Polarisation of Raman lines in some inorganic acids

Summary

The polarisation of Raman lines of nitric, iodic, sulphuric, selenic and selenious acids is investigated making use of the usual double-image prism method. All the lines of the concentrated nitric acid except one at 1538 which is attributed to the deformation oscillation of the hydroxyl group, show varying degrees of depolarisation which are less than the limiting value 6/7. The lines due to the nitrate ion possess a state of polarisation which is in agreement with the plane equilateral structure attributed to it. From the results of polarisation six lines belonging to the undissociated nitric acid molecule are identified with the known modes of oscillation of an unsymmetrical pyramidal model having a structure

. The state of polarisation of the Raman lines of the H₂SO₄ molecule and its dissociation products, HSO₄′ and SO₄″ ions generally supports the assumption that all the three forms are tetrahedral and that HSO₄′ ion is the most distorted of the three. Structural formulæ of the three species and the mechanism of dissociation from one form to the other are discussed in relation to the degree of depolarisation of the breathing frequency. Polarisation of the Raman lines of selenic acid is analogous to those of sulphuric acid. The results of polarisation of the bands of iodic acid as well as selenious acid generally support the previous assignments of these bands to different molecular species.