Infrared Absorption Spectrum of Hydrogen Cyanide

Abstract

The absorption spectrum of HCN in the gas phase has been investigated by a grating spectrometer with high resolution in the region 3\ensuremath{\mu} - 15\ensuremath{\mu}. Five of the infrared bands were studied, and of these the very intense fundamental at 14\ensuremath{\mu} has been resolved into individual lines forming $P$, $R$ and $Q$ branches. The first harmonic of the band at 7\ensuremath{\mu} 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\ensuremath{\mu}, 4.760\ensuremath{\mu}, and 4.794\ensuremath{\mu} forming the second harmonic of the same vibration. There is a sharp single maximum at 3.57\ensuremath{\mu} with another very strong double band at 3.04\ensuremath{\mu}.Form of the HCN molecule. The HCN molecule is linear with fundamental frequencies ${\ensuremath{\nu}}_{2}=712$ ${\mathrm{cm}}^{\ensuremath{-}1}$, ${\ensuremath{\nu}}_{3}=3289$ ${\mathrm{cm}}^{\ensuremath{-}1}$ and ${\ensuremath{\nu}}_{1}$ approximately 2100 ${\mathrm{cm}}^{\ensuremath{-}1}$. The latter has not been observed directly since the band associated with it is of extremely low intensity, but the combination ${\ensuremath{\nu}}_{1}+{\ensuremath{\nu}}_{2}$ explains the band at 3.57\ensuremath{\mu}. The moment of inertia determined from the fine structure of the 14\ensuremath{\mu} band is 18.68\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}40}$g ${\mathrm{cm}}^{2}$.