Raman-spectral depolarisation ratios of ions in concentrated aqueous solution. The next-to-negligible effect of highly asymmetric ion surroundings on the symmetry properties of polarisability changes during vibrations of symmetric ions. Ammonium sulphate and tetramethylammonium bromide

Depolarisation ratios ρ have been measured for the Raman spectra of solutions of composition (NH₄)₂ SO₄ · 11H₂O and (CH₃)₄NBr · 29D₂O. Even though the former's vibration spectrum shows clear evidence of lowered ion symmetries (presence of ν₁ of SO₄^2- in the IR spectrum, IR versus R ν_max shifts for ν₃ and ν₄ of SO₄^2- and ν₄ of NH₄⁺) ν₁ of SO₄^2- has (apparent) ρ of only 0.014, while ν₂, ν₃ and ν₄ of SO₄^2- and ν₄ (probably also ν₂) of NH₄⁺ have ρ in the range 0.73-0.77; within the experimental error and base line uncertainty the latter are equal to 0.75, i.e. to ρ_max with the geometry of the optics used. For (CH₃)₄NBr symmetric N⁺-C stretching has ρ 0.012; all-in-phase C-H stretching and four overtones in Fermi resonance with it have ρ in the range 0.02-0.035, but the deviation from zero here is in part due to underlying or overlapping depolarised bands. The sufficiently well isolated antisymmetric CH stretching and degenerate CH bending bands again have ρ in the range 0.74-0.76. These results show that the selection rules in respect of ρ, which apply strictly only to isolated molecules, are for practical purposes still valid for molecules in strongly symmetry-distorting external environments in the liquid phase. More specifically: (A) During vibrations in which quasi-spherical intramolecular symmetry is retained, the externally caused aspherical component of the polarizability ellipsoid does not change aspherically to a sufficient extent for an appreciably intense anisotropic Raman band to appear. (B) During intramolecularly anti-symmetric vibrations of symmetric molecules, the portion of the externally caused distortion of the polarizability ellipsoid that fails to cancel over a whole vibration period is not large enough to give rise to an appreciably intense isotropic component of the Raman band. This means in practice ρ for these Raman bands is still ρ_max, even for concentrated aqueous solutions.