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tokens: such quartet at 173.2 GHz that corresponds to the transitions gave us an opportunity to assign the whole series of this kind of transitions and get an initial approximation for the rotational constants of CHNH (at this stage the tunneling parameters were kept fixed at the values of the main methylamine isotopologue, Motiyenko et al. 2014 ). Obtained in this way, the initial model allowed us to continue assigning the spectra in a usual iterative manner by adding new assigned lines to the fit, refining the Hamiltonian model, and producing new predictions. The Doppler-limited resolution of our spectrometers provided an opportunity to resolve nuclear quadrupole hyperfine splittings for 898 rotational transitions. Typically, as for the main methylamine isotopologue, a resolved pattern of the hyperfine structure was observed as a doublet with an approximately 2-to-1 ratio in intensities. The stronger doublet component contains unresolved hyperfine transitions with selection rules and , whereas the weaker doublet component corresponds to the transition. At the same time, a number of hyperfine components with selection rules were also assigned. To provide the hyperfine-free frequencies we used in our analysis of the rotation-torsion-wagging spectrum of the ground state of C methylamine, we fitted the frequencies of the individual hyperfine components using the model described above. The quadrupole hyperfine parameters , and that we obtained from the fitting of the hyperfine patterns of the rotational transitions are presented in Table 1 . Table 2 Main rotational and torsion-wagging splitting parameters (all in MHz except of which is unitless) of the ground torsional state of C and C methylamines. Notes. (a) . Fig. 1 Predicted (in red) and observed (in blue) rotational spectrum of C methylamine between 882 and 908 GHz dominated by -type series of transitions with . A slight inconsistency between predicted and observed spectrum, which may be visible for some strong lines, is due to source power and detector sensitivity variations. Weak unassigned lines are due to higher excited vibrational states. In total, the dataset for the rotational spectra of CHNH in the ground vibrational state includes 2721 rotational transitions with an upper value of . This set of rotational transitions was fitted to the effective Hamiltonian of the group-theoretical high-barrier tunneling formalism that was developed for methylamine by Ohashi Hougen (1987) . The fit adopted in the present study as the “best” achieved the root-mean-square (rms) deviation of 0.039 MHz that corresponds to the weighted rms deviation of 0.73. The Hamiltonian model includes 75 parameters. The values of the molecular parameters obtained from the final fit are presented in Table 1 . In Table 2 , the main rotational and torsion-wagging splitting parameters for CHNH and CHNH ( Motiyenko et al. 2014 ) are compared. From the comparison we see that the values of the main tunneling splitting parameters in