The changes in the Raman spectra of aqueous solutions of I, in HI (1:l mole ratio) with changing concentration, have been shown to be a result of ion-pairing: H++I,=H+I;.
The anomalous high frequency stretching band of the solute at 172 cm -' for these solutions arises mainly from stretching of the stronger I-I bond in the H+I; ion-pair and not from vibrational modes of I2 or I;. Ion-pair dissociation constants estimated from the Fuoss equation, combined with the known 1s ion-pair and not from vibrational modes of I2 or I;. Ion-pair dissociation constants estimated from the Fuoss equation, combined with the known I; and I; formation constants, account for the intensity changes of the stretching bands. The spectra of solutions of Br,/HBr at 1:l mole ratio may be interpreted in the same way but other Br species are present. The spectra of the isolated X; ions in solution exhibit a shoulder to high frequency of the symmetric stretching band, Y,. In the spectrum of the I; ion, this peak is assigned to one of the two frequencies, resulting from Fermi resonance between Y, and 2v2 but, in the case of the Br; ion, this peak may be due to v,, which becomes Raman active as a result of disymmetric solvation. The consequences of such ion-pairing for the nature of I, dissolved in polymers are discussed.