Infrared Absorption Spectrum of Hydrogen Cyanide

The absorption spectrum of HCN in the gas phase has been investigated by a grating spectrometer with high resolution in the region 3{μ} - 15{μ}. Five of the infrared bands were studied, and of these the very intense fundamental at 14{μ} has been resolved into individual lines forming P, R and Q branches. The first harmonic of the band at 7{μ} has two maxima, very nearly symmetrical and equal in intensity, and no Q branch. Although fine structure is indicated, it is not possible to determine the position of successive individual lines. Three other maxima, less intense and unsymmetrical are found at 4.728{μ}, 4.760{μ}, and 4.794{μ} forming the second harmonic of the same vibration. There is a sharp single maximum at 3.57{μ} with another very strong double band at 3.04{μ}.Form of the HCN molecule. The HCN molecule is linear with fundamental frequencies ν₂=712 cm^-1, ν₃=3289 cm^-1 and ν₁ approximately 2100 cm^-1. The latter has not been observed directly since the band associated with it is of extremely low intensity, but the combination ν₁+ν₂ explains the band at 3.57{μ}. The moment of inertia determined from the fine structure of the 14{μ} band is 18.68×{}10^-40g cm².