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Valence-bond approach, predictions

Table 2.4 shows a comparison of the experimental and PPP-MO calculated electronic spectral data for azobenzene and the three isomeric monoamino derivatives. It is noteworthy that the ortho isomer is observed to be most bathochromic, while the para isomer is least bathoch-romic. From a consideration of the principles of the application of the valence-bond approach to colour described in the previous section, it might have been expected that the ortho and para isomers would be most bathochromic with the meta isomer least bathochromic. In contrast, the data contained in Table 2.4 demonstrate that the PPP-MO method is capable of correctly accounting for the relative bathochromicities of the amino isomers. It is clear, at least in this case, that the valence-bond method is inferior to the molecular orbital approach. An explanation for the failure of the valence-bond method to predict the order of bathochromicities of the o-, m- and p-aminoazobenzenes emerges from a consideration of the changes in 7r-electron charge densities on excitation calculated by the PPP-MO method, as illustrated in Figure 2.14. [Pg.41]

The molecular orbital methods discussed in the previous section have highlighted the electronic factors responsible for the bending of the nitrosyl ligand, but the arguments do little to help predict when such bending will occur or estimate the equilibrium concentrations of the two possible isomers (26) and (27) in solution. A valence-bond approach, coupled with the inert-gas rule, provides a simple way of estimating the relative stabilities of the linear and bent structures. The valence-bond equivalent of the stabilization of the + tt- )... [Pg.344]

Molecular orbital theory differs from valence bond theory in that it does not require the electrons involved in a bond to be localized between two of the atoms in a molecule. Instead, the electron occupies a molecular orbital, which may be spread out over the entire molecule. As in the valence bond approach, the molecular orbital is formed by adding up contributions from the atomic orbitals on the atoms that make up the molecule. This approach, which does not explicitly model bonds as existing between two atoms, is somewhat less appealing to the intuition than the valence bond approach. However, molecular orbital calculations typically yield better predictions of molecular structure and properties than valence bond methods. Accordingly, most commercially available quantum chemistry software packages rely on molecular orbital methods to perform calculations. [Pg.1072]

Because this proved to be a powerful method for predicting reactivity and sometimes offered information where Pauling s valence-bond methods did not, there arose two camps those who regarded the valence-bond approach as the method, and those who regarded... [Pg.329]

Rutherford, J. S. (1998). Theoretical prediction of bond valence networks II. Comparison of the graph matrix and resonant bond approach. Acta Cryst. B54, 204-10. [Pg.265]

The VSEPR approach is largely restricted to Main Group species (as is Lewis theory). It can be applied to compounds of the transition elements where the nd subshell is either empty or filled, but a partly-filled nd subshell exerts an influence on stereochemistry which can often be interpreted satisfactorily by means of crystal field theory. Even in Main Group chemistry, VSEPR is by no means infallible. It remains, however, the simplest means of rationalising molecular shapes. In the absence of experimental data, it makes a reasonably reliable prediction of molecular geometry, an essential preliminary to a detailed description of bonding within a more elaborate, quantum-mechanical model such as valence bond or molecular orbital theory. [Pg.12]

Clearly these predictions do not agree with the results (Table 5) and it seems that despite the low spin density on oxygen, the perturbation model is unsatisfactory. Presumably the approach used for the nitrocompounds is better and for those who like to visualize valence bond structures the ketone model... [Pg.307]

Recently, Friesner et al.124 proposed a method referred to as J2 theory to predict accurate thermochemical data. This approach is based on the generalized valence bond-localized Moller-Plesset method (GVB-LMP2) and includes parameters that depend on the number of electron pairs and whether the pairs are a or 7t types. Thus, the parameterization in the J2 method is molecule dependent. The GVB-LMP2 method scales as n3 as opposed to n6 or n7 for the MP4, QCISD, or CCSD methods, so J2 is much faster than G2. The J2 method... [Pg.179]

This simple method of deducing the structure of molecules is called Valence Shell Electron Pair Repulsion Theory (VSEPRT). It says that all electron pairs, both bonding and nonbonding, in the outer or valence shell of an atom repel each other. This simple approach predicts (more or less) the correct structures for methane, ammonia, and water with four electron pairs arranged Lctrahedrally in each case. [Pg.83]

One of the interesting successes of the molecular orbital approach to bonding in diatomic molecules is the fact that molecules such as 02 and B2 are correctly predicted to be paramagnetic, but the valence bond structures for these molecules are unsatisfactory. Properties for many diatomic species are shown in Table 2.5. [Pg.35]

The empirical valence bond model has shown good predicting power if one defines the bond multiplicity to be two for compounds such as CePd or BaPd and three for compounds such as LaRh, Ylr, or RhTh. The limitations of the model could also be shown (40) when applied to assumed quadruple bond formation, as in RuV or ThRu or to double bond formation as in CePt or ThPt. The case of ThRu (41) shows that quadruple bonding is approached where suitable valence states are available, but not fully achieved, apparently because of the directional requirements of the bonding in a diatomic molecule. The examples of CePt and ThPt (40) show the calculated values that have been based on an assumed double bond to be too low. Platinum has no suitable valence state for triple bond formation, but apparently forms triple bonds with other... [Pg.116]

Simple valence-bond theory provides another approach to construct correlation diagrams and to predict regions of diabatic surface crossings.20,338... [Pg.179]


See other pages where Valence-bond approach, predictions is mentioned: [Pg.42]    [Pg.20]    [Pg.104]    [Pg.83]    [Pg.64]    [Pg.50]    [Pg.39]    [Pg.128]    [Pg.71]    [Pg.104]    [Pg.60]    [Pg.366]    [Pg.128]    [Pg.238]    [Pg.310]    [Pg.530]    [Pg.56]    [Pg.110]    [Pg.161]    [Pg.15]    [Pg.9]    [Pg.258]    [Pg.270]    [Pg.258]    [Pg.47]    [Pg.114]    [Pg.100]    [Pg.138]    [Pg.38]    [Pg.1251]    [Pg.268]    [Pg.47]    [Pg.523]    [Pg.308]   
See also in sourсe #XX -- [ Pg.104 ]

See also in sourсe #XX -- [ Pg.104 ]




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Bond valence prediction

Valence bond approach

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