The electronic absorption and magnetic circular dichroism spectra of the hexaquochromium(III) ion

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Abstract
The absorption and magnetic circular dichroism (MCD) spectra of a single crystal of [K(H2O)6][Al(Cr)(H2O)6](SO4)2, with an aluminium : chromium ratio of approximately 10 : 1, have been measured in the range 14 500–33 000 cm-1 at various temperatures between 5 and 300 K. An analysis of the temperature dependence of the zeroth moment of absorption and first moment of MCD for the spin-allowed transitions 4 A 2g → 4 T 2g and 4 A 2g → 4 aT 1g shows that the vibrational modes responsible for the greater part of the absorption intensity are of t 1u symmetry. The magnitude of the temperature-dependent contribution to the first MCD moment also indicates that the spin-orbit splitting of both 4 T configurations is much smaller than expected, and possible origins of this phenomenon are discussed. The relatively large zeroth moments which dominate the MCD spectrum even at low temperatures are shown to arise from second-order interactions between the 4 A 2g and 4 T 2g states. The spin-forbidden transitions 4 A 2g → 2 aEg and 4 A 2g → 2 aT 2g exhibit complex vibronic fine structure, whose interpretation is discussed.