Dominance of dipole coupling over isotope shift effects for internal crystal modes: ν3 of KNO3 (III)

The occurrence of well-separated and sharp transverse and longitudinal optic mode Raman features for the nitrate asymmetric stretching mode of KNO3 (III) presents an exceptional opportunity to observe the effect of isotopic dilution on the dynamics of a crystal mode based on an intensely infrared active internal mode. It has been found that for most of the range of isotopic dilution with 15N, which is known to cause an ∼32 cm−1 isotope shift for isolated NO—3 units, the KNO3 (III) Raman spectrum for backscattering along the trigonal axis contains only two relatively sharp (TO and LO) features, as observed for the 14N sample, but displaced in a regular manner towards the pure 15N sample values. The sharpness of the Raman features relative to the ν3 features observed for the orientationally disordered KNO3 (I), suggests that the spectra can be discussed as though the k?0 selection rule is operating, even though translational symmetry is clearly destroyed by isotopic mixing. It has also been shown that the atr ν3 infrared band of a 50:50 isotopic mixture behaves as though based on a single resonant mode. The results can be interpreted by assuming that dipole forces cause the nitrate units to vibrate with an ’’average’’ resonant frequency, a result that may have interpretive value in the analysis of vibrational spectra of disordered systems.