The electronic absorption and magnetic circular dichroism spectra of cobalt (II) bromate hexahydrate

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Abstract
The magnetic circular dichroism (M.C.D.) and absorption spectra between 14 000 and 37 000 cm-1 of a single crystal of cubic [Co(H2O)6] (BrO3)2 have been measured at various temperatures between 5 and 300 K. The most important contributions to the M.C.D. b and c parameters of the vibronic d-d transitions of the cobalt (II) hexaquo ion have been calculated and an assessment of the magnetic dipole contributions has been made. The form of the temperature variation of the dipole strength and the signs of the M.C.D. c-terms show that the intensities of the 4 a T 1g → 4 A 2g and 4 b T 1g transitions are provided mainly through the two cobalt-oxygen t 1u vibrations. Semi-quantitative calculations of b and c for the latter transition are in reasonable agreement with the experimental values. A comparison of the spectra with ‘complete’ crystal field/spin-orbit coupling calculations leads to an assignment of many spin-forbidden bands. It is shown in particular, that the two relatively intense bands above the 4 a T 1g → 4 b T 1g transition arise from strong second-order spin-orbit interactions between the components of 2 b T 1g and 4 b T 1g .

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