Infrared Spectra of HCl and DCl in Solid Rare Gases. II. Polymers

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Abstract
Dimers and trimers of HCl, of DCl, and of mixed isotopic composition are identified from their infrared absorption spectra in rare-gas host matrices. One dimer and at least two different trimers have been found. The vibrational frequencies of these aggregates can be quantitatively explained on the basis of a model where: (1) the molecules are situated at substitutional sites in the close-packed rare-gas crystal; (2) nearest-neighbor intermolecular resonance interactions, identical in magnitude with those in the pure HCl or DCl crystal, act to split the states of the aggregate into components about a ``center of gravity''; (3) the position of the ``center of gravity'' depends upon the size but not the shape of the aggregate, decreasing in frequency toward the pure crystal limit as the aggregate grows. From an explanation of a portion of the trimer spectra, the sign of the intermolecular coupling constant was found to be positive. The relative intensities of the various spectral components are used to estimate the intermolecular orientations within the aggregate. While these estimates, which are based on an ``oriented gas model,'' should be viewed with caution, the data suggest that the relative nearest-neighbor orientation approaches that of the crystal as the polymer size increases. The orientation in the dimer is very different from that in the crystal while the orientation in the trimer is intermediate between these two extremes. There is no obvious relationship between the HCl dimer observed in the rare-gas matrix and that observed by Rank, Sitaram, Glickman, and Wiggins in the gas phase.