Millimeter and submillimeter wave spectra of 13C methylamine

Context. Methylamine (CH₃NH₂) is a light molecule of astrophysical interest, which has an intensive rotational spectrum that extends in the submillimeter wave range and far beyond, even at temperatures characteristic for the interstellar medium. It is likely for ¹³C isotopologue of methylamine to be identified in astronomical surveys, but there is no information available for the ¹³CH₃NH₂ millimeter and submillimeter wave spectra.
Aims: In this context, to provide reliable predictions of ¹³CH₃NH₂ spectrum in millimeter and submillimeter wave ranges, we have studied rotational spectra of the ¹³C methylamine isotopologue in the frequency range from 48 to 945 GHz.
Methods: The spectrum of ¹³C methylamine was recorded using conventional absorption spectrometers. The analysis of the rotational spectrum of ¹³C methylamine in the ground vibrational state was performed on the basis of the group-theoretical high-barrier tunneling Hamiltonian that was developed for methylamine. The available multiple observations of the parent methylamine species toward Sgr B2(N) at 1, 2, and 3 mm using the Submillimeter Telescope and the 12 m antenna of the Arizona Radio Observatory were used to make a search for interstellar ¹³CH₃NH₂.
Results: In the recorded spectra, we have assigned 2721 rotational transitions that belong to the ground vibrational state of the ¹³CH₃NH₂. These measurements were fitted to the Hamiltonian model that uses 75 parameters to achieve an overall weighted rms deviation of 0.73. On the basis of these spectroscopic results, predictions of transition frequencies in the frequency range up to 950 GHz with J ≤ 50 and K_a ≤ 20 are presented. The search for interstellar ¹³C methylamine in available observational data was not successful and therefore only an upper limit of 6.5 × 10¹⁴ cm^-2 can be derived for the column density of ¹³CH₃NH₂ toward Sgr B2(N), assuming the same source size, temperature, linewidth, and systemic velocity as for parent methylamine isotopic species.

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