Local site symmetry

Other features in the absorption spectrum are due to local site symmetry about the absorber that dictates the intensity of the various molecular transitions. Figure 2 compares two compounds, VgO and V naphthenate. The considerable differences in peak intensities, particularly in the pre-edge absorption at "5465 eV, are due to symmetry changes resulting from different local geometries. 

For isomer counting, the problem to be solved is to determine the number of distinct isomers corresponding to decoration of atoms in a parent molecule. Substitution of an atom by another isotope of the same element or by a different chemical species, addition of a structureless ligand or functional group oriented so as to preserve the local site symmetry, all are to be treated as aspects of the same decoration process. 

The first high temperature phase transition from the Cap2 is displacive, and is therefore a classic cooperative J-T distortion in which d-state degeneracies are lifted when the basal plane distortions are taken into account [7,8]. The second transition represents a further and more significant reduction in local site symmetry. This symmetry is sufficiently low so that all d-state degeneracies are lifted, and in addition the local coordination of Hf is reduced from eightfold to sevenfold [9]. The definition of a cooperative J-T effect in [7] clearly includes the monoclinic phases of Hf02 and... 

We apply an approach based on the local site symmetry that has been used in classic texts on molecular orbital theory [11,12], and subsequently addressed in a more formal way for TM bonding in octahedral and tetrahedral arrangements, as for example in Ti02, and in cubic titanate and manganite perovskites by F.A. Cotton [13]. 

High pressure applied to a-FcaOs causes the expected pressure-induced change in the second-order Doppler shift [9]. The electron density at the iron is effectively independent of pressure. However, the sign of the quadrupole splitting reverses at 30 kbar, the data suggesting that, in addition to a spin-flip transition, there is also some alteration in the local site symmetry. 

The fluorescence spectra of the europium adducts also provide information about the geometry of the complex. The lower the local site symmetry about the europium ion, the more numerous the lines in the spectrum. This fine structure results from internal Stark splitting of the ionic levels. The maximum number of lines into which each transition will be split as a function of geometry has been tabulated by several authors (II, 15, 32). The observation of a Do- Do transition for all the complexes reported indicates that none possesses a center of symmetry. 

Heteropolyanion structures in which the local site symmetry of each metal atom is approximately C41, (i.e. each metal has one terminal oxygen) frequently undergo partial hydrolysis to lose an group and generate a so-called lacunary structure, see for example... 

A certain amount of MCD work has been carried out for lanthanides in aqueous solution, from which a number of deductions can be made about the local site symmetry (Gbrller-Walrand and Godemont 1977a,b Gorller-Walrand et al. 1982). 

The methods described above yield greatly enhanced resolution, and as such help to determine the types and amovmts of distinct sites in a material. This resolution gain comes at the price of discarding the information available from the interaction anisotropies. This information typically relates to the local site symmetry, for example, distortions in the bond angles, number of nearest neighbors, and so forth. 

Fig. 8.4 AIMASNMR spectrum fm BagAli4Si3i. The figures show the local site symmetry of the framework atoms with the 6c, 16i, and 24 k indicated by blue, purple and green, respectively. The cation atoms ate indicated by orange and yellow for the 2a and 6d sites, respectively (Reprinted with permission from [36]. Copyright 2006, American Chemical Society.)...

T.J. Bastow, C.T. Forwood, M.A. Gibson, M.E. Smith, Local site symmetry and electronic structure of trialuminide and related intermetaUic alloys probed by sohd-state NMR, Phys. Rev. B 58 (1998) 2988-2997. 

---