Coordination symmetry

Chemical shift anisotropy 5.5,5 xxr yy zz Line-shape analysis MAS-sidebands Coordination symmetry... 

Nuclear electric quadrupole QCC (quadrupole coupling constant), (asymmetry parameter) Line-shape analysis, nutation NMR Coordination symmetry... 

Figure 2.9 Schematic structure of two dif- (d) cube. The dpp parameter defines the aver-ferent polyoxometalate complexes with inter- age distance between the two oxygen-based esting coordination symmetry (a) LnW10 and square planes. The djn is the average 0-0 (b) [LnPd"12(AsvPh)8032]5-, and their coor- distance within the oxygen-based square dination polyhedra (c) square antiprism and planes.

A number of other spectroscopies provide information that is related to molecular structure, such as coordination symmetry, electronic splitting, and/or the nature and number of chemical functional groups in the species. This information can be used to develop models for the molecular structure of the system under study, and ultimately to determine the forces acting on the atoms in a molecule for any arbitrary displacement of the nuclei. According to the energy of the particles used for excitation (photons, electrons, neutrons, etc.), different parts of a molecule will interact, and different structural information will be obtained. Depending on the relaxation process, each method has a characteristic time scale over which the structural information is averaged. Especially for NMR, the relaxation rate may often be slower than the rate constant of a reaction under study. 

Fig. 4. Schematic ligand-field splitting patterns of rf-orbitals in various coordination symmetries...

ESEM results on the interaction of silica-exchanged Cu(II) with a range of adsorbates showed that one or two adsorbate molecules were able to coordinate to the Cu depending on the chemical interaction, polarity and size (7[2). Differences in A, were observed for N- and O-coordinated ligands, but these seem to reflect a change in coordination symmetry and not a difference in adsorbate ligand number. N-coordinated ligands form approximately square planar... 

It is clear from the above observations that pyridine molecule interacts on the catalyst surface in the following three modes (1) interaction of the N lone pair electron and the H atom of the OH group, (2) transfer of a proton from surface OH group to the pyridine forming a pyridinium ion (Bronsted acidity), and (3) pyridine coordination to an electron deficient metal atom (Lewis acidity). Predominant IR bands, vga and vigb, confirms that the major contribution of acidity is due to Lewis acid sites from all compositions. Between the above two modes of vibrations, Vsa is very sensitive with respect to the oxidation state, coordination symmetry and cationic environment [100]. A broad feature for v a band on Cu containing... 

The electronic spectrum (36) of the pol5uner is dominated by a very broad ultraviolet band, with shoulders at 280 and 470 m/t, which tails into the visible region and is responsible for the deep brown color of the polymer. Very weak crystal field excitations are found at 640 and 880 m. From the latter transition one can estimate that for high-spin Fe +, Dq = 1100 cm i. This value is typical of Fe3+ in octahedral coordination with oxygen ligands, but the X-ray evidence (see below) indicates that the coordination is tetrahedral, so that Dq seems anomalously high. However, the coordination symmetry is actually lower than tetrahedral, since both hydroxide and oxide ligands are involved. 

Thus we can directly deduce the XANES spectrum of the product state from the measured transient signal and the reactant state XAS, if we knowXO Alternatively, we can derive fit), if we know the exact shape of the product state XAS, P(E,t). The details are given in ref. 14. The resulting spectrum for the [Ruln(bpy )(bpy)2]2+ species is shown in Fig. 4a (trace P). It contains an energetic shift of all features by 1.2 eV, together with the photoinduced appearance of the A feature, as expected. The A-B splitting (4 eV) and the B /A intensity ratio (ca. 2.3) is indeed close to the values observed for [Ru"(NH3)6]3+ complex [18,20], which has the same valency and a similar coordination symmetry (Oh versus D3) as the bipyridine complex. 

The situation becomes more complicated, however, when each oxygen belonging to an Fe coordination polyhedron is bound to different populations of next-nearest cations in their own coordination sites as a result of extensive atomic substitution of Fe2, Fe3, Ti4, etc., for Mg and Al. Such chemical and bonding variations for different mineral structures and the variability of inductive effects from adjacent cations are probably sufficiently strong to account for large ranges of isomer shifts exhibited within a specific coordination symmetry in silicate minerals. 

The valence and coordination symmetry of a transition metal ion in a crystal structure govern the relative energies and energy separations of its 3d orbitals and, hence, influence the positions of absorption bands in a crystal field spectrum. The intensities of the absorption bands depend on the valences and spin states of each cation, the centrosymmetric properties of the coordination sites, the covalency of cation-anion bonds, and next-nearest-neighbour interactions with adjacent cations. These factors may produce characteristic spectra for most transition metal ions, particularly when the cation occurs alone in a simple oxide structure. Conversely, it is sometimes possible to identify the valence of a transition metal ion and the symmetry of its coordination site from the absorption spectrum of a mineral. 

Coordination symmetry of iron and cobalt in staurolite The crystal field spectra of Fe2+ ions surrounded by oxygen in regular octahedral sites normally contain absorption bands centred near 1,000 nm or 10,000 cm-1 (see fig. 3.2). By changing from octahedral to tetrahedral coordination, absorption bands for tetrahedral Fe2+ ions would, according to eq. (2.7), be predicted to occur at (% x 10,000) or 4,444 cm-1 (2,250 nm) if the iron-oxygen distances remain identical in the two coordinations. Tetrahedrally coordinated Fe2+ ions in spinel, MgAl204, for example, produce an absorption band near 4,830 cm"1 (2,070 nm) ( 5.3.3). 

Molecular mechanics as a minimization of strain energy makes a rigid distinction between steric and electronic effects. Electronic effects are introduced in the form of empirical constants such as characteristic bond lengths and angles, the corresponding force constants, torsional rigidity of even-order bonds, planarity of aromatic systems and the coordination symmetry at transition-metal centres. These constants are accepted, without proof, to summarize the ensual of electronic interactions and used without further optimization. 

Polarisation lowers the energy of anions in layer and chain structures because generally in these the anions are in asymmetric environment and they experience a strong net electric field from neighbouring ions. In such a situation polarisation stabilises an ion. This is not possible in compounds with coordination symmetry. The occurrence in layer structure of disulphide and dichloride may be attributed to stabilisation effect. The stabilisation is not possible in oxides and fluorides (smaller anions). 

If the bonding forces are directional, as in a covalent crystal, the energy minimum is attained upon the saturation of the directional covalent bonds. In these structures, atoms have relatively few neighbors, as, for example, in diamond and NiAs. The symmetry of these structures is defined by the coordination symmetry of the constituent atoms. It is interesting to note that the volume per atom in structures with different types of bonds differs only slightly, on the average, since the covalent bonds are usually slightly shorter than the ionic or metallic ones. 

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