EDP Sciences
Free Access
Issue
A&A
Volume 600, April 2017
Article Number A44
Number of page(s) 5
Section Atomic, molecular, and nuclear data
DOI https://doi.org/10.1051/0004-6361/201629880
Published online 28 March 2017

© ESO, 2017

1. Introduction

The search for prebiotic molecules in interstellar space has been attractive to many researchers for a long time. Among the building blocks of the constituent materials of life, the first report of a trial for the detection of glycine in the interstellar medium (ISM) with a radio telescope was published by Brown et al. (1979). For more than three decades since then, the detection of glycine, the simplest amino acid, has been unsuccessful (Hollis et al. 2003a, 2003b; Kuan et al. 2003; Snyder et al. 2005; Cunningham et al. 2007; Jones et al. 2007; Belloche et al. 2008, 2011); however, 16 predicted that ALMA would be able to detect gas-phase glycine released from dust grains at kinetic temperatures above 100 K. To avoid difficulties in detecting complex molecules toward hot cores, due to the high density of spectral lines, 26 claimed that photo-desorbed glycine may instead be detectable in low-mass prestellar cores such as L1544.

Various kinds of formation mechanisms of glycine in the ISM have been proposed. A Strecker-type reaction (Strecker 1850) may be one of the potential mechanisms. In this reaction scheme, aminoacetonitrile (NH2CH2CN) and methanimine (CH2NH) are the key intermediate molecules, and both molecules have already been detected mainly by spectral observation in the millimeter-wave region (e.g., Belloche et al. 2008, 2011; Godfrey et al. 1973; Qin et al. 2010; Turner et al. 1999; Tenenbaum et al. 2010; Martin et al. 2006; Suzuki et al. 2016). We have extended spectroscopic information of these species to the terahertz region to make it possible to carry out future searches in higher frequency regions, since the spectral line intensities would reach a maximum in typical interstellar condition (Dore et al. 2012; Motoki et al. 2013, 2014). The column densities of these precursor molecules were reported to be on the order of 1014−1016 molecules cm-2 (Belloche et al. 2008, 2011; Godfrey et al. 1973), whereas the upper limit of glycine has so far been estimated to be 1.4 × 1017 cm-2 toward Sgr B2 LMH (Hollis et al. 2003b) and 8−121 × 1014 cm-2 toward OMC-1 (Hollis et al. 2003a). Rigorous evaluations of the consistency between the observational results and chemical models are needed, which may lead to a call for other formation mechanisms.

Another possible glycine precursor, hydantoin (imidazolidine-2, 4-dione, C3H4N2O2), came to our attention since this molecule was detected in carbonaceous chondrites together with several kinds of amino acids by gas chromatography combined with mass spectrometry (Shimoyama & Ogasawara 2002). Hydantoin, whose structure is shown in Fig. 1, is a five-membered heterocyclic compound; it is known to arise from prebiotic molecules such as glycolic acid and urea (e.g., M.-Salván & M.-Yaseli 2012), and to give the simplest amino acid glycine by hydrolysis under acidic condition (e.g., Marcellus et al. 2011).

From the molecular structural point of view, hydantoin itself is not a chiral molecule since one of the carbon atoms constituting the five-membered ring (C5 position) is bonded with two hydrogen atoms. The asymmetric substitution of either of the hydrogen atoms to the corresponding carbon atom (the so-called 5-substituted hydantoins) introduces enantiomerism.

Spectroscopic information on hydantoin is fairly limited. The molecular structure of solid hydantoin was derived by X-ray crystallography (Yu et al. 2004). Infrared and Raman spectroscopy in the solid phase were conducted to observe vibrational spectra for normal (Lebedev et al. 1969; Saito & Machida 1978; Christiani et al. 1985; Sohar 1968; Ildiz et al. 2012) and for N- and C-deuterated isotopologues (Saito & Machida 1978). Low-lying vibrational modes were identified by Saito & Machida (1978). Recently, infrared spectra were recorded for matrix isolated hydantoin by Ildiz et al. (2013), but these low-lying vibrational states were not detected, even though theoretical calculations suggested their existence. Other computational studies were reported by using molecular mechanics and density functional methods to calculate molecular geometry, dipole moment, vibrational frequencies, etc. (e.g., Ildiz et al. 2012; Belaidi et al. 2015). To the best of our knowledge, no gas phase data have been obtained by high-resolution spectroscopy. Lack of millimeter-wave transition information prevents us from searching for hydantoin in the ISM.

In this paper we report pure rotational spectra of hydantoin observed in the millimeter-wave region for the first time. Molecular constants for the ground vibrational state and for the two low-lying vibrational excited states are provided. Spectral line catalogues for these three states provide accurate rest frequencies up to at least 300 GHz, where the line intensity is at their maximum at 100 K. The estimated frequency accuracy is less than <0.1 MHz, corresponding to a velocity resolution of <0.1 km s-1 at 300 GHz.

