Multiplet structures properties

A tremendous number of various fragments are used in structure-property studies atoms, bonds, topological torsions , chains, cycles, atom- and bond-centered fragments, maximum common substructures, line notation (WLN and SMILES) fragments, atom pairs and topological multiplets, substituents and molecular frameworks, basic subgraphs, etc. Their detailed description is given below. 

The actinide element series, like the lanthanide series, is characterized by the filling of an f-electron shell. The chemical and physical properties, however, are quite different between these two series of f-electron elements, especially in the first half of the series. The differences are mainly due to the different radial extension of the 4f- and 5f-electron wavefunctions. For the rare-earth ions, even in metallic systems, the 4f electrons are spatially well localized near the ion sites. Photoemission spectra of the f electrons in lanthanide elements and compounds always show "final state multiplet" structure (3), spectra that result from partially filled shells of localized electrons. In contrast, the 5f electrons are not so well localized. They experience a smaller coulomb correlation interaction than the 4f electrons in the rare earths and stronger hybridization with the 6d- and 7s-derived conduction bands. The 5f s thus... 

The implicit assumption in the previous model is that atomic structure has little influence on the degree of / electron localisation in the solid. There are difficulties in reconciling this view with any persistence of atomic multiplet structure, and with the fact that changes of localise tion are clearly associated with certain regions of the Periodic Table. For this reason, various attempts have been made to explore what properties of atoms might survive in solids and, perhaps, provide a quasiatomic mechanism to drive changes of valence. 

Finally we note that there exist techniques for the measurement of long-range coupling constants derived from the HSQC experiment that aim at producing multiplet structures devoid of the complex phase properties associated with the HMBC method [86]. In the ideal case, crosspeak multiplets contain only in-phase proton homonuclear couplings and the anti-phase heteronuclear couplings of interest such that "7ch values can be measured directly from these anti-phase splittings. A number of refinements of the HSQC experiment are required for this to be the case [87, 88], and whilst the direct... 

In sect. 1, we considered how particular mechanisms affect the behaviour of d and f orbitals in free atoms. We now turn to the question of how these atomic properties are modified when dealing with the atom in the solid. A crucial aspect of rare earth physics is the persistence of quasi-atomic spectral multiplet structure in the solid. To explain this, we first note that a collapsed orbital, localized in the inner reaches of the atom, will tend to survive as a localized atomic orbital in the solid, whereas an orbital which lies in the outer reaches of the atom (or in the outer well of the double-well potential described above) will be completely modified, and may hybridize with the conduction band. 

In these oxides, the 6s and 5d and, to a lesser extent, the 4f electrons of the rare earth atom are mainly responsible for electrical transport and structural properties, whereas the localized 4f electrons govern the magnetic properties. X-ray absorption spectroscopy (XAS) offers the important advantage of simultaneously probing the 4f and the ds conduction states in these oxides. In XAS, the dipole selection rules are strictly obeyed and this facilitates the identification of the spectral features. Generally, the 3d—>4f (Mjv-v) or 4d—>4f (Niv-v) absorption transitions are studied. In these absorption processes the excited 4f electron participates directly in the transition. The resulting multiplet structure is observed to provide a finger-print of the 4f population of the rare earth atom. The modification in the valence band electron distribution introduced by the delocalization of a 4f electron is probed by the transition of a 2p (Ln-m) electron in the vacant sd conduction states. In this case the 4f electron does not participate direct in the transition. 

Another feature evident in Table 11 is that the virtually similar magnetic properties of chemically related complexes may nevertheless invoke essentially different conditions of the electron spin lattice relaxation. Thus, while complex provides the frequent picture of a widely spaced first-order spectrum devoid of any multiplet structure (except, in part, for H ), complexes displays a clean first-order spectrum in which the individual multiplet patterns of all equivalent H atoms (including Hq ) remain. One important reason for this (for mono-substituted cyclohexanes) rather exceptional feature is probably that, owing to steric congestion, the cyclohexyl ring in cannot invert, even around room temperature, between its two chair conformations carrying the N atom in either the equatorial or axial a-position. The very informative nature of the NMR spectrum of the CeHii part of has made it possible to unequivocally assign the complete spectrum. 

X-ray diffraction work (11,15) shows that there is an ionomer peak at 4°C which is absent in the acid precursor. This low, broad peak is not affected by annealing or ion type and persists up to 300°C. Since the 4°C peak corresponds to a spacing of about 2.5 nm, it is reasonable to propose a structural feature of this dimension in the ionomer. The concept of ionic clusters was initially suggested to explain the large effects on properties of relatively sparse ionic species (1). The exact size of the clusters has been the subject of much debate and has been discussed in a substantial body of literature (3,4,18—20). A theoretical treatment has shown that various models can give rise to supramolecular structures containing ionic multiplets which are about 10 nm in diameter (19). 

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