2011) prove that, by warming ice analogues in astrophysical-like conditions, methanimine participates in the Strecker synthesis to form aminoacetonitrile (NH2CH2CN; recently detected in Sgr B2(N) by Belloche et al. 2008), which is a possible precursor of glycine, the simplest amino acid.

In reality, CH2NH has been found in several “hot cores” associated with massive star-forming regions (Dickens et al. 1997; White et al. 2003; Qin et al. 2010), where it is more abundant than in dark clouds because of its release to the gas phase from the heated grain mantles (Dickens et al. 1997). However, wether this partially saturated species is formed in the gas phase or on the dust grains has not been clearly established, as outlined by Turner et al. (1999) in their comparison of the observations of CH2NH in one translucent molecular cloud with those in the L183 pre-stellar core. As far as circumstellar envelopes are concerned, discussing their recent detection of methanimine in the carbon-rich IRC+10216, Tenenbaum et al. (2010) assume a gas-phase formation, by the association of CH with NH3, because CH2NH is present in the outer envelope. In addition, the molecule was not detected by Schilke et al. (2001) in their submillimetre survey of the Orion KL hot core (IRc2); this confirmed previous findings (Dickens et al. 1997), suggesting that the CH2NH emission observed towards Orion KL is associated with the “compact ridge”, where lower temperature and intermediate density favour radiative association.

It is noteworthy that methanimine is one of the more than 50 molecules identified in extragalactic environments: it has been detected in the ultraluminous infrared galaxy Arp 220 (Salter et al. 2008), and tentatively in the starburst galaxy NGC 253 (Martín et al. 2006). Furthermore, the rotational 4-mm-rest-frame absorption line of CH2NH was identified in the spectral line survey of a high-z molecular absorber, located at z = 0.89 in front of the quasar PKS 1830-211 (Muller et al. 2011). Finally, there is the hypothesis that CH2NH is present in the atmosphere of Titan, where it might have been formed either from NH and CH3 (Redondoa et al. 2006), by ion-chemistry (Vuitton et al. 2007), or via the N(2D) + CH4 reaction (Balucani et al. 2009).

Methanimine is a light molecule and its rotational spectrum shows increasing intensity and spectral density with frequency at submillimetre wavelengths. At the typical kinetic temperatures of the molecular gas in high-mass star-forming regions (50–200 K, see e.g., Dickens et al. 1997), many strong transitions of CH2NH fall at frequencies higher than 400 GHz. Chemically rich regions will soon be the target of ALMA observations in this wavelength regime (bands 8 and 9, which will both be available in the forthcoming cycle 1), and are already being extensively surveyed by Herschel/HIFI (e.g., the HEXOS key programme, Bergin et al. 2010). With their improved sensitivity, high resolution, and the wideband coverage, these telescopes are expected to deliver a wealth of spectral data, hence the availability of submillimetre rest-frequencies with small uncertainties is crucial to unravel the chemical complexity and infer, from the observational data, the source physical parameters.

The laboratory rotational spectrum of methanimine was first recorded in the gas phase by Johnson & Lovas (1972) up to 123 GHz. In addition to the main species, several isotopologues (CNH, H2C15NH, H2CND, D2CNH, and D2CND) were later detected by microwave spectroscopy (Pearson & Lovas 1977) in order to determine the molecular structure of CH2NH. Recently, several transitions falling in the range of 64–172 GHz were recorded in the laboratory study of Dore et al. (2010): the main focus was to study the magnetic hyperfine structure due to the interactions of 14N and the three 1H nuclear spins, which could be resolved by means of the Lamb-dip spectroscopy (Costain 1969; Winton & Gordy 1970).

The present laboratory investigation was undertaken to extend rotational frequency measurements of CH2NH to submillimetre wavelengths, and makes it possible to build a list of very accurate rest-frequencies for astrophysical purposes in the THz region with 1σ uncertainties smaller than 0.01 km s-1 in radial equivalent velocity.

