Vibration Frequencies of NO2− and NO3− Ions in KBr Crystals

Abstract

Infrared‐absorption spectra of doped KBr crystals at temperatures of approximately 8°K were measured in the wavelength range 2 to 15 μ. From the absorption spectra of crystals doped with NO2 − and NO3 − ions, and with ions enriched in the isotopes15N and 18O, frequencies of normal vibrations of six isotopic NO2 − molecules, and eight isotopic NO2 − molecules were determined. These frequency values were used to calculate a set of force constants for NO2 − and NO3 − ions in the KBr lattice, using the most general harmonic force field (GFF). In mdyn/Å units, values obtained for NO2 − were: fr =7.50, frr =1.66, f rα=0.39, f α=1.67, and for NO3 −: fr =7.62, frr =1.43, (f α—f αα)=1.09, (f rα—f rα′)=−0 .48, f γ=1.506. In crystals doped with divalent metal ions as well as NO2 − and NO3 − ions, new absorption bands appeared. These were perturbations of the vibrational frequencies of NO2 − and NO3 − ions, caused by association between divalent metal ions and NO2 − and NO3 − ions. In the case of NO3 −, the doubly degenerate asymmetric stretching band was split into two nondegenerate bands by the perturbation, which is thus almost certainly caused by a nearest‐neighbor association between metal and nitrate ions. While the other frequency shifts of nondegenerate vibrations can give no information about the symmetry of the surrounding lattice, they can also be explained by a nearest‐neighbor interaction between divalent metal and NO2 − and NO3 ions.