Vibrational circular dichroism in transition-metal complexes. 3. Ring currents and ring conformations of amino acid ligands

Effects of electron configuration and coordination number on the vibrational circular dichroism spectra of metal complexes of trans-1,2-diaminocyclohexane

“…16 Very recently, Nicu et al revisited these spectra and showed that the strong effects on the observed VCD arise from the charge transfer that is inherent in the donor-acceptor type of interactions between the Cl À counter-ions and the N-H bonds. 17 Other metal complexes that have been studied using VCD spectroscopy are, for instance, amino acid complexes of cobalt(III), 18,19 complexes with p-donor ligands, 20,21 and the tris(b-diketonato) complexes of transition metals. The latter were systematically studied by Sato and co-workers with regard to the influence of metal ions on the VCD spectra, 22 as well as the spectral differences among several isomers of mixed-ligand complexes.…”

Symmetry-Dependent Vibrational Circular Dichroism Enhancement in Co(II) Salicylaldiminato Complexes

“…have intrinsically strong VCD signals, often referred to as enhanced VCD. ,, In a different and more general approach, one may conceive a moiety capable of generating enhanced VCD signals when linked to a molecular or biomolecular species, thus functioning as a local structural probe. − The smallest imaginable probe is a metal ion bound to a ligand, whose (usually weak) VCD signals may be enhanced upon binding. In fact, Co­(II), ,− Co­(III), − Ni­(II), ,, Cu­(II), other M­(III) (M = metal), , and several Ln­(III) ions (Ln = lanthanide) , have been demonstrated to be endowed with VCD-enhancing properties. The common feature of all mentioned metal complexes is the presence of unpaired d or f electrons responsible for low-lying electronic states (LLESs).…”

“…The common feature of all mentioned metal complexes is the presence of unpaired d or f electrons responsible for low-lying electronic states (LLESs). The currently accepted theory proposed by Nafie in 2004 relates the VCD enhancement to the communication between ligand-centered vibrational transitions of the ground-state and metal-centered LLES, , which requires going beyond the Born–Oppenheimer approximation. , LLES theory has replaced previous interpretations based on ring current theory − , and on the Fano-type interference mechanism. ,, …”

A novel correlation of vibrational circular dichroism spectra with the electronic ground state for Δ-SAPR-8-cesium-tetrakis((+)-heptafluorobutyryl-camphorato)lanthanide(iii) complexes

“…§ The VCD couplet at 1550 cm À1 for the D-Cs-Ln complexes of the group (A) appears to originate from the exciton-like interaction described by the coupled oscillator mechanism between the C-C stretching modes in the chelates as claimed for D-[M(acac) 3 ] complexes. 20 Thus, a negative-positive VCD couplet observed for the group (A) complexes ensures the stereospecifically formed D-SAPR-C 4 -8 absolute configuration. [11][12][13][14][15][16][17] This is supported by the DFT calculation with the structure optimization of the D-SAPR-Cs-Lu complex (Fig.…”

“…In terms of the coupled oscillator mechanism, 20,21 the disruption of the oppositely signed couplet may arise from the achiral disposition of the transition dipoles (chelates) and/or lengthening of the dipole-dipole (chelate-chelate) distance which cause weakening of the VCD intensities. However, this is not the case, since the positive VCD doublet intensities at 1550 cm À1 for the group (B) are as strong as the couplet intensities for the group (A) and the chiral absolute configuration for the group (B) is the same as that for the group (A) as observed in the ECD spectra.…”