Infra-Red Absorption Spectrum of Crystalline Sodium Nitrite

Abstract

THERE are very few data available in the infra-red absorption spectra of solids which enable an application of the modern ideas of point group symmetry to the classification of proper vibrations. We have recently examined the absorption spectrum of sodium nitrite in the solid state. The substance has already been shown to be orthorhombic, and thus biaxial, with considerable separation between the indices of refraction. There are two molecules in the unit cell, and there will be eight particles in the basic group: accordingly we shall expect 3(8 1) = 21 proper vibrations, of which six will be inner vibrations, and fifteen external or lattice vibrations. Generally, some of the inner vibrations will coincide in pairs, but if the nitrite group is angular, its symmetry will be C₂, which is the same as that of the crystal group¹, and we may expect considerable separation between the pairs. These conditions do not occur in the free ion, and A. Langseth and E. Walles² have obtained a clear and satisfying Raman spectrum for the aqueous solution with ₁ = 1331, ₃ = 1240, ₂ = 813 cm.¹. In our case all the fundamental frequencies and overtones are doubled, bands occurring at ₂ = 707 and 834 (10); ₃ = 1127, 1220 (14); ₁ = 1330 and 1392 (10); 2₂ = 1378, 1626 (4); 2₃ = 2252, 2439 (2) ; 2₁ = 2646, 2762 (3). The numbers in brackets give an approximate measure of the relative intensities. The bands at 707 cm.¹ and 834 cm.¹ have faint wings on each side, which may perhaps be explained by the trichroism of the substance.