Significance of both polarizability and polarizing power of cations in nitrate vibrational spectra

Vibrational frequencies of the ${\mathrm{NO}}_3^{-}$ ion in matrix‐isolated monomers of several nitrates MNO₃ are shown to correlate with the polarizing power of the cation ${\mathrm{M}}^{+},\mathrm{P}{\mathrm{\equiv \; (z/r)/S}}_{\mathrm{eff}}$ , properly defined as the quotient, rather than the product, of the ionic potential, z/r, and a factor $S_{\mathrm{eff}}{\text{= (5z}}^{\text{1.27}}{\mathrm{)/(r}}^{\text{(1/2)}}\mathrm{I)}$ (Ahrens, 1954) representing the efficiency with which the electron shell of the cation screens its nuclear charge. For molten nitrates MNO₃, this correlation breaks down: in the condensed state, the polarizability of the cation, a, becomes important as the result of the mutual enhancement of the polarization of a given cation by several anions. A correlation of ν₁ with the product of cation polarizing power and polarizability, P a, is presented.