Electronegativity alternation

Within the ipsocentric model the electronegativity alternation that characterises X2m+iY2m+i heterocycles therefore plays a key role in the balance between aromaticity and localisation. This conclusion is supported by simple inspection of the symmetry product of HOMO and LUMO in the halved symmetry group. The properties of the product can be seen by descent in symmetry from the cylindrical group. In Dooh the pair of functions with angular momentum A spans the representation Exg/u with g for even A, u for odd A. In this group, the product for successive values of A obeys... 

As the atom (A) to which H is bonded becomes more electronegative the polarization H—A becomes more pronounced and H is more easily lost as H An alternative approach to the same conclusion is based on the equation for proton transfer especially with regard to the flow of electrons as shown by curved arrows... 

This tendency is related to the polarization properties of the monomer substituents (42). Monomers that are dissimilar in polarity tend to form alternating monomer sequences in the polymer chain. An example is the monomer pair acrylonitrile—styrene. Styrene, with its pendent phenyl group, has a relatively electronegative double bond whereas acrylonitrile, with its electron-withdrawing nitrile group, tends to be electropositive. 

The Q and e values of VP are 0.088 and —1.62, respectively (125). This indicates resonance interaction of the double bond of the vinyl group with the electrons of the lactam nitrogen, whence the electronegative nature. With high e+ monomers such as maleic anhydride, VP forms alternating copolymers, much as expected (126). With other monomers between these Q and e extremes a wide variety of possibiHties exist. Table 14 Hsts reactivity ratios for important comonomers. 

In each case the left-hand atom of the pair as written is the least electronegative. Since dipole moments have direction as well as magnitude it is necessary to add the moments of each bond vertically. For this reason the individual dipole moments cancel each other out in carbon tetrachloride but only partially in chloroform. In other molecules, such as that of water, it is necessary to know the bond angle to calculate the dipole moment. Alternatively since the dipole moment of the molecule is measurable the method may be used to compute the bond angle. 

Every alternating atom in the PDMS chain is the electronegative oxygen, which makes it a polar polymer. The authors observed that at low loadings of PDMS, PVC is destabilized. This is to be expected according to the alternative model for degradation and stabilization of PVC put forward by Naqvi. But for compositions with 50% or more PDMS, both polymers are stabilized. No possible explanation can be given for this reversal in thermal stability at this time. 

Theories based on cr-effects consider that the more electronegative a ligand the more polarization of the metal occurs, weakening the bond trans to it. This can alternatively be viewed in terms of electronic transmission via a <7-type (p) orbital shared by the two ligands. 

Another seven-membered ring that shows some aromatic character is tropone (44). This molecule would have an aromatic sextet if the two C=0 electrons stayed away from the ring and resided near the electronegative oxygen atom. In fact, tropones are stable compounds, and tropolones (45) are found in nature. However, analyses of dipole moments, NMR spectra, and X-ray diffraction measurements show that tropones and tropolones display appreciable bond alternations. ... 

An example of quantum mechanical schemes is the oldest and most widely used Mulliken population analysis [1], which simply divides the part of the electron density localized between two atoms, the overlap population that identifies a bond, equally between the two atoms of a bond. Alternatively, empirical methods to allocate atomic charges to directly bonded atoms in a reasonable way use appropriate rules which combine the atomic electronegativities with experimental structural information on the bonds linking the atoms of interest. A widely used approach included in many programs is the Gasteiger-Hiickel scheme [1]. 

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