Structure property relationships in Prussian Blue analogues and hydrogen bond mediated metal complexes.
PhD thesis, University of Glasgow.
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Within this work the effects of metal identity are investigated on the magnetic properties of a range of materials, in which different transition metals are shown to produce significantly different calculated magnetic coupling values when incorporated into the same structure. The model structures composed of transition metals mediated by hydrogen halide ligands of the form FHF-, ClHCl- and FHCl- enable the calculation of magnetic coupling values via hydrogen bonds (HBs), providing insight into a scarcely studied topic. The employment of the well known HF=35% functional, provides results anticipated to represent potential experimental trends. Interesting results are also reported which indicate a relationship between the binding energy of the atom and the level of coupling displayed. The quantum nature of the H atom, often difficult to localise, are also accounted for by use of a shooting algorithm in order to solve the one dimensional Schrödinger equation in relation to the hydrogen positions within the isotropic lattice. Hydrogen positions were fixed during geometry optimisations in order to obtain the E(x) potential energy curve required to allow this to be achieved. Both the <J> in which proton motion is incorporated and J coupling values from the optimised ground state structures, in which the atomic positions including the H positions underwent a full geometry optimisation, are presented.
The electronic effects of incorporating different group one and group two metals into the related Prussian Blue (PB), Prussian Yellow (PY) and Prussian White (PW) model lattices are also presented. More specifically band gaps within these complexes are calculated from projected density of state (DOS) plots of the electrons within the atomic orbitals from a converged calculation in which the B3LYP functional (HF=20%) is employed. This functional has previously been shown to provide band gap energies in good agreement with experimental values. The results obtained confirm that the value of ~3eV, associated with an absorption in the yellow region, lies within the PY band gap range, indicated by the HOMO and LUMO orbitals within the DOS plot. Band gaps calculated directly from SCF energy differences do not agree well with experimental values or trends for the various group one PB complexes.
A number of related metal containing bromanilic acid (BA) and chloranilic acid (CA) complexes are synthesised, crystallised and their structures, as determined by X-ray single crystal diffraction, reported. The structural packing of the molecules is analysed by defining a number or recurring unit within the structures in order to obtain key similarities. The structures are also broken down in to lower dimensional units composed of the individual components within the asymmetric unit and built up into higher dimensional units such as clusters, chains and planes that all intersect with one another in order to produce the overall three dimensional structures. In most cases the point of intersection is on a key symmetry element such as an inversion centre.
Particularly short M-M contact distances are observed in a number of the complexes. This proves interesting within the related isomorphous complexes in which the main effect of incorporating a larger metal cation into the same structure is in the ionic repulsions between the metals. This is particularly interesting in more extended structures in which the metals exist in chains or planes that run along a particular direction of the unit cell. The specific increase in cell parameter can in most cases be explained as a consequence of the repulsion between the shortest M-M contacts. The effect is also observed in structures in which the metals exist as isolated clusters, and also displays some interesting consequential effects on other interactions.
A variable pressure neutron single crystal diffraction experiment on a short asymmetric HB is also reported, in which one lattice is compressed significantly more than the other two, an effect that is explained by the change in contact distances with pressure.
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