Coupling between normal coordinates in the ground and excited states of coordination compounds. Electronic spectroscopy and theoretical models

Effects of coupling between normal coordinates can be analyzed from absorption and emission spectra. The spectroscopic phenomena reviewed here arise from interactions between electronic states with different equilibrium structures, a situation that leads to coupled coordinates. They have received some attention in the past for special categories of coordination compounds, such as Jahn Teller active high-symmetry transition metal complexes and mixed valence compounds. We use model systems outside these established categories and illustrate the spectroscopic effects with calculations involving two normal coordinates and two electronic states. These models show directly how coordinate coupling arises as a consequence of the non-crossing rule for states of identical symmetry and they are easily generalized to other molecular systems with energetically close electronic states. Electronic structure calculations are presented to further illustrate the effects of coupling. The examples analyzed are based on experimental results reported for trans-dioxo complexes of rhenium(V) and chromium(III) fluorides. (C) 2002 Elsevier Science B.V. All rights reserved.