Knowledge Bank

Vibrational circular dichroism spectra have been obtained for $\Delta$-and A-bis ( acetylacetonato) (L-alaninato) cobalt(III) in the hydrogen stretching region between 3400 and $2800cm-1$ and the mid-infrared region between 1600 and $1180cm-1$. The bisignate VCD spectrum centered at $1522cm-1$, due to the doubly degenerate antisymmetric CC stretching mode of the acetylacetonato rings, is interpreted on the basis of the coupled oscillator intensity mechanism. Remaining VCD features are explained primarily in terms of vibrational currents in rings formed by an intramolecular hydrogen bond or transition metal ligation. It is shown, based on the appearance of a large negative VCD band in the NH stretching region of the $\Delta$-complex, which does not appear in the $\Delta$-complex, that an intramolecular hydrogen bond forms between the L-alaninato ligand and one of the acetylacetonato ligands in the $\Delta$-complex, but forms only weakly if at all in the $\Delta$-complex.