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

Abstract

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 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.