Millimeter-wave spectroscopy of hydantoin, a possible precursor of glycine^⋆

¹ Department of Environmental Science, Faculty of Science, Toho University, 2-2-1 Miyama, 274-8510 Funabashi, Japan
e-mail: ozeki@env.sci.toho-u.ac.jp
² Department of Physics, Faculty of Science, University of Toyama, 3190 Gofuku, 930-8555, Toyama, Japan
³ National Astronomical Observatory of Japan, 2-21-1 Oosawa, 181-8588 Mitaka, Japan
e-mail: kaori@sci.u-toyama.ac.jp
⁴ Astronomy Data Center, National Astronomical Observatory of Japan, 2-21-1 Oosawa, 181-8588 Mitaka, Japan
e-mail: masatoshi.ohishi@nao.ac.jp

Received: 11 October 2016
Accepted: 12 January 2017

Abstract

Context. Hydantoin (Imidazolidine-2, 4-dione, C₃H₄N₂O₂) is a five-membered heterocyclic compound that is known to arise from prebiotic molecules such as glycolic acid and urea, and to give the simplest amino acid, glycine, by hydrolysis under acidic condition. The gas chromatography combined with the mass spectrometry of carbonaceous chondrites lead to the detection of this molecule as well as several kinds of amino acids.

Aims. The lack of spectroscopic information, especially on the rotational constants, has prevented us from conducting a search for hydantoin in interstellar space. If a rotational temperature of 100 K is assumed as the kinetic temperature of a star-forming region, the spectral intensity is expected to be at its maximum in the millimeter-wave region. Laboratory spectroscopy of hydantoin in the millimeter-wave region is the most important in providing accurate rest frequencies to be used for astronomical research.

Methods. Pure rotational spectra of hydantoin were observed in the millimeter-wave region using the frequency modulated microwave spectrometer at Toho University. Solid hydantoin was heated to around 150 °C to provide appropriate vapor pressure. Quantum chemical calculations suggest that the permanent dipole moment of this molecule lies almost along the b-molecular axis, so that spectral search for b-type R-branch transition has been conducted.

Results. Rotational and centrifugal distortion constants up to the fourth order for the ground vibrational state of hydantoin were accurately determined by measuring 161 b-type transitions in the frequency range between 90 and 370 GHz. In addition, we succeeded in assigning 230 satellite lines, which were attributed to the two vibrationally excited states. The spectral intensity ratio of these lines indicates that these states correspond to the low-lying (approximately 150 cm^-1 above the ground state) vibrational modes.

Conclusions. The frequency catalog of hydantoin in the millimeter-wave range was created for the ground state and for the two low-lying excited states, and are ideal for a future astronomical research. The 1σ frequency accuracy is lower than 100 kHz for the lines with upper-state energy below 200 cm^-1, corresponding to a velocity resolution of 0.1 km s^-1 at 300 GHz