Varying spin state composition by the choice of capping ligand in a family of molecular chains: detailed analysis of magnetic properties of chromium(III) horseshoes

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Electronic and Molecular Structure of Transition Metal Complexes

The work of the group is centred on the interplay between the electronic and molecular structure of transition metal complexes. Current projects address the role of Jahn-Teller coupling in determining the structural and magnetic properties of complexes with anisotropic ligands such as water and imidazole. We seek to demonstrate that in order to obtain a satisfactory understanding of metal–water and metal–imidazole interactions, which are central to the functionality of the majority of biological systems, both the electronic and vibrational co-ordinates of the system must be considered

Metal(III)-water interactions

This thesis is devoted to the study of metal(III)-water interactions by a variety of physical techniques. Studies are undertaken in the solid state where the tervalent hexa-aqua-cation is held rigidly within the lattice, thus facilitating its study. The RbMᴵᴵᴵ(SO₄)₂.12H₂O alums, where Mᴵᴵᴵ is Al, Ga, In, Ti, V, Cr, and Fe, have been characterised by Raman spectroscopy. The Ti and V rubidium alums are of a different structural modification to the other alums studied. This is related to a change in the mode of water co-ordination leading to a larger trigonal field stabilisation energy for the Ti and V hexa-aqua-cations. The Raman spectra of the RbTi(SO₄)₂.12H₂O alum are anomalous. Soft modes are observed indicating the onset of a phase transition. The Raman spectra are interpreted in conjunction with the published EPR and magnetic data for the [Ti(OH₂)₆]³⁺ cation. A model in which the [Ti(OH₂)₆]³⁺ cation is subject to a dynamic Jahn-Teller distortion, freezing out into a static distortion at temperatures approaching 4 K, is proposed. A polarised neutron diffraction experiment has been performed on CsMo(SO₄)₂.12D₂O. This is the first report of such a study for any tervalent hexa-aqua-cation. Information on the spatial spin distribution within the [Mo(OD₂)₆]³⁺ cation is obtained. Spin density is found to occupy t2g-like orbitals on the metal with some spin transferred to the ligand where it is concentrated in a molecular orbital normal to the plane of the water molecule. Negative spin is also found in the molybdenum-oxygen bonding region indicative of electron correlation effects. These observations are interpreted in terms of simple concepts of chemical bonding

Constructing, solving and applying the vibronic Hamiltonian

The Jahn–Teller effect is shrouded in mysticism and cynicism. To paraphrase a remark that a colleague recently relayed, “For every anomalous spectrum, structural distortion or novel physical property, there is a vibronic Hamiltonian and ensuing explanation that few can appreciate or comprehend.” The aim of this article is to provide a basic introduction to the Jahn–Teller effect, pitched at a level that undergraduates in chemistry can understand, with an emphasis on how to calculate a given experimental quantity. We show that armed with just a little group theory and matrix mechanics, vibronic Hamiltonians can be readily constructed, solved, and the molecular property of interest extracted from the eigenvalues and eigenfunctions. The manifestation of the Jahn–Teller effect does indeed come in many shapes and forms, three signatures of which are briefly discussed. (1) The vibronic energy spectrum is best revealed by spectroscopy and two examples are taken from the literature that elucidate the intricate energy-level pattern of the E ⊗ e vibronic interaction. (2) ‘The Ham effect’, ‘Ham factors’ and ‘Ham quenching’ are now common parlance in spectroscopy and the phenomenon is aptly illustrated by the magnetic and spectroscopic data of the titanium(III) and vanadium(III) aqua ions. (3) The plasticity of the co-ordination sphere is the quintessential feature of transition metals exhibiting strong Jahn–Teller coupling. We show how a concomitant description of structural and spectroscopic data can be obtained employing a model in which the potential energy surface resulting from the cubic Jahn–Teller Hamiltonian is perturbed by anisotropic strain

Vibronic Interactions and the Jahn-Teller EffectTheory and Applications /

XXII, 446 p.online resource