Submillimetre-wave Spectroscopy of Unstable Imines of Astrophysical Interest: CH2NH and CH2CNH.

Methanimine (CH2NH) and ketenimine (CH2CNH) are unstable molecules which have been detected in different astronomical sources. Presently, the published rotational data for these species are limited to frequency regions which are well below the spectral ranges covered by Herschel/HIFI and the ALMA bands 9 and 10. We have extended the recordings of rotational lines of CH2NH and CH2CNH into the submillimetre-wave region, in order to improve the precision of ground-state rotational and centrifugal distortion constants and make it possible to build a list of very accurate rest-frequencies for astrophysical purposes in the THz region (1σ uncertainties lower than 0.01 kms−1 in radial equivalent velocity). The investigation was carried out using a source-modulation microwave spectrometer based on Gunn oscillators equipped with various frequency multipliers. The free-space cell of the spectrometer was coupled to a high temperature reactor which allowed for the production of these species by gas-phase pyrolysis of 1,2 diaminoethane (for CH2NH), or isoxazole (for CH2CNH). The spectra were recorded in the frequency range 80–630 GHz, and many tens of new ground-state transition frequencies were accurately measured for each species. The hyperfine structure due to the 14N nucleus was very often detected, and it was accounted for in the analysis. The complete sets of quartic and sextic centrifugal distortion constants could be accurately determined for CH2NH, while less satisfactory results were obtained for CH2CNH, whose ground-state rotational spectrum is perturbed by a centrifugal resonance. Further analyses are currently in progress to achieve a better modeling of the observed perturbation.