Chemical electronegativity

Table 4. Quantum chemical electronegativity and hardness indices for atoms bonded in diatomic molecules. Absolute electronegativity and hardness of molecules, calculated by the same method, is also shown. After Ref. [31]...

A rigorous quantum-mechanical calculation of some of the energy derivatives is unique to DFT alone [52]. The first and second derivatives with respect to the number of electrons, 0E/6N and 0 E/0N, recognised respectively as measures of chemical electronegativity [60] and hardness [61,62], are amenable to a rigorous calculation [52, 55,63,64]. For a system of N electrons characterised by an external potential v(f) (arising, for example, from the nuclei in an atom, molecule or cluster), the energy density functional can be expressed as... 

Putz, M. V. (2008). Absolute and Chemical Electronegativity and Hardness, Nova Publishers Inc., New York. 

TABLE 4.3 Experimental First Ionization Potential First Electron Affinity EA, and Their Associated Chemical Electronegativity... 

FIGURE 4.13 The comparative trend of tire atomic finite difference chemical electronegativity Xj, and chemical hardness from Table 4.3 (upper) with respect to the atomic absolute electronegativity zT and and hardness from Table 4.8 (middle),... 

Putz MV (2008b) Absolute and chemical electronegativity and hardness. Nova Science, New York... 

Putz, M. V. (2007a). Unifying absolute and chemical electronegativity and hardness density functional formulations through the chemical action concept In Hoffman, E. O. (Ed.), Progress in Quantum Chemistry Research, Nova Science Pubhshers Inc., New York,pp. 59-121. 

Previous studies with a variety of datasets had shown the importance of charge distribution, of inductive effect), of r-electronegativity, resonance effect), and of effective polarizability, aeffi polarizability effect) for details on these methods see Section 7.1). All four of these descriptors on all three carbon atoms were calculated. However, in the final study, a reduced set of descriptors, shown in Table 3-4, was chosen that was obtained both by statistical methods and by chemical intuition. 

The Q-equilibrate method is applicable to the widest range of chemical systems. It is based on atomic electronegativities only. An iterative procedure is used to adjust the charges until all charges are consistent with the electronegativities of the atoms. This is perhaps the most often used of these methods. 

The opening paragraph of this chapter emphasized that the connection between structure and properties is what chemistry is all about We have just seen one such con nection From the Lewis structure of a molecule we can use electronegativity to tell us about the polarity of bonds and combine that with VSEPR to predict whether the mol ecule has a dipole moment In the next several sections we 11 see a connection between structure and chemical reactivity as we review acids and bases... 

The decreased shielding caused by electronegative substituents is primarily an inductive effect and like other inductive effects falls off rapidly as the number of bonds between the substituent and the proton increases Compare the chemical shifts of the pro tons m propane and 1 mtropropane... 

The most obvious feature of these C chemical shifts is that the closer the carbon is to the electronegative chlorine the more deshielded it is Peak assignments will not always be this easy but the correspondence with electronegativity is so pronounced that spec trum simulators are available that allow reliable prediction of chemical shifts from structural formulas These simulators are based on arithmetic formulas that combine experimentally derived chemical shift increments for the various structural units within a molecule... 

NMR The electronegative oxygen of an alcohol decreases the shielding of the car bon to which it is attached The chemical shift for the carbon of the C—OH is 60-75 ppm for most alcohols Carbon of a C—S group is more shielded than carbon of C—O... 

H NMR The chemical shift of the proton m the H—C—O—C unit of an ether is very similar to that of the proton m the H—C—OH unit of an alcohol A range of 8 3 2-4 0 IS typical The proton m the H—C—S—C unit of a sulfide appears at higher field than the corresponding proton of an ether because sulfur is less electronegative than oxygen... 

Section 16 18 An H—C—O—C structural unit m an ether resembles an H—C—O—H unit of an alcohol with respect to the C—O stretching frequency m its infrared spectrum and the H—C chemical shift m its H NMR spectrum Because sulfur is less electronegative than oxygen the H and chemical shifts of H—C—S—C units appear at higher field than those of H—C—O—C... 

The chemical shift is related to the part of the electron density contributed by the valence electrons, ft is a natural extension, therefore, to try to relate changes of chemical shift due to neighbouring atoms to the electronegativities of those atoms. A good illustration of this is provided by the X-ray photoelectron carbon Is spectmm of ethyltrifluoroacetate, CF3COOCH2CH3, in Figure 8.14, obtained with AlXa ionizing radiation which was narrowed with a monochromator. 

Substitution of fluorine for hydrogen in an organic compound has a profound influence on the compound s chemical and physical properties. Several factors that are characteristic of fluorine and that underHe the observed effects are the large electronegativity of fluorine, its small size, the low degree of polarizabiHty of the carbon—fluorine bond and the weak intermolecular forces. These effects are illustrated by the comparisons of properties of fluorocarbons to chlorocarbons and hydrocarbons in Tables 1 and 2. 

Oxygen usually exhibits a valence of —2 in combination with other chemical elements to form compounds such as oxides. Most elements combine with oxygen, which is highly electronegative, in more than one ratio because of the variety of valences exhibited by the other element, or because of the existence of compHcated molecular stmctures. An extended discussion of oxides is available in the Hterature (13). 

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