Acentric vibrational modes

Figure 3.15 Vibrational modes of an octahedron. Note the acentric modes t1b and x2u which aid vibronic coupling.

Figure 3.20 Vibrational modes, u, to u7, of atoms in a square planar site such as the [FeOJ coordination environment in gillespite (from Hitchman, 1985). Note the acentric a2u, p2u ar>d eu modes that facilitate electronic transitions within Fe2+ ions by vibronic coupling.

The CLF intensity model (Chapter 7.11) parametrizes local transition moments using parameters. For each M—L bond, parity mixing is deemed to arise from A = ligand-based functions. For acentric complexes, the local transition moments are appropriately summed to yield global moments and polarizations. For vibronically sourced intensity, the local moments are multiplied by the ligand displacements for each vibrational mode and then summed. If these displacements are known, from a prior normal coordinate analysis, the contributions from different modes to the total static- and vibronic-sourced intensity is set and no extra parametrization is introduced. The model can then be used for both acentric, near-centric, and centric complexes and the relative contributions are decided computationally by the geometry and vibrational displacements. If the vibrational displacements are not known then the relative contributions from different modes and the relative static-to-vibronic intensities are parametrized. 

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