2. Experimental

Methanimine was produced by pyrolysis with the same apparatus employed in this laboratory to produce other molecules of astrophysical interest, such as HC5N (Yamada et al. 2004), HC7N (Bizzocchi & Degli Esposti 2004), and C3O (Bizzocchi et al. 2008). The precursor used was ethylenediamine (NH2CH2CH2NH2; Hamada et al. 1984), which was heated to 1150 °C while flowing through a quartz tube (50 cm long and 1 cm in diameter) inserted in a 30 cm long cylindrical furnace. The quartz tube was connected to the gas inlet of the double-pass absorption cell, a glass tube 3 m long and 10 cm in diameter equipped with a wire grid polarizer and a roof mirror (Dore et al. 1999), through which the pyrolysis products were continuously pumped. The very effective production and the strong signals of methanimine allowed us to use a low flow in the reactor in order to record the spectra with a pressure in the cell no higher than 10 mTorr (=1.33 Pa).

Measurements were carried out in the frequency range of 329–629 GHz, by employing phase-locked Gunn oscillators (Radiometer Physics GmbH, J. E. Carlstrom Co) as primary radiation source working in the range of 75–115 GHz, and power at higher frequencies was obtained using harmonic multiplication. Two phase-lock loops allowed the stabilization of the Gunn oscillator with respect to a frequency synthesizer, which was driven by a 5-MHz rubidium frequency standard. The frequency modulation of the radiation was obtained by a sine-wave (at 6 kHz or 1.666 kHz for Lamb-dip measurements) modulating the reference signal of the wide-band Gunn synchronizer; the signal, detected by a liquid-helium-cooled InSb hot electron bolometer (QMC Instr. Ltd. type QFI/2), was demodulated at 2-f by a lock-in amplifier.

3. Analysis

CH2NH is a near prolate (κ = −0.937) planar asymmetric rotor, where all the nuclei lie in the ab plane and the C–N bond is almost aligned with the a principal axis, thus the dipole moment has components along the a and b axes1, and a-type and b-type transitions occur in its rotational spectrum. The newly observed transitions are listed in Table 1: there are R (ΔJ =  +1) and Q (ΔJ = 0) a-type lines and R, Q, and P (ΔJ = −1) b-type lines. Some of them are partially split by the electric quadrupole coupling of the 14N nucleus: the three ΔF = ΔJ hyperfine components either overlap or one is more or less partially resolved from the other two. For transitions appearing as a main blended peak, possibly with a shoulder, the unperturbed line frequency was recovered by a line shape analysis (Dore 2003) of the spectral profile, which is modelled as a sum of hyperfine components with their frequency shift and intensity fixed at the values accurately predicted using the hyperfine constants known from our previous work (Dore et al. 2010). In addition, for the transitions appearing as a doublet, the two frequencies were obtained by a line shape analysis, as is shown in Fig. 1.

thumbnail Fig. 1

Hyperfine doublet of the J = 92,8 ← 91,9 rotational transition of CH2NH; total integration time 310 s at 194 kHz/s with time constant of 10 ms. The spectral profile has been fit to a sum of three hyperfine components; fit residuals are shown at the bottom of the plot.

Table 2

Spectroscopic constants of CH2NH.

Table 3

Calculated hyperfine frequenciesa of CH2NH in the ALMA band 10 (780−970 GHz).

The measured line frequencies fall in the submillimetre-wave region and allow us to extend the centrifugal analysis carried out in Dore et al. (2010). Present and previous hyperfine data2 were fitted together, employing Pickett’s SPFIT fitting program (Pickett 1991), to an asymmetric rotor Hamiltonian using Watsons S-reduced Ir representation (Watson 1977). The rotational constants, as well as the 14N hyperfine constants, were refined, and the complete sets of quartic and sextic distortion constants plus two octic ones were determined; all of them are reported in Table 2, where we compare with previous determinations by millimetre-wave (Dore et al. 2010) and IR (Halonen & Duxbury 1985) spectroscopy.

