Methylamine is considered a model molecule exhibiting two large amplitude motions: CH₃ internal rotation and NH₂ inversion. These two large amplitude motions are strongly coupled and give rise to a rich rotation-inversion-torsion structure in the vibrational states.

In the current work a recent progress is reported on assignment and fitting the pure rotational spectrum of methylamine in the ground vibrational state and in the first and the second torsional states. It was possible to assign in the IR spectrum the complete ^rR and ^rQ series for higher values of K and J, for all symmetry species.

The high resolution IR spectra were recorded in the range 40-360 cm^-1 with a resolution of 0.00125 cm^-1 using Bruker IFS-120HR spectrometer at the University of Oulu. About 8300 new infrared rotational transitions have been assigned in the ground, first and second torsional states using an original Loomis-Wood program dedicated to methylamine [1].

For each vibrational state a joint fit of rotational transitions of all symmetry species has been carried out. The transitions assigned in this project and published earlier were fit to a single state model based on the group theoretical formalism of Hougen and Ohashi [2]. The standard deviation of the fit for the ground state with K≤20 and J≤50 to 78 parameters was 0.00031 cm^-1. The standard deviation of the fit for the first excited state with K≤20 and J≤45 was 0.00066 cm^-1 and 89 parameters were necessary. For the second excited torsional state with K≤14 and J≤40 and with 95 parameters a standard deviation of 0.00207 cm^-1 was obtained. Only for the ground state the standard deviation is close to the experimental accuracy.

[1] W. Łodyga, M. Kręglewski, P. Pracna, S. Urban, J. Mol. Spectrosc., 2007, 243, 182-188 (www.lww.amu.edu.pl).
[2] N. Ohashi, J.T. Hougen, J. Mol. Spectrosc., 1987, 121, 474-501.