Theory of Natural Optical Activity in Crystalline NiSO4· 6H2O

The optical activity of crystalline NiSO4· 6H2O is exclusively a property of the solid state, arising from a dissymmetric crystal environment. The rotatory strengths and dipole strengths associated with electronic transitions within the Ni2+ 3d manifold of states are calculated to second order in perturbation theory. It is assumed that optical activity is induced in the ligand-field bands of the octahedral NiO6 clusters by the perturbing influence of the neighboring crystal environment which has C2 symmetry. A crystal-field representation is adopted so that the crystal perturber sites (the H atoms of the water ligands, and O and S atoms of the SO42− anions) are treated as point charges ``seen'' by the NiO6 chromophoric sites. The signs and relative magnitudes of the rotatory strengths calculated for the 3A2g→3T2g, 3A2g→3T1g(F), and 3A2g→3T1g(P) cubic transitions of Ni2+ are in substantial agreement with experiment. It is concluded that the CD spectrum of crystalline NiSO4· 6H2O in the near infrared, visible, and near ultraviolet regions can be understood in terms of a model on which the optical activity is assumed to arise from dissymmetric perturbations of individual Ni2+ sites rather than from chiral exciton motion over the helically arranged Ni(H2O)62+ clusters.