Where does the Raman optical activity of [Rh(en)₃]³⁺ come from? Insight from a combined experimental and theoretical approach

Abstract

Backscattered Raman optical activity (ROA) spectra are measured for Δ-and Λ-tris-(ethylenediamine)rhodium( chloride in aqueous solution. In addition, the spectra of the four possible conformers in the Λ configuration are investigated by ab initio calculations. The Λ() conformer is in best agreement with experimental spectra and examined in more details. The two most stable conformers according to the calculations are not compatible with the experimental ROA spectrum. Insights into the origin of observed band intensities are obtained by means of group coupling matrices. The influence of the first solvation shell is explored via an ab initio molecular dynamics simulation. Taking explicit solvent molecules into account further improves the agreement between calculation and experiment. Analysis of selected normal modes using group coupling matrices shows that solvent molecules lead to normal mode rotation and thus contribute to the ROA intensity, whereas the contribution of the Rh can be neglected.

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

“…After having analyzed the electronic excitations playing a role in the (resonance) ROA spectra, we are now in a position to investigate these spectra. First, we compare our results to the ones obtained by Humbert-Droz et al 19 In Fig. 3, we show both the experimental as well as the theoretical off-resonance spectrum obtained by these authors and compare it to the off-resonance spectrum obtained in this work using an excitation wavelength of 800 nm.…”

“…In their study, 19 Humbert-Droz et al analyzed in detail the different configurations and conformations possible for [Rh(en) 3 ] 3+ . Therefore, we will not attempt to repeat such an analysis in this work.…”

“…The apparent ruggedness of the two spectra taken from Ref. 19 is in part due to the fact that it was extracted directly from the paper by Humbert-Droz et al, 19 which introduced minor artifacts. Also note that Humbert-Droz et al adopted a different computational methodology; most notably, instead of the PBE0 density functional, they used the B3LYP [43][44][45] exchange-correlation functional (as implemented in Gaussian 46 ).…”

“…Thereby it also complements the detailed study published by Humbert-Droz et al on the same compound, which focused exclusively on the off-resonance case. 19 This paper is organized as follows: First, we give a short introduction into the theoretical background of the calculation of (resonance) ROA spectra. Then, in Section 3, we explain the computational methodology followed in this study.…”

Localized molecular orbitals for calculation and analysis of vibrational Raman optical activity

“…Whereas proteins [15][16][17][18][19][20][21][22], carbohydrates [23][24][25], and polymers [26,27] have been studied more frequently (see reviews in Refs. [28][29][30][31][32]), the field of chiral metal complexes is a relatively new field of research [22,[33][34][35][36]. A review about ROA of coordination compounds and solids can be found in Ref.…”