2. Experiment

Commercially available hydantoin (Sigma Aldrich, 98% purity) was used without further purification. The sample powder of approximately 20 grams was set inside a glass absorption cell (100 cm in length and 6 cm in diameter), which was then heated to 150 °C. Hydantoin (melting point 220 °C) does not decompose at this temperature. The recorded spectra in the present study were observed when we heated the compound above 100 °C, and were reproducible after repeated heating. The pressure of the cell was kept at around 1 Pa.

All spectral lines were measured with the 23 kHz-source frequency modulated spectrometer at Toho University, Japan. The details of our spectrometer have been described extensively in our previous study (Motoki et al. 2013). Millimeter-wave radiation in the frequency range of 90370 GHz was provided with the state-of-the-art frequency multiplier chains (VDI Inc.) driven by a microwave synthesizer (Agilent 8257D). The InSb photo conductive detector that operates at 4 K (QFI-3BI, QMC) were used for detection. The 40 dB preamplified signal is detected at twice the modulation frequency, giving a second-derivative line shape. An example of the measured spectrum is shown in Fig. 1; it is clear that the signal-to-noise ratio of the spectra of hydantoin is sufficient. The frequency measurement error in this frequency region is approximately 30 kHz.

thumbnail Fig. 1

Observed and fitted spectra of hydantoin (solid and dashed lines, respectively) at 93 GHz (NKaKc = 270 27 ← 261 26,NKaKc = 271 27 ← 260 26,). The spectrum was recorded by integrating 400 scans at a 0.5 Hz repetition rate and a 1 ms time constant of the lock-in amplifier.

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The molecular structure of hydantoin has been calculated by several computational studies (e.g., Belaidi et al. 2015) which are in close agreement with the structure obtained by X-ray crystallography (Yu et al. 2004). We also conducted the DFT calculations of hydantoin with the B3LYP/aug-cc-pVQZ basis set (Gaussian 09 et al. 2016) to obtain the dipole moment component along each molecular axis. Rotational A, B, and C constants of the molecule were obtained of 6567, 2291, and 1717 GHz, respectively. The permanent dipole moment lies almost along the molecular b-axis (μ(b) = 2.655 D and μ(a) = 0.169 D). Hence a series of b-type R-branch transitions were expected to be predominantly observed in the millimeter-wave region.

Table 1

Selected observed transition frequencies of hydantoin in its ground and excited vibrational states around 130 GHz.

3. Spectral analysis

The search for spectral lines in the frequency range between 138 and 149 GHz showed that a bunch of intense lines appeared repeatedly with a frequency spacing of approximately 3340 MHz. The number of the observed intense lines in an individual bunch is larger than expected when we attribute them to the spectral lines in the ground vibrational state. As mentioned in the Introduction, data obtained using infrared and Raman spectroscopy (Saito & Machida 1978), and via several theoretical calculations (Ildiz et al. 2012, 2013; Belaidi et al. 2015), suggest that there are at least two low-lying vibrational excited states (below 300 K, or 200 cm-1) for hydantoin. Initial assignments were made possible by assuming that the observed intense lines are due to the b-type R-branch transition in the ground state and in two excited vibrtaional states. The observed data were analyzed by using the SPFIT/SPCAT suite of programs (Pickett 1991). The rotational constants and the fourth order centifugal distortion constants were successfully obtained by using the conventional Watson S-reduced Hamiltonian for an asymmetric top molecule. Some of the observed and calculated frequencies are given in Table 1. The number of measured lines in the present study and the fitted lines, as well as the determined molecular constants for the ground state and the two excited vibrational states, are summarized in Table 2. The standard deviation of the fit was 39 kHz, which is of the same order as our experimental frequency accuracy.

Table 2

Molecular constants of hydantoin in its vibrational ground state and two excited states.

4. Results and discussion

We measured pure rotational spectra of hydantoin in its vibrational ground state and in two excited states up to 370 GHz. Fundamental frequencies of the two low-lying vibrational modes in the gas phase have not been directly observed yet. Ildiz et al. (2012, 2013) presented theoretically predicted infrared spectra of hydantoin by using the DFT(B3LYP) and MP2 levels of approximation, and showed that Ar-matrix isolated infrared spectra observed above 500 cm-1 were fully assigned with the help of the above information. Their calculation also showed that there are two low-lying vibrational modes below 200 cm-1, both of which are designated to be torsional motions of a five-membered ring. These low-lying modes were also reported by other computational studies (Ildiz et al. 2013; Belaidi et al. 2015). However, the vibrational frequencies of these modes strongly depend on the level of approximation and on the basis sets. For example, our calculation gave the vibrational energies of the two lowest modes to be 55.2 cm-1 and 146.7 cm-1, while the corresponding values of 29 cm-1 and 133 cm-1 were calculated by Ildiz et al. (2013), and 128 cm-1 and 144 cm-1 by Belaidi et al. (2015). We see a large discrepancy in frequency especially for the lowest mode, which is presumably due to a large anharmonicity of the potential surface.

