Orbital Electronegativity Values

Using the definitions of the orbital electronegativity presented in eqs. (3.5, 4.10 and 4.11) together with the latest experimental values for the ionisation potentials, electron affinities and term values for the atoms, an extensive reevaluation of the atomic valence state promotion energies and electronegativities has been carried out[28-30]. The results obtained are presented in Chart 1 and in more detail in Table 1. 

With these results for the electronegativity values calculated, obtained as a potential in [V], we give also the corresponding values in Pauling Units [PU], using the conversion 

In addition we give the corresponding hardness parameters [34] in units of [V/e] and a parameter 61, which permits the determination of electronegativities as a function of the charge on an atom, to be described below. These parameters are given here already as they were obtained directly in the recalculation of the electronegativity values. 

Electronegativity Values, %, in Pauling Units and Hardness Parameters, r) in [V/e] 

Some additional comments to the results presented in Chart 1 and Table 1 are in order. Detailed calculations have been possible only for those elements included in Table 1. In this table we report here only in the case of carbon different values for different valence states. Corresponding results for a large number of valence states for different elements have been documented elsewhere[28-30]. Here we have chosen for the different elements just those results corresponding to the most probable valence states. To this end we report here the values for the elements of the main groups 5 through 7 with 15%, 10% and 5% s-character per bonding orbital, respectively. A somewhat more detailed discussion of these results has been published recently [41]. 

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The PEOE method leads to only partial equalization of orbital electronegativities. Thus, each atom of a molecule retains, on the basis of Eq. (12), a residual electronegativity that measures its potential to attract further electrons. It has been shown that the values of residual electronegativities can be taken as a quantitative measure of the inductive effect [35]. 

The coefficients a, b, and c (with a > 0, b > 0) for this charge dependence can be derived from the electronegativity values of a given atomic orbital in the neutral state, and in the positive and negative ions, which in turn are derived from the relevant IP s and EA s 37). Thus, the latter are the fundamental data on which the whole method is based. 

Fig. 23. Comparison between FMO-parameters and charge/electronegativity values. cu refers to the coefficient of orbital i at atom j...

Titanium dioxide differs from silica mainly in two respects (1) the Ti + ions are octahedrally coordinated in all three modifications of TiOji (2) the Ti—0 bond is more pronouncedly ionic than the Si—O bond. Using Pauling s electronegativity values (297), one calculates a 63% ionic character for the Ti—0 single bond versus 50% for Si—O. In SiOj, there is certainly some double bond character involving 3d orbitals of the Si atom, causing lowered ionic character. Therefore, characteristic differences should be expected regarding the surface chemistry. 

Pople argued that this would better account for the tendency of an atomic orbital to both lose and gain electrons and hence "would better represent the departure of an atom from neutrality in a molecular environment . The value of the orbital electronegativity i (I+A) is determined, as described above, from appropriate spectroscopic data. 

With simple aromatic compounds (Tables XVII and XVIII) the effect of structure on sensory qualities is easily recognizable, too. In order to determine electrophilic and nucleophilic centers in aromatic compounds their charge distributions were calculated. The values for the atomic charges were obtained from a computer program which is based on a model for partial equalization of orbital electronegativity (26, 27). This model was extended to1T-systems (28). 

Table 1 Atomic Electronegativity Parameters. The parameters 6° in [V], 61 and c in [V/e] are defined through eqs.(6.6 and 6.7). In the last two columns the Mulliken and Pauling electronegativity values, and P respectively, are given. For the inert gases, the electronegativity values given are those for doubly occupied orbitals.

Table 2.2 shows the values of average orbital electronegativities for some oxides and hydroxides, calculated according to the equation 2.17. One... 

For quantitative estimations of acid-base properties, the above-described ideas of electronegativity are used. Besides electronegativities of atoms. Table 2.2 shows also the data of orbital electronegativities of the oxides and hydroxides. These data suggest that small values characterize basic properties while large values refer to acidic properties intermediate values are typical for compounds with amphoteric properties. 

It was mentioned above that the concept of average orbital electronegativity is used to characterize acid-base properties. According to this concept, acidity increases with increasing the numerical value of electronegativity while the basicity decreases, correspondingly. 

Figure 6 lists the electronegativity values for several elements. In a molecule such as H2, the values of the two atoms in the bond are equal. Because each one attracts the bonding electrons with the same force, they share the electrons equally. A nonpolar covalent bond is a covalent bond in which the bonding electrons in the molecular orbital are shared equally. 

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