Cyanide Ion—Environmental Perturbation of Its Vibrational and Rotational Motion When Isolated in Alkali Halides

The spectrum of the cyanide ion isolated in 12 different alkali halides has been investigated as a function of pressure up to 50 kbar at 100°K. Observed frequency shifts are used to optimize frequency perturbation calculations. The general form of the potential experienced by ions isolated in alkali halides is discussed, and the particular terms dominating the CN−-ion frequency shifts are indicated. Pressure-induced band-contour changes are used together with calculations on the dependence of the energy of the system on the cyanide-ion axis orientation, to obtain a better understanding of the hindered rotational motion exhibited by this ion in many of the crystals. Appreciable displacement of near-neighbor ions is shown, and preferred CN−-ion axis orientation of <001> in NaCl-type structures and <110> in CsCl-type structures is indicated.