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An infrared study of small molecules in inert matrices Shurvell, Herbert F.

Abstract

Infrared absorption spectra of HC1 and HBr, suspended in solid argon, krypton and nitrogen, were recorded in order to obtain information on intermolecular forces. SO₂ in argon and nitrogen, and CO in argon were also studied. The spectra were observed in the temperature range from liquid helium temperatures up to the melting point of the matrix. The halogen acids gave more complicated spectra in the noble gas matrices than in nitrogen. This has been correlated with the different thermal properties of the matrix materials. Matrix to solute ratios from 100 to 800 to 1 were used and evidence was found for solute-solute interactions, arising from incomplete isolation of solute molecules at the lower ratios. During the warm-up period at the end of an experiment, additional peaks appeared in the spectra. It is suggested that these new peaks were due to clusters of solute molecules produced by diffusion of the solute through the lattice. Semi-empirical calculations were carried out to estimate shifts of vibrational frequencies of the trapped molecules. From these calculations it was concluded that repulsive intermolecular forces play an important part in determining the magnitude, and direction of the shifts. A first order perturbation calculation was made, using a Lennard-Jones' potential, to determine the effect of the matrix on the rotational energy levels of a trapped molecule. Spectra of the clathrate-hydrates of SO₂, H₂S and krypton were recorded at liquid nitrogen temperatures, and the SO₂ hydrate was studied in the temperature range from 4° to 120° K. The spectrum of the water skeletal vibrations exhibited several interesting features. The assignment of the 1600 cmˉ¹ and 2200 cmˉ¹ peaks to v₂ and v₂ + vR was confirmed and a new peak at 2410 cmˉ¹ was observed. A lattice mode in the spectrum of the S0₂ hydrate was observed in combination with V₃ of SO₂.

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