Raman spectral study of solutions of N2O4 and N2O5 in nitric acid

Abstract

Solutions of N2O4 and N2O5 in anhydrous and aqueous nitric acid were examined by Raman spectroscopy to gain a more detailed knowledge of the species present in these solutions and to assess the applicability of Raman spectroscopy as an analytical measurement technique. Solutions of N2O4 in HNO3 show evidence of the presence of the associated species, 3NO+·NO3- (N4O62+), which had been identified previously in a solid compound. The effects of water on the spectra of these solutions were also examined. The nitronium cation, NO2+, is the dominant species in solutions of N2O5 in anhydrous HNO3, and the spectra of concentrated solutions of N2O5 exhibit a band that has not been reported previously. This weak band, which may be the same as a band found in the spectra of solids containing the nitronium cation, has been attributed to a second vibrational mode of the molecule. This indicates a nonlinear conformation of the NO2+ ion, and suggests that it is strongly associated with other species in solution. The Raman data indicate that N2O5 in HNO3 is completely ionized at concentrations to at least 3.0 mol l-1 (21 wt%). © US Government work and, as such is in the public domain in the USA