Extension of the millimeter- and submillimeter-wave spectral databases of deuterated methyl cyanides (CH₂DCN and CHD₂CN)^⋆

¹ Université de Toulouse, UPS-OMP, IRAP, Toulouse, France
e-mail: linh.nguyen80@yahoo.fr; adam.walters@irap.omp.eu
² CNRS, IRAP, 9 avenue du Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
³ Laboratoire de Physique des Lasers, Atomes, et Molécules, UMR CNRS 8523, Université de Lille 1, 59655 Villeneuve d’Ascq Cedex, France
⁴ Sciences Chimiques de Rennes, École Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, Avenue du Général Leclerc, CS 50837, 35708 Rennes Cedex 7, France
⁵ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), UMR 5274, 38041 Grenoble Cedex 9, France

Received: 24 July 2012
Accepted: 28 January 2013

Abstract

Context. The study of deuterated abundant molecules is recognized as important in understanding molecular complexity in star-forming regions.

Aims. We seek to assign the laboratory millimeter and submillimeter wave rotational spectra of the CHD₂CN and CH₂DCN deuterated isotopologues of methyl cyanide over a wide frequency range in order to provide precise spectral predictions for observations.

Methods. Using the solid-state submillimeter-wave spectrometer in Lille, we measured and assigned 723 new lines for CHD₂CN and 307 new lines for CH₂DCN. The observed rotational transition frequencies were fitted with the ASFIT program to determine the spectroscopic parameters. The prediction of transition frequencies was performed using the SPCAT program.

Results. Measurements for both isotoplogues were taken up to 945 GHz and are made available at the CDS. For CHD₂CN this is significantly higher than the previous range of measurement up to 40 GHz. For CH₂DCN many more lines of high K_a have been measured than previously.

Conclusions. Our work confirms the recent analysis for CHD₂CN given in the CDMS database and extends the number of determined molecular parameters from 10 to 19. For CH₂DCN, 3 new parameters including D_K have been determined, and the uncertainty on parameters has been decreased by a factor of approximately 2. For both isotopologues the measured data show shifts in the frequency of some high K_a transitions that are attributed to interactions with a low-lying vibrational state. The availability of more directly measured data and the increase in confidence of the predictions to higher quantum numbers and frequencies will be helpful for the radio astronomical detection of deuterated isotopologues of methyl cyanide in the interstellar medium.