Fundamental Absorption Bands of the Deuteroammonias

The four fundamental vibration bands of ND₃ have been observed in infrared absorption, ν₃ being a double band as in NH₃. The frequencies of the parallel vibrations, expressed in cm^-1, are ν₁=2420, ν₃=749.2 and 745.8, and of the perpendicular vibrations, ν₂=2556 and ν₄=1191.3. The rotational structure for ν₃ is completely resolved. The perpendicular bands consist of many components with zero branches spaced at intervals of 1.7 cm^-1 in ν₂ and 5.2 cm^-1 in ν₄. The former cannot be resolved: they coalesce into a broad central absorption region, but the spacing of the most intense rotation lines in the composite band indicates the interval between centers. These frequency intervals agree well with the predictions of Johnston and Dennison. The moments of inertia for ND₃ are determined from the structure of the band ν₃, by comparison with observations upon NH₃. They are C=8.985×10^-40 g cm² with respect to the symmetry axis, and A=5.397×10^-40 g cm² for any perpendicular axis. From these the molecular dimensions are obtained. The height of the pyramid is 0.360×{}10^-8 cm and the distance between hydrogen atoms is 1.645×{}10^-8 cm. New information regarding the potential curve is obtained (1) from observations upon the doubling of the first excited levels in the four varieties of ammonia, and (2) from the positions of higher excited levels in ND₃. Four vibration levels are located which lie above the central potential hump, and correspond to oscillations of the nitrogen atom from side to side through the plane formed by the hydrogen atoms.