Raman Spectra of Solid (NH 4 ) 2 CrO 4 and (ND 4 ) 2 CrO 4 Obtained by a Rotating Cell Technique

The Raman spectra of polycrystalline (NH₄)₂CrO₄ and (ND₄)₂CrO₄ have been obtained by a sample rotation technique where the uncompressed solid is contained in a glass cylindrical cell. The apparatus is a commerically available sample rotator for liquids, which was modified for the described solid sampling technique. The Raman spectra of (NH₄)₂CrO₄ and (ND₄)₂CrO₄ are discussed in relation to their uniquely monoclinic structure, in contrast to the β-K₂SO₄ structure found for (NH₄)₂SO₄ and many alkali metal chromates and sulfates. The hydrogen bonding in (NH₄)₂CrO₄ is described, and its role in determining both the structure and the Raman spectra is discussed. The data suggest a barrier to NH₄⁺ rotation of approximately 3.70 kcal/mol, indicating that the ammonium ions are not freely rotating on the time scale of the Raman experiment (10⁻¹³ sec).