4. Discussion

This paper extends the laboratory study of the rotational spectrum of methanimine into the submillimetre-wave region (329−629 GHz). Several different types of transitions were recorded: for the a-type spectrum, the complete K-structure of the J = 9 ← 8 transition, the three lines J = 100,10 ← 90,9, 101,10 ← 91,9, and 110,11 ← 100,10, and three transitions (J = 24, 25, and 27) of the Q band with Ka = 3; the b-type spectrum is very sparse, and very informative, therefore ΔJ = 0, ± 1 transitions were observed, sampling states with the rotational quantum number J ranging from 1 to 36, and its projection Ka from 0 to 9.

These measured rotational frequencies, added to the variety of transitions previously measured in the millimetre-wave region, allowed us to determine rotational and centrifugal distortion constants with an accuracy higher than previously obtained, by almost an order of magnitude for the quartic and sextic distortion constants; in addition, two octic constants were determined. Transition frequencies predicted using these high-precision spectroscopic constants have uncertainties on average one order of magnitude smaller than predictions obtained with previous constants.

As an example of the small uncertainty attainable in the prediction of rest-frequencies, Table 3 reports a portion of the catalogue file obtained by running Pickett’s SPCAT program  (Pickett 1991) (with correlations between the spectroscopic constants considered), where predictions along with their 1σ uncertainties are listed for the 30 strongest transitions at 50 K in the ALMA band 10 (780−970 GHz). The frequency precision is of the order of a few parts in 109 (1 part in 109 corresponds to to 0.0003 km s-1 in radial velocity), and the lowest uncertainties are for transition frequencies with a relatively small centrifugal-distortion contribution. It is obvious that the predicted uncertainties are model-dependent (in addition to the predicted transition frequencies), in the sense that additional centrifugal terms, indeterminable with present data, can cause these to increase. However, the spectroscopic constants presently determined from measurements up to 629 GHz should allow a fairly accurate prediction of the rotational spectrum of methanimine at higher frequency, particularly for b-type transitions with small centrifugal contributions. Figure 2 shows the rotational spectrum up to 1 THz computed at 50 K, where it is apparent that at this temperature the strongest transitions of CH2NH lie in the ALMA bands 9 and 10, and the Herschel/HIFI bands 1–3.

thumbnail Fig. 2

Simulation of the rotational spectrum of CH2NH. Intensities are computed at 50 K.

1

μa = 1.3396 D and μb = 1.4461 D (Allegrini et al. 1979).

2

The hf components of the 51,4 ← 51,5 transition, reported by Johnson & Lovas (1972), were measured again by Lamb-dip spectroscopy and two of them were split by magnetic interactions due to the three protons. Frequencies in MHz for the F'N ← FN components are: 4 ← 4: 79   280.1688, 79   280.1970; 6 ← 6: 79   280.6682; and 5 ← 5: 79   282.7901, 79   282.8120.

Acknowledgments

Financial support from MIUR (PRIN 2009 funds, project “Spettroscopia molecolare per la Ricerca Atmosferica e Astrochimica: Esperimento, Teoria ed Applicazioni”) and from the University of Bologna (RFO funds) is gratefully acknowledged. L.B. gratefully acknowledges support from the Science and Technology Foundation (FCT, Portugal) through the Fellowship SFRH/BPD/62966/2009.

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All Tables

Table 2

Spectroscopic constants of CH2NH.

In the text
Table 3

Calculated hyperfine frequenciesa of CH2NH in the ALMA band 10 (780−970 GHz).

In the text

All Figures

thumbnail Fig. 1

Hyperfine doublet of the J = 92,8 ← 91,9 rotational transition of CH2NH; total integration time 310 s at 194 kHz/s with time constant of 10 ms. The spectral profile has been fit to a sum of three hyperfine components; fit residuals are shown at the bottom of the plot.

In the text
thumbnail Fig. 2

Simulation of the rotational spectrum of CH2NH. Intensities are computed at 50 K.
In the text

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