Electronegativity equilibration

Polarization of the surrounding by the metal ion (either formal or effective) charge can be fit into the electronegativity equilibration scheme by ascribing new electronegativities to the atoms in the external field induced by the ion. However, it must be realized that neither of the characteristic quantum features of polarizability, such as the alternating polarity law [27] can be reproduced by classical methods. 

Smith DW (1998) Group electronegativities from electronegativity equilibration. Faraday Trans 94 201-205... 

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 rate of enolate-carbonyl equilibration " is dependent on the forward and backward rates of proton exchange. Proton exchange from a carbon-based acid is known to be slower than that of a more electronegative atom donor (in particular, O and N atoms) . For a series of closely related molecules usually the more acidic a given molecule is, the faster the rate of proton transfer (high kreu note that thermodynamic and kinetic parameters are not related). For example, benzocyclobutanone (10) is less acidic and the rate of deprotonation is substantially slower (10 times) than the related benzocyclopentanone (12) due to its enolate (11) having unfavourable anti-aromatic character. Deprotonation of the simplest cyclobutanone (13) clearly does not lead to an unfavourable anti-aromatic enolate (14) . By assuming the internal strain of 14 is similar to that of 11, cyclobutanone (13) is evidently 10 " times more acidic than benzocyclopentanone (12). By the same vain, the more acidic propanone (15) has a faster rate of deprotonation (10 times) than the less acidic ethyl acetate (16) . ... 

For group IV metals, the ease of redistribution parallels the size of the central atom. Thus for systems in which M = Sn and X and Y are alkyl, aryl, hydrogen, or electronegative groups such as halogens or alkoxy groups, equilibration can often be reached under very mild conditions, that is, <200 °C in the absence of a catalyst. The redistribution of groups on silicon, however, is a feasible process only in the presence of a catalyst, normally a Lewis acid such as aluminum chloride, and at elevated temperatures. 

Cyclic (4p-2p)n conjugation contributes to the lowering of the inversion barrier in 167, however, where the value AG = 145kJmoC at 151 °C was determined by an analysis of kinetic data for the equilibration of a 72 28 diastereomeric mixture of the arsindole. The effect will be maximal in the planar transition state. Total line-shape analysis of the temperature-dependent silyl-methyl NMR resonances in 168 indicated a cooperative effect between cyclic (p-p)n delocalization and substituent atom electronegativity, since the diminution in barrier height was greater than that afforded by either effect alone. 

Obviously, the charges are geometry dependent because of the last term in Eq. [8]. The EEM scheme derived for a molecule can be extended to macromolecu-lar systems where equilibration of electronegativities occurs within parts of the system rather than in the system as a whole. Such an extension leads to an (N + M) X (N + M) matrix equation, where N is the total number of atoms and M the number of separate parts in the system. Equation [8] can also be adapted to include the long-range character of the electrostatic interactions and the periodicity of zeolite structures. 

The products can equilibrate under the reaction conditions therefore the individual yields depend on the reactant ratios and the reaction time. Moreover, the individual product yields can be varied by the addition of one of the products Very likely, the electronegative fluoroalkyl groups prevent complete conversion to phosphonium salts by severely reducing the uncleophilic properties of the products. A radical mechanism was suggested. 

Despite the wide belief that hydrogen transfer between electronegative atoms, such as N or O, is a very fast process and that the presence of individual tautomers cannot, therefore, be detected on the NMR time scale, it is possible to slow down this equilibration rate by carefully controlling structural factors in a series of DHP. 

The preference of electronegative substituents at Cl of pyranose rings for the axial orientation has been known for 30 years [13]. This preference is the reverse of what would be expected in cyclohexane chemistry, and is termed the anomeric effect. It is responsible for the exclusive production of a-glycopyranosyl halides when these are equilibrated under strongly acidic conditions (Fig. 3, I). 

---