Rotational spectroscopy of methylamine up to 2.6 THz^⋆

¹ Laboratoire de Physique des Lasers, Atomes, et Molécules, UMR CNRS 8523, Université de Lille 1, 59655 Villeneuve d’Ascq Cedex, France
e-mail: roman.motienko@univ-lille1.fr
² Institute of Radio Astronomy of NASU, Chervonopraporna, 4, 61002 Kharkov, Ukraine
³ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena CA 91109-8099, USA

Received: 4 December 2013
Accepted: 20 January 2014

Abstract

Context. Methylamine (CH₃NH₂) is the simplest primary alkylamine that has been detected in the interstellar medium. The molecule is relatively light, with the 50 K Boltzmann peak appearing near 800 GHz. However, reliable predictions for its rotational spectrum are available only up to 500 GHz. Spectroscopic analyses have been complicated by the two large-amplitude motions: internal rotation of the methyl top and inversion of the amino group.

Aims. To provide reliable predictions of the methylamine ground state rotational spectrum above 500 GHz, we studied its rotational spectrum in the frequency range from 500 to 2650 GHz.

Methods. The spectra of methylamine were recorded using the spectrometers based on Schottky diode frequency multiplication chains in the Lille laboratory (500−945 GHz) and in JPL (1060−2660 GHz). The analysis of the rotational spectrum of methylamine in the ground vibrational state was performed on the basis of the group-theoretical high barrier tunneling Hamiltonian developed for methylamine by Ohashi and Hougen.

Results. In the recorded spectra, we have assigned 1849 new rotational transitions of methylamine. They were fitted together with previously published data, to a Hamiltonian model that uses 76 parameters with an overall weighted rms deviation of 0.87. On the basis of the new spectroscopic results, predictions of transition frequencies in the frequency range up to 3 THz with J ≤ 50 and K_a ≤ 20 are presented.