AbstractThe totally symmetric ν1vibrational modes of the anionsN3−and NO3−have been studied in aqueous solution with very high precision. The effect of different alkali metal cations (Li+, Na+, K+, Rb+and Cs+) has been systematically recorded as a function of the solution concentration. The experimental results show that the anion ν1mode frequencies are dependent on both the cation present and on the solution concentration. Extrapolation of the experimental data yielded precise values of the anion ν1frequencies in the limit of infinite dilution. These frequencies are 1342.7 ± 0.1 cm−1for N3−and 1047.4 ± 0.1 cm−1for NO3−. When Li+or Na+was present in solution with the azide anion, the ν1frequency was shifted to a higher value than the infinite dilution frequency, with both cations having approximately the same effect, whereas when K+, Rb+or Cs+was present the ν1frequency was decreased, the largest decrease being caused by Cs+, followed by Rb+, with the smallest decrease being caused by K+. In the case of the nitrate anion, the cations Na+, K+, Li+and Rb+were all responsible for an increase in the NO3−ν1frequency with respect to the infinite dilution value, with the magnitude of the perturbation decreasing from Na+to Rb+. WhenCs+was present the NO3−ν1frequency was observed to decrease by a very small amount. It is suggested that for the vibrationally excited azide anion, the cation perturbation occurs through a form of polarizability interaction. However, the cation‐nitrate anion interaction is thought to occur through the effect that the hydrated cation has on the vibrationally excited nitrate a