Absorption Bands of Hydrogen Cyanide Gas in the Near Infrared

Abstract

The absorption spectrum of gaseous hydrogen cyanide has been investigated by photographic methods in the region $\ensuremath{\lambda}7000\ensuremath{-}9200$. Two weak bands of very simple structure were found, having $P$ and $R$ branches but no $Q$ branches. The band at $\ensuremath{\lambda}7912$ is apparently a harmonic of a fundamental band at 3.04\ensuremath{\mu}, and the very weak band at $\ensuremath{\lambda}8563$ is a combination band. The hydrogen cyanide molecule is linear in the normal state, and has a moment of inertia $I=18.79\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}40}$ g\ifmmode\cdot\else\textperiodcentered\fi{}${\mathrm{cm}}^{2}$. The distance of separation of the carbon and nitrogen atoms is estimated to be 1.15\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ cm. Hydrogen cyanide is discussed in regard to its three fundamental oscillations which have frequencies 3290, 2090, and 710, respectively, and in regard to its dissociation energy and dissociation products. The evidence requires a molecular structure represented by the formula HCN, and shows that the normal molecule is built from a normal hydrogen atom and a normal CN radical. The absorption of cyanogen gas has also been investigated in the photographic infrared, but no absorption bands could be detected.