Far-infrared spectrum of the internal rotation in methylamine

The far-infrared spectra of the Q branches of hindered rotation of CH₃NH₂ were studied in the 80 to 280 cm^?1 spectral region with resolutions up to 0.12 cm^?1. Through the theoretical analysis 11 of the 12 b-type parameters I_a1, I_a2, V₃, V₆, k₁, k₂, k₃, k₄, k₅, k₆, k₇, D_KK are determined using a nonlinear least-squares fit. The agreement between the observed and the final theoretical spectrum is satisfactory considering that only Q-branch lines were studied. Experimentally more lines, especially at low frequencies, were observed which probably belong to P- and R-branch lines of clusters thereof.