The Effect of Electronegativity

The effect of differing electronegativities on the MOs of the simple two s-orbital system shown above is shown in Fig. 2.5 for LiH. 

The 0-MO, which is closer to the energy level of the s-AO of the more electronegative hydrogen, is composed of a larger proportion of this AO than of the higher energy lithium s-AO. In contrast, the situation is reversed for the a -MO, which is closer in energy to the s-AO of the more electropositive lithium. This situation can be expressed in the LCAO expression for the two MOs  

X and the coefficients Cj - C4 determine the relative weights of the AOs in the MOs. These coefficients are normalised, so that the total probabilty of finding an electron in the orbital is one. In this case this normalisation is defined by  

Interactive orbital interaction diagram between two s-AOs MOs dipole moment, bond energy and atomic charges as a function of bond distance and electronegativity difference. 

The interaction diagrams in Sects. 2.1 and 2.2 dealt with 5-AOs. Because they are spherically symmetric, s-orbitals have no directional preferences. In contrast, p-AOs interact very differently depending on their relative directions. Two p-AOs can, for instance, form a- and a -MOs, as shown in Fig. 2.6. 

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Electronegativity The effect of electronegativity on acidity is evident m the following senes involving bonds between hydrogen and the second row elements C N O and F... 

The effect of electronegative modifiers on the activation energy of CO desorption, Ed, and on the corresponding pre-exponential factor, vd, can be quantified by analysis of the TPD spectra at very low CO coverages. The... 

The effect of electronegative additives on the adsorption of ethylene on transition metal surfaces is similar to the effect of S or C adatoms on the adsorption of other unsaturated hydrocarbons.6 For example the addition of C or S atoms on Mo(100) inhibits the complete decomposition (dehydrogenation) of butadiene and butene, which are almost completely decomposed on the clean surface.108 Steric hindrance plays the main role in certain cases, i.e the addition of the electronegative adatoms results in blocking of the sites available for hydrocarbon adsorption. The same effect has been observed for saturated hydrocarbons.108,109 Overall, however, and at least for low coverages where geometric hindrance plays a limited role, electronegative promoters stabilize the adsorption of ethylene and other unsaturated and saturated hydrocarbons on metal surfaces. 

Brenninkmeijer, C. A. and Mook, W. G., The Effect of Electronegative Impurities on C02 Proportional Counting, Ninth International Radiocarbon Conference, University of California, Los Angeles and San Diego, 1976. 

Fig. 42. The effect of electronegativity on the 2py- a interaction. H is assumed to have an artificially greater electronegativity than H. The 2py-H interaction is stronger than the 2py-H interaction...

The structures of some stable stannylenes, such as several amino-, " alkoxy-, and arylthio-substituted intermediates, have been revealed by X-ray crystallography. They are monomeric crystals and the tin atom has the coordination number 2. The divalent tin in such compounds is stabilized by the effects of electronegativity of the ligand atoms and by the donation of the lone-pair electrons to the vacant 5p orbital of the tin. Although the first monomeric dialkyl- and diaryl- stannylenes in... 

The effects of electronegativity differences on the bonding in heteronuclear diatomic molecules... 

Brook and coworkers have studied the effects of electronegative groups on silicon on the reactivities of vinylsilanes115. Unlike the substitution reaction of 1-trimethylsilylprop-1-ene with acetyl chloride (equation 51a) the major product of the reaction of fi-trichlorosilylstyrene with phenylacetyl chloride arises from addition, as shown in equation 51b. 

De Rider, M. L., Wilkens, S. J., Waddell, M. J., Bretcher, L. E., Weinhold, F., Raines, R. T., and Markely, J. L. (2002). Collagen stability Insights from NMR spectroscopic and hybrid density functional computational investigations of the effect of electronegative substituents on prolyl ring conformations. J. Am. Chem. Soc. 124, 2497-2505. 

Figure 3-1 illustrates the effect of electronegativity differences on bond polarity. 

Wetmore SD, Schofield R, Smith DM, Radom L (2001) A theoretical investigation of the effects of electronegative substitution on the strength of C-H- N hydrogen bonds. J Phys Chem A 105 8718-8726... 

The ortho coupling in benzene derivatives varies over the relatively small range of 6.7-8.5 Hz, depending on the resonance and inductive effects of the substituents. The presence of heteroatoms in the ring expands the range at the lower end down to 2 Hz, because of the effects of electronegativity (pyridines) and of smaller rings (furans and pyrroles). 

It is important to note that to derive a Karplus equation that accounts for the effects of electronegative substituents would require a significantly larger amount of experimental data. 

Ratier, M., Pereyre, M., Davies, A.G., and Sutcliffe, R., Regioselectivity in the ring opening of 2-alkylcyclopropylmethyl radicals. The effect of electronegative substituents, J. Chem. Soc., Perkin Trans. 2, 1907, 1984. 

In the study of coupling constants a large lore has also been accumulated. The qualitative behavior of the H-H coupling constant in saturated methylene groups is predicted by molecular orbital theory,12 and the effects of electronegative element substitution, angle deformation, and adjacent ir-bonded groups observed experimentally confirm the theory. 

Thus, it is comprehensible that the electronic effects which modify the polarity of the bonds have an influence upon the chemical shifts. Table 15.3 illustrates the effect of electronegativity upon the position of the methyl signal in the halomethane series CHj-X where the chemical shift of the carbon atom signal increases with the electronegativity of the halogen X. 

The effect of electronegativity and angiotensin-converting enzyme inhibition on the kinin-forming capacity of polyacrylonitrile dialysis membranes. Biomaterials, 29, 1139-1146. 

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