Polarization induction

Where the positive charge on the atom adjacent to the nucleus is real rather than formal, i.e. NR3 rather than N02, there is evidence that its effect on o complex stability is exerted through a field effect (cf. p. 22) operating through space, in addition to any polar (inductive) effect operating through the bonds. The deactivating effect of Y on the nucleus declines, i.e. the overall rate of substitution increases, in the approximate order ... 

The secondary /3-deuterium KIEs observed for the reaction of the same substrate with hydroxide ion and with tris(hydroxymethyl)methylamine in aqueous solution at 25°C were small, i.e. kH/kD = 1.09 0.01 and 1.10 0.01, respectively. While Kresge argued that the EIE was primarily due to hyperconjugation, the secondary /3-deuterium KIEs were attributed partly to hyperconjugation and partly to a polar (inductive) effect. The rate constants for the evaluation of both the EIE and the KIEs were determined in separate kinetic runs by following the increase in the absorbance due to the nitronate ion by UV spectroscopy. 

Xrx is a parameter characterizing the homologous series RX. The values of /j,r are direct measures of the polar inductive effects of alkyl groups relative to that of methyl and correlate well with Taft s a values. Substituent-induced IP shifts can thus be handled by linear free energy relationships (LFER) of the Hammett pcr-type. 

The two papers cited above also give explicit expressions for the polarization (induction) energies between polar molecules. They are expressed in terms of permanent moments and static polarizabilities of the interacting molecules. Both are observable quantities that can be measured by experiment. 

Taft LFERs. Both a and measure resonance effects on electron distributions. Relationships using these parameters generally do not work very well for ortho substituents and for substituents on aliphatic compounds because they influence reaction rates by polar (inductive) and steric effects. Separation of... 

Taft [53] utilised as a model of steric and polar inductive effects the hydrolysis of... 

Figure 18.6 Dependence of the polarization (induction) and donor-acceptor energies and their sum for the water dimer. The data are taken from Ref. [46].

The explanatory models are based on file basic concepts of electrical theory—potential, conductance, polarization, induction, etc. Knowledge about the physical mechanisms behind these phenomena is used to provide understanding of similar phenomena in biological materials, and the models are largely influenced by theories concerning the relationship between microanatomy and fundamental electrical properties. It is vital that these models only include discrete electric components for which the essential mode of operation is known. The models are explanatory because one believes that the components of the model represent isolated anatomical structures or physical processes, such that the dominating electrical property can be explained by means of the properties of the component. 

Semiempirical methods (and this also holds for many forcefields) mainly suffer from a poor description of the electrostatic interactions which, e.g., is demonstrated by their notoriously bad performance for hydrogen-bonded systems (for which even the simplest GGAs work very well). Because also their description of polarization (induction) contributions to binding is deficient, these methods can only be recommended for nonpolar systems or as parts in ONlOM-type approaches to describe the outer regions of a quantum chemical system. 

Vy = o = zero-frequency contribution due to polar/ induction forces (Keesom/Debye) ... 

To establish more accurate fed estimations, Benin et developed a multiparameter approach, in which logfed is described using the Hammett constants corresponding to polar inductive/field (alkyl radical (eqns [15] and [16] for TEMPO- and SGl-based alkoxyamines, respectively) ... 

Noteworthy points in the chapter on Electrophilic Substitution are the surprising findings, based on n.m.r. parameters of a model system, that the polar inductive effect of alkyl groups attached to p -carbon is zero (ref. 12). The correlation of the ortfto-directing effect in the metallation of aromatics (refs. 76—79) and a detailed study of the acylation of the pyrrole anion rationalizing the results on the HSAB principle (refs. 265, 266) are worthy of mention. 

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