Resonance Effects in the Raman Optical Activity Spectrum of [Rh(en)₃]³⁺

Raman optical activity spectra of Λ-tris­(ethylenediamine)-rhodium­(III) {[Rh­(en)₃]³⁺} have been calculated at 16 on-, near-, and off-resonant wavelengths between 290 and 800 nm. The resulting spectra are analyzed in detail with a focus on the observed resonance effects. Because several electronically excited states are involved, the spectra are never monosignate, as is often observed in resonance Raman optical activity spectra. Most normal modes are enhanced through these resonance effects, but in several cases, de-enhancement effects are found. The molecular origins of the Raman optical activity intensity for selected normal modes are established by means of group coupling matrices. In general, this methodology allows one to produce an intuitive explanation for the intensity behavior of a given normal mode. However, due to the complex electronic structure of [Rh­(en)₃]³⁺, there are some intriguing resonance effects the origins of which could not be fully clarified in terms of group coupling effects. Therefore, simple and general rules that predict how the intensity of a specific normal mode is affected by resonance effects are difficult to devise.