Internal Rotation Spectra of Methylamines. I. CH3NH2 and CH3ND2

Infrared absorption spectra of methylamine (CH₃NH₂) and methylamine‐d₂ (CH₂ND₂) have been observed in the gaseous states in the region of 500–80 cm^–1 $\text{(20–125 μ)}$ . A complicated, irregular structure is observed all over the spectral region, which is attributable to the internal rotation coupled with an over‐all rotation of the molecule. The internal and over‐all rotation energy levels were calculated on the basis of the previously determined molecular parameters and a potential function for the internal rotation $\mathrm{(x)}$ with the form of $\mathrm{V(x)\hspace{0.17em}=\hspace{0.17em}}\frac{1}{2}{H}_3\text{(1‐}\cos \text{3x)\hspace{0.17em}+\hspace{0.17em}}\frac{1}{2}{H}_6\text{(1‐}\cos \text{6x)}$ . The frequencies and intensities for radiative transitions between these energy levels were then calculated. On comparing the results of the calculation with the observed spectra, it has been found that we need to revise the values of the potential constants and one of the molecular constants $C_2$ (the moment of inertia of the methyl group around its own symmetry axis) from what were determined previously with microwave data. The new set of values is $H_3\text{\hspace{0.17em}=\hspace{0.17em}683.2}{\mathrm{cm}}^{\text{−1}}$ , $H_6\text{\hspace{0.17em}=\hspace{0.17em}−2.0}{\mathrm{cm}}^{\text{−1}}$ , and $C_2{\text{\hspace{0.17em}=\hspace{0.17em}5.273\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−40}}\mathrm{g}·{\mathrm{cm}}^2$ for CH₃NH₂. For CH₂ND₂, a set of values: $H_3\text{\hspace{0.17em}=\hspace{0.17em}671.6}{\mathrm{cm}}^{\text{−1}}$ , $H_6\text{\hspace{0.17em}=\hspace{0.17em}0}$ , and $C_2{\text{\hspace{0.17em}=\hspace{0.17em}5.273\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−40}}\mathrm{g}·{\mathrm{cm}}^2$ , has been proposed. The band center of the fundamental band $\text{(n\hspace{0.17em}=\hspace{0.17em}1←0)}$ is found to be at 263.9 cm^− 1 for CH₃NH₂ and at 224.5 cm^− 1 for CH₃ND₂.