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Symmetry site terms

The CHEMiCAL ABSTRACTS stereochemical notation system is composed of four parts the site symmetry term, the configuration number, the chirality label, and the ligand stereochemical label. [Pg.406]

The site symmetry term indicates the idealized geometry of the central atom and is a letter code derived from the geometric terms used to describe the coordination polygon. The coordination number is given to define further the site symmetry... [Pg.406]

The utility of the CAS notation to describe the stereochemistry of coordination compounds is illustrated in the following examples. The first example of the complete notation is the cobalt compound to which the CIP priority numbers have been assigned previously (see Figure 12). The cobalt is an octahedral-six coordinate thus, the site symmetry term is 0C-6. [Pg.412]

The next example (see Figure 13) is the tris(ethylenedia= mine)cobalt(III) complex. The site symmetry term is octahedral, six coordinate or OC-6, and the configuration number is 11. The nitrogen atoms are all equivalent, and the chelation defines a right-handed or A helix to give the complete notation 0C-6-11-A. I have indicated the primes on the chelating ligands in this structure to illustrate how to determine the... [Pg.412]

Inspection of Equation 1.23 and consideration of the properties of 3-y and 6-j symbols confirm that only even A--values contribute to crystal field splitting. Further, it indicates that mixing between levels belonging to different / multiplets can only occur if terms with k site symmetry of the lanthanide, in much the same way as discussed above for the Stevens formalism. [Pg.14]

Electronic Structure Determination, Cox and Beaumont have studied the polarized x-ray absorption edge of a single crystal of Zn 2 (23), in which the Zn has tetragonal (D ) site symmetry. The observed anisotropic K-absorption edges were explained in terms of a ls+4p and a ls-K4p, 4p ) transition. [Pg.416]

Loss of degeneracy of energy levels of molecular entities due to lower site symmetry created by a crystalhne environment this term is not synonymous with hgand field splitting. See Ligand Field Splitting Comm, on Photochem. (1988), Pure and Appl. Chem. 60, 1055. [Pg.177]

The cross terms (df)0(d,), with i j in Eq. (10.3) do not appear in the case of the isolated atom for which the electron density is the sum of the square of the atomic orbitals. In the molecular case, the cross terms will only be nonzero for orbitals belonging to the same representation of the point group of the molecule, like the eg orbitals in the case of trigonal site symmetry discussed above. In the square-planar point group D4h(4/m mm), the orbitals have alg, blg, b2g, and eg symmetry, and no such mixing occurs. [Pg.216]

The vibrational displacements corresponding to the anharmonic terms in the potential are most pronounced in the directions away from the stronger bonding interactions, in which restoring forces are weaker. Thus, for the tetrahedral site symmetry of the diamond structure, the anharmonicity causes a larger mean-square displacement in directions opposite to the covalent bonds. At lower... [Pg.241]

More quantitatively, the effect of the thermal motion follows from the anharmonic thermal motion formalisms discussed in chapter 2. In the bcc structure, the relevant nonzero anharmonic term in the one-particle potential is the anisotropic, cubic site-symmetry allowed, part of uJuku um in expression (2.39). The modified potential for the cubic sites is given by (Willis 1969, Willis and Pryor 1975)... [Pg.263]

When sites are inequivalent, or if orbitals belong to different symmetries as in a multiorbital spdf basis case of further sections, it is necessary to add to the Hamiltonian an on-site energy term... [Pg.514]

The Correlation Field Approximation. In some cases it is not possible to explain experimental observations in terms of the site symmetry approximation, whereby the surroundings of a given molecule are treated as static. A clear example is provided by the crystalline form of the trans isomer of [(C5H5)Fe(CO)2]2, which has the centrosymmetric structure and the IR spectrum shown in Figure 10.13. The trans molecule (other isomers exist) has inherent Cy, symmetry when rotational orientation of the C5H5 rings about... [Pg.344]

One of plausible candidates for the entropy source is a dynamic structural disorder in the HS phase, which should be settled down in the LS phase. The crystallographic data for [Mn(taa)] [11] provide a clue, i.e., the presence of C3 axis in the HS molecule. An Mn(III) ion in the 5E state is a well-known Jahn-Teller ion [19]. Since the C3 site-symmetry cannot lift the orbital degeneracy of the 5E term (Fig. 1(b)), it is likely that the Mn ion is subjected to the E e Jahn-Teller effect, which gives rise to three energetically equivalent deformation structures. The apparent C3 symmetry should be observed in a time-averaged structure over three deformed structures. [Pg.622]

When S > 1/2 and the metal site symmetry is lower than Oh or Td, there is a term in the spin Hamiltonian, in addition to the Zeeman term (gfill), that will split the (2S + 1 )MS spin degeneracy even in the absence of a magnetic field.32 This is shown in Equation 1.8, where D in the first term describes the effect of an axial distortion of the ligand field (z / x y) and E in the second term accounts for the presence of a rhombic ligand field y). [Pg.15]

Crystal field splitting The removal of a degeneracy of the energy levels of molecular entities or ions due to the lower site symmetry created by a crystalline environment. This term is sometimes incorrectly used synonymously with the term ligand field splitting. [Pg.306]


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See also in sourсe #XX -- [ Pg.406 , Pg.407 ]




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