Infrared Absorption Spectra of Isotopic Dimeric Hydrogen Chloride Molecules

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Abstract
The spectra of HCl and of HCl mixed with DCl at 195°K have been observed in the region of the HCl fundamental band. In the case of HCl three prominent lines occur in the null gap. Smaller complex features were observed in the region extending from R(0) to P(3). The intense lines at 2891.2, 2889.8, and 2889.0 cm−1 had relative intensities of 1:0.6:0.1. These lines have been shown to be due to the HCl dimer with a heat of formation of 2.1 kcal/mole. Treating the HCl–HCl molecule as a coupled nearly resonant system only two lines of relative intensity of 1:0.3 would be expected corresponding to the dimers (H35Cl–H35Cl, H35Cl–H37Cl) and (H37Cl–H35Cl, H37Cl–H37Cl) for the resolution possible with these broad features. If these are Q branches corresponding to a slightly bent dimer, the wide spread features would correspond to the other branches of the band. With the HCl—DCl mixture two additional lines were observed of separation 2.1 cm−1 and relative intensity 1:0.34. These are attributed to the dimers (H35Cl—D35Cl, H35Cl—D37Cl) and (H37Cl—D35Cl, H37Cl—D37Cl). Assuming nonresonant coupled oscillators, the measured frequencies can be computed within experimental error using a reduced mass for the molecule calculated by adding to the 35Cl or 37Cl the ΔH/D0 fraction of the mass of DCl, where ΔH is the heat of formation and D0 the dissociation energy of HCl.

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