Vibrational circular dichroism observed in crystalline α-NiSO4 · 6H2O and α-ZnSeO4 · 6H2O between 1900 and 5000 cm−1

Axial circular dichroism (CD) of crystalline α‐NiSO₄ · 6H₂O and α‐ZnSeO₄ · 6H₂O is measured between 1900 and 5000 cm⁻¹ at room temperature. Three CD bands with rotational strengths of 7 × 10⁻⁴⁴, 3 × 10⁻⁴⁴, and 1 × 10⁻⁴² erg · cm³ per molecule of water are observed at 5000, 4050, and 2300 cm⁻¹, respectively, in α‐NiSO₄ · 6H₂O. These CD bands are due to combination bands of H₂O vibrations in the crystals. Substitution of H₂O with D₂O on α‐NiSO₄ · 6H₂O shifts the 5000 cm⁻¹ band to a frequency lower by the expected factor of $\sqrt{2}$ . The 5000 and 4050 cm⁻¹ CD bands are also observed in α‐ZnSeO₄ · 6H₂O with the same intensities as in α‐NiSO₄ · 6H₂O. Their positions are, however, shifted by 50 cm⁻¹ toward higher energy. Due to strong absorption, the CD of fundamental vibrational modes of H₂O in the crystals could not be measured. The CD is interpreted as a consequence of the linear k dependence of the effective charge of E mode optical phonons near k = 0.