Theory of natural optical activity in crystalline Cu²⁺:ZnSeO₄.6H₂O

Abstract

The chiroptical properties associated with the d-d transitions of Cu²⁺ ions in crystalline Cu²⁺:ZnSeO₄.6H₂O are examined on a theoretical model which is based on a crystal-field representation of the Cu²⁺ d-d states, and which includes explicit consideration of pseudo Jahn-Teller distortions within the manifold of Cu²⁺ ligand field states. In Cu²⁺:ZnSeO₄.6H₂O, the Cu²⁺ ions are located at sites of C ₂ symmetry. However, the six oxygen atoms (from H₂O) coordinated directly to Cu²⁺ form a near perfect octahedron so that the ‘crystal field’ as felt by the Cu²⁺ 3d electrons deviates only slightly from cubic (O_h ) symmetry. This results in a ‘nearly degenerate’ electronic ground state which is subject to pseudo Jahn-Teller distortion. Calculations are carried out which incorporate both the crystal field and vibronic coupling (pseudo Jahn-Teller interactions) aspects of the fundamental theoretical model. The signs and magnitudes of the computed rotatory strengths are in qualitative agreement with experimental observations, and the temperature dependence observed for the ‘net’, or total, rotatory strength associated with the Cu²⁺ d-d transitions can be accounted for by the existence of vibronic coupling between the nearly degenerate components of the electronic ground state.