The microwave spectrum of methyl isothiocyanate

New measurements of microwave transitions of CH₃NCS in the region 14–40 GHz are reported. Assignments of several sets of lines in terms of the transitions expected for a pseudosymmetric-top molecule are suggested and a tentative extension of the analysis to cover all the k = 0 lines in our spectra and in the region 5–11 GHz is proposed. Preliminary calculations using a two-dimensional anharmonic oscillator model lead to a bending potential with a central hump of some 160 cm^?1, and a minimum-energy configuration at a CNC bond angle of about 147°. A correlation between rovibrational energy levels of a two-dimensional anharmonic oscillator and those of an asymmetric-top molecule with a bent skeleton and freely rotating methyl group is proposed.