Accurate rovibrational energies for the first excited torsional state of methylamine

The first excited torsional band of methylamine, \nub{15}, has been reassigned in a high resolution spectrum in the region from 40 to 360 cm$^{-1}$. Over 12400 transitions with a resolution of 0.00125 cm$^{-1}$ with K from 0 up to 16 and J from 0 up to 40 have been assigned for all six symmetry species. A global fit of the infrared, pure rotational and microwave data has been carried out and the band centre was determined at 264.5825(60) cm$^{-1}$. The combined data were fit to a single state model based on the group theoretical formalism of Hougen and Ohashi resulted in the total standard deviation of 0.004 cm$^{-1}$ for the infrared spectrum and 0.40 MHz for microwave spectrum. From the same spectrum the upper state combination differences produced data for the rotational structure of the ground state which could be fitted with the standard deviation of 0.0003 cm$^{-1}$. The fit to ground state rotational transitions in the MHz region gave the standard deviation of 0.21 MHz. Although the precision of the energies calculated for the excited torsional state is not fully satisfactory it allows us to assign several thousands of lines in the hot bands \nub{15}-2\nub{15}, \nub{15}-3\nub{15} and \nub{15}-4\nub{15} , which are quite intense in the spectrum recorded at room temperature