The Infrared Absorption Spectrum of Hydrogen Chloride in Solution

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Abstract
The near infrared absorption bands of HCl in nonionizing solvents in the neighborhood of 3.5μ and 1.8μ, measured with a 3600 line per inch grating, have the following characteristics; they show no rotational structure; they usually have two components of unequal intensities; the absorption region is displaced towards lower frequencies compared with the gas. The displacement is independent of concentration up to mole fraction 0.1, but increases with diminishing temperature in certain solvents. The interval between the components varies directly with the dielectric constant of the solvent. In solution, the relative displacement of the more intense component from the origin of the gas band is approximately the same for the 3.5μ and 1.8μ bands, as is also the interval between components. A simple relation exists approximately between the frequency displacement from the gas origin and the dielectric constant of the solvent, of the form Δv/v=C[(D—1)/(2D+1)], a deduction of which formula has been made by J. G. Kirkwood. The more intense component, whose relative shift from the gas origin is within the limits of error the same as the corresponding shift in the Raman spectrum, is interpreted as originating in the internuclear vibration, and it is tentatively suggested that the other component represents a combination between such vibrational transitions and some slow intermolecular vibration between HCl and solvent molecules. The effect of the medium on the anharmonic character of the mechanical and electrical vibrations in the HCl molecule is qualitatively discussed.