We compared the spectral intensity of the same observed rotational transitions in the vibrationally excited states relative to that in the ground state, as shown in Fig. 2. Observed relative intensities were 1, 0.60, and 0.53 at 420 K for the ground state, vibrationally excited state 1, and excited state 2, respectively. The vibrational energy for these excited states are estimated to be 140 and 170 cm-1 for excited states 1 and 2, respectively. Any other series of spectral lines with larger intensities than those of these assigned excited states were not found in the present study; therefore, these two excited states must be due to the two lowest vibrational excited states.

thumbnail Fig. 2

Comparison of spectral intensity of hydantoin in its vibrational ground state (left) and two excited states (center and right) at 150°C, observed at 121 GHz (NKaKc = 350 35 ← 341 34, NKaKc = 351 35 ← 340 34). Each spectrum was recorded by integrating 400 scans with a 0.5 Hz repetition rate and a 1 ms time constant of the lock-in amplifier.

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The vibration-rotation partition functions of hydantoin for several temperatures were calculated by taking the two lowest vibrational excited states into account; they are listed in Table 3. With our calculated dipole moment (μ(b) = 2.655 D and μ(a) = 0.169 D), and the molecular constants obtained in the present study, the spectral intensity distribution is estimated and is shown in Fig. 3. The main spectral pattern consists of a series of b-type transitions, whose intensities are reach their maximum at around 300 GHz, at a temperature of 100 K, as shown in Fig. 3.

thumbnail Fig. 3

Spectral intensity distribution of hydantoin at 100 K.

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Table 3

Vibration-rotation partition function of hydantoin.

Given the partition function and the dipole moment along the b-axis for hydantoin, the expected spectral intensity can be calculated when we conduct a search for this molecule in the interstellar space. The integrated intensity for the low-Kab-type R-branch transition (NKaKc = 390 39 ← 381 38,NKaKc = 391 39 ← 380 38 at 135.5 GHz) is estimated to be 0.044 K km/s, assuming a column density of 1013−1015 cm-2.

5. Conclusions

Pure rotational spectral lines of hydantoin in its ground vibrational state and the two lowest vibrational excited states were observed for the first time. The obtained millimeter data were analyzed and the molecular constants were obtained. The rotational partition function including the partition function of the two lowest vibrational excited states at various temperatures were calculated. The frequency error for spectral lines with upper-state energy below 200 cm-1 is <100 kHz, corresponding to a velocity resolution of 0.1 km s-1 at 300 GHz, where the spectral line intensities are at their maximum with the temperature condition of 100 K. Our provided spectral line list will be cataloged in a spectral line database (Toyama Microwave Atlas, ToyaMA)1, also available at the CDS, and is valid for astronomical searches for hydantoin with millimeter to submillimeter-wave telescopes such as ALMA.


Acknowledgments

H.O. thanks the Futaba Electronics Memorial Foundation for its financial support in constructing the spectrometer at Toho University. K.K. thanks Koichi Nozaki for helping ab initio calculations. This study was supported by JSPS KAKENHI (Grants-in-Aid for Scientific Research), grant numbers JP15H03646, JP15K05027.

References

All Tables

Table 1

Selected observed transition frequencies of hydantoin in its ground and excited vibrational states around 130 GHz.

Table 2

Molecular constants of hydantoin in its vibrational ground state and two excited states.

Table 3

Vibration-rotation partition function of hydantoin.

All Figures

thumbnail Fig. 1

Observed and fitted spectra of hydantoin (solid and dashed lines, respectively) at 93 GHz (NKaKc = 270 27 ← 261 26,NKaKc = 271 27 ← 260 26,). The spectrum was recorded by integrating 400 scans at a 0.5 Hz repetition rate and a 1 ms time constant of the lock-in amplifier.

Open with DEXTER
In the text
thumbnail Fig. 2

Comparison of spectral intensity of hydantoin in its vibrational ground state (left) and two excited states (center and right) at 150°C, observed at 121 GHz (NKaKc = 350 35 ← 341 34, NKaKc = 351 35 ← 340 34). Each spectrum was recorded by integrating 400 scans with a 0.5 Hz repetition rate and a 1 ms time constant of the lock-in amplifier.

Open with DEXTER
In the text
thumbnail Fig. 3

Spectral intensity distribution of hydantoin at 100 K.

Open with DEXTER
In the text

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