Optically Active Crystal Vibrations of the Alkali-Metal Nitrates

The infrared and Raman spectra for the three alkali‐metal nitrates LiNo₃, NaNO₃, and KNO₃ have been measured in the region from 1600 to 30 wavenumbers, and a normal‐coordinate analysis has been made for these compounds with a special emphasis on the rotational and translational lattice modes in the crystal. All three compounds were studied as $D_{\text{3d}}^6$ models and a modified Urey–Bradley force field was used in the calculations. The intramolecular and interaction force constants for all three nitrates are given. The effect of the interaction potential constants on the NO₃− ion is discussed as well as the effect of the metal ion–nitrate ionic interaction in the crystal from a comparison of the calculated force constants. Although there are large changes in the rotational and translational lattice frequencies when comparing one alkali‐metal nitrate with another, the lack of force constant dependence is interesting.