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Vibrational spectra of the ammonium halides and the alkali-metal borohydrides McQuaker, Neil Robert

Abstract

Using both infrared and Raman techniques the vibrational spectra of selected polymorphs of the ammonium halides; NH₄C1, ND₄C1, NH₄Br, ND₄Br, NH₄I and ND₄I, have been recorded in the spectral region 4000 - 50 cm⁻¹. The Raman spectra of NH₄F and ND₄F were also recorded. Although all the foregoing crystals have been the subject of previous spectroscopic investigations some important new features are observed. These include (i) Raman active longitudinal modes and (ii) previously unreported internal and external modes. In addition more meaningful assignments are made for some previously reported spectral lines. Included are assignments involving (i) the non-degenerate NH₄⁺ stretching mode in combination with acoustical modes and (ii) the transverse and longitudinal components of the triply degenerate NH₄⁺ bending mode in combination with the librational mode. A study analagous to that involving the ammonium halides was made of the following alkali-metal borohydrides: LiBH₄, LiBD₄, NaBH₄> NaBD₄, KBH₄, KBD₄, RbBH₄ and CsBH₄. From the vibrational spectrum of potassium borohydride recorded at 10°K it is possible to infer that the ordered phase associated with this salt has a cubic structure compatible with the 2 space group Td² . The vibrational spectrum of the ordered tetragonal phase of sodium borohydride allows the placing of the seven BH₄⁻ (D₂d) internal vibrational modes. In addition a mode of translatory origin is observed and a mode of rotatory origin is inferred from a line assigned as the second overtone of a librational mode. In the case of lithium borohydride seven of the nine BH₄⁻ (C₅) modes are observed. Six modes of translatory origin appear in the infrared and a mode of rotatory origin is inferred from a line assigned as the second overtone of a librational mode. The structure of potassium borohydride at room temperature is discussed and evidence in support of a structure intermediate between the ordered cubic Td² phase and the disordered cubic 0h⁵ phase of sodium borohydride is given. Finally, the F matrix associated with an undistorted XY₄ (Td) (XY₄ = NH₄⁺, BH₄⁻) ion and the G matrix associated with the same ion which has undergone a slight angular distortion to give an XY₄(D₂d) ion is used to calculate the contribution of the kinetic energy to the spectrum of the XY₄ ( D₂d) ion. It is found that the order of appearance in the spectrum of the B₂ and E components associated with the two F₂ vibrations is correctly predicted.

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