Internal-Rotational Spectra of Methylamines. II. The Fundamental Torsional Band of CH2DNH2

The infrared absorption spectrum of CH₂DNH₂ gas has been examined in the 310–80‐cm⁻¹ region. A relatively simple fine structure observed in the 300–260‐cm⁻¹ region was assigned to that of the fundamental band designated as $\text{n\hspace{0.17em}=\hspace{0.17em}1\hspace{0.17em}←\hspace{0.17em}0}$ , where $n$ is the vibrational quantum number of the torsional oscillation of the CH₂D group with respect to the amino group. It is shown that, in the ground state $\text{(n\hspace{0.17em}=\hspace{0.17em}0)}$ and in the first excited state $\text{(n\hspace{0.17em}=\hspace{0.17em}1)}$ , there are well‐defined trans and gauche rotational isomers and that each of the gauche levels at $\text{n\hspace{0.17em}=\hspace{0.17em}1}$ splits into two with a separation of about 2.8 cm⁻¹ due to the resonance between the two equivalent gauche positions. It is shown, on the other hand, that, in the higher excited states with $\text{n\hspace{0.17em}≥\hspace{0.17em}2}$ , the internal rotation couples strongly with over‐all rotation, so that the fine structures of the hot bands $\text{(n\hspace{0.17em}=\hspace{0.17em}2\hspace{0.17em}←\hspace{0.17em}1, 37\hspace{0.17em}←\hspace{0.17em}2}$ , etc.) are apparently complicated. Such an energy‐level structure of the internal rotation in CH₂DNH₂ is mainly attributable to the situation that the mode of coupling of the torsional oscillation with other vibrations is different in the trans and gauche forms, rather than to the situation that the moments of inertia are the functions of the internal‐rotation angle $\mathrm{(x)}$ .