Spectra of the Alkali Halides. II. The Infrared Spectra of the Sodium and Potassium Halides, RbCl, and CsCl

The infrared spectra of NaCl, NaBr, NaI, KCl, KBr, RbCl, CsCl in the gas phase have been measured in the region of the fundamental vibration frequency. The vibrational frequencies calculated from the observed spectra are: $$\begin{array}{c}\mathrm{NaCl}{\mathrm{\hspace{0.17em}\omega }}_e\text{=366\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}},\\ \mathrm{NaBr}{\mathrm{\hspace{0.17em}\omega }}_e\text{=302\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}},\\ \mathrm{NaI}{\mathrm{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\omega }}_e\text{=258\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}},\\ \mathrm{KCl}{\mathrm{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\omega }}_e\text{=281\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}},\\ \mathrm{KBr}{\mathrm{\hspace{0.17em}\hspace{0.17em}\omega }}_e\text{=213\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}},\\ \mathrm{RbCl}{\mathrm{\hspace{0.17em}\omega }}_e\text{=228\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}},\\ \mathrm{CsCl}{\mathrm{\hspace{0.17em}\omega }}_e\text{=209\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}.\end{array}$$ This data, together with previous data on the lithium halides, permits extrapolations to be made to determine the vibrational frequencies of the following halides: $$\begin{array}{c}\mathrm{KI}{\mathrm{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\omega }}_e\text{=173\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}},\\ \mathrm{RbBr}{\mathrm{\hspace{0.17em}\omega }}_e\text{=166\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}},\\ \mathrm{RbI}{\mathrm{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\omega }}_e\text{=128\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}},\\ \mathrm{CsBr}{\mathrm{\hspace{0.17em}\omega }}_e\text{=139\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}},\\ \mathrm{CsI}{\mathrm{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\omega }}_e\text{=101\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}.\end{array}$$ The classical ionic model for the gaseous alkali halides is found to be in quantitative agreement with experiment. Some aspects of the problem of dimers in the gas phase are discussed.