Raman intensity de‐enhancement in nontotally symmetric vibrations of copper(II) acetate by forbidden ligand‐field transitions

The Raman spectrum of the binuclear transition metal complex copper(II) acetate monohydrate is examined with various excitation wavelengths in the spectral range 17 500 to 25 000 cm−1. Nontotally symmetric vibrations are predominant in the Raman spectrum, and the depolarized Raman bands observed at 320 and 948 cm−1 exhibit marked intensity de-enhancement as the excitation wavelength approaches the dimer-associated, Laporte-forbidden ligand-field transition at 26 500 cm−1. The intensity de-enhancement observed for the nontotally symmetric vibrations of copper(II) acetate is in contrast to that observed for other centrosymmetric transition metal complexes studied thus far in which only the totally symmetric vibrations exhibit antiresonance effects. The excitation profiles are calculated for the Raman bands of copper(II) acetate using a model in which the scattering intensity arises from two terms Iα|Be+Bf|2, where Bf couples two charge-transfer states at 35 000 and 40 000 cm−1 respectively, and Be couples the ligand-field state to the charge-transfer state at 35 000 cm−1. The calculated profiles are in good agreement with those observed and predict that the system origin for the ligand-field transition is located near 21 000 cm−1.

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