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 rho 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 nu1 of SO4(2-) in the IR spectrum, IR versus R nu(max) shifts for nu3 and nu4 of SO4(2-) and nu4 of NH4+) nu1 of SO4(2-) has (apparent) rho of only 0.014, while nu2, nu3 and nu4 of SO4(2-) and nu4 (probably also nu2) of NH4+ have rho 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 rho(max) with the geometry of the optics used. For (CH3)4NBr symmetric N+-C stretching has rho 0.012; all-in-phase C-H stretching and four overtones in Fermi resonance with it have rho 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 rho in the range 0.74-0.76. These results show that the selection rules in respect of rho, 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 rho for these Raman bands is still rho(max), even for concentrated aqueous solutions.
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Protein Chemistry Group, John Curtin School of Medical Research, Australian National University, GPO Box 334, ACT 2601, Canberra, Australia.
D000645:Ammonium Sulfate / Q000737:chemistry*; D004563:Electrochemistry; D007477:Ions; D008956:Models, Chemical; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*; D012996:Solutions; D013055:Spectrophotometry, Infrared; D013059:Spectrum Analysis, Raman; D014867:Water
D007477:Ions; D000644:Quaternary Ammonium Compounds; D012996:Solutions; D014867:Water; C027917:tetramethylammonium; D000645:Ammonium Sulfate