Effect polar

In the same work [5], Pedulli and coworkers also measured the value, referred to as path (i), for a wide range of substrates, demonstrating that the HAT reaction 

Second key point PINO behaves as a good catalyst for the hydrogen atom abstraction from C-H bonds (path i). 

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Flenk J, Schattke W, Carstensen FI, Manzke R and Skibowski M 1993 Surface-barrier and polarization effects in the photoemission from GaAs(IIO) Phys. Rev. B 47 2251... 

Figure 7-23. The polarizing effect ofd-orfeitals. a) The energy of the. t-MO can be lowered slightly by shifting the centers of the AOs slightly towards the center of the bond, b) The lobes ca n be directed towards each other by mixing soine d-character into the p-orbitals. The effect is exaggerated for clarity.

Studies on solvent effects on the endo-exo selectivity of Diels-Alder reactions have revealed the importance of hydrogen bonding interactions besides the already mentioned solvophobic interactions and polarity effects. Further evidence of the significance of the former interactions comes from computer simulations" and the analogy with Lewis-acid catalysis which is known to enhance dramatically the endo-exo selectivity (Section 1.2.4). 

For the methyl-substituted compounds (322) the increase in AG and AHf values relative to the unsubstituted thiazole is interpreted as being mainly due to polar effects. Electron-donating methyl groups are expected to stabilize the thiazolium ion, that is to decrease its acid strength. From Table 1-51 it may be seen that there is an increase in AG and AH by about 1 kcal mole for each methyl group. Similar effects have been observed for picolines and lutidines (325). 

Application of Snyder s theory of linear chromatographic adsorption (171) gives the variation in adsorption energy of the thiazole nitrogen atom as a function of this steric hindrance for silica and alumina (see Table III-22). These results show that alumina is more sensitive toward steric effects while silica shows a higher selectivity in the case of polar effects. 

If polarity effects a/one are considered, it is possible to make some sense out of the tendency toward alternation. 

The way out of this dilemma is easily stated, although not easily acted upon. It is not adequate to consider any one of these approaches for the explanation of something as complicated as these reactions. Polarity effects and resonance are both operative, and, if these still fall short of explaining all observations, there is another old standby to fall back on steric effects. 

In the next section we shall consider an attempt to combine both resonance and polarity effects. 

That these expressions do combine resonance and polarity effects can be seen as follows ... 

It is experimentally easy to generate Raman spectra using polarized light and to observe the partial depolarization of the spectra. Bands of totally symmetric vibrations are strongly polarized in Hquid or solution spectra. AH other bands in Hquid or solution are depolarized. Polarization effects are essential to elucidate stmctures, but are usuaHy ignored in most other appHcations. Details of the theory and experimental procedure can be found in the Hterature (15,16). 

Alfrey and Price proposed a means of predicting monomer reactivity in copolymerization from two parameters, (a measure of resonance) and e (a measure of polar effects) (8). These parameters have been related to the reactivity ratios by equations 15—17. 

Af-Oxidation of pyrazines appears to result in increased shielding of the a and a carbon resonances by 6-11 p.p.m., whereas the /3 and /3 carbon atoms are deshielded by 3-4 p.p.m., a trend similar to that observed with substituted pyridines. These results have been qualitatively explained in terms of resonance polar effects (80OMR(l3)l72). 

Selective chlorination of the 3-position of thietane 1,1-dioxide may be a consequence of hydrogen atom abstraction by a chlorine atom. Such reactions of chlorine atoms are believed to be influenced by polar effects, preferential hydrogen abstraction occurring remotely from an electron withdrawing group. The free radical chain reaction may be propagated by attack of the 3-thietanyl 1,1-dioxide radical on molecular chlorine. 

Greater deviations which are occasionally observed between two reference electrodes in a medium are mostly due to stray electric fields or colloid chemical dielectric polarization effects of solid constituents of the medium (e.g., sand [3]) (see Section 3.3.1). Major changes in composition (e.g., in soils) do not lead to noticeable differences of diffusion potentials with reference electrodes in concentrated salt solutions. On the other hand, with simple metal electrodes which are sometimes used as probes for potential controlled rectifiers, certain changes are to be expected through the medium. In these cases the concern is not with reference electrodes, in principle, but metals that have a rest potential which is as constant as possible in the medium concerned. This is usually more constant the more active the metal is, which is the case, for example, for zinc but not stainless steel. 

Resistances in and of electrolytes are exclusively measured with low or audio frequency ac so as not to falsify the results with polarization effects. Measurement is mainly by four electrodes, which eliminates voltages due to the grounding field resistances of the measuring electrodes. 

The indirect method is performed in the field with the arrangement shown in Fig. 3-14. In both cases the measurement is made with ac to suppress polarization effects. 

One of the most efficient ways to treat this problem is to combine the ab initio MO method and the RISM theory, and this has been achieved by a slight modification of the original RISM-SCF method. Effective atomic charges in liquid water are determined such that the electronic structure and the liquid properties become self-consistent, and along the route of convergence the polarization effect can be naturally incorporated. 

Polarization effects are another feature of Raman spectroscopy that improves the assignment of bands and enables the determination of molecular orientation. Analysis of the polarized and non-polarized bands of isotropic phases enables determination of the symmetry of the respective vibrations. For aligned molecules in crystals or at surfaces it is possible to measure the dependence of up to six independent Raman spectra on the polarization and direction of propagation of incident and scattered light relative to the molecular or crystal axes. 

We will address this issue further in Chapter 10, where the polar effects of the substituents on both the c and n electrons will be considered. For the case of electrophilic aromatic substitution, where the energetics of interaction of an approaching electrophile with the 7t system determines both the rate of reaction and position of substitution, simple resonance arguments are extremely useful. 

It is always important to keep in mind the relative nature of substituent effects. Thus, the effect of the chlorine atoms in the case of trichloroacetic acid is primarily to stabilize the dissociated anion. The acid is more highly dissociated than in the unsubstituted case because there is a more favorable energy difference between the parent acid and the anion. It is the energy differences, not the absolute energies, that determine the equilibrium constant for ionization. As we will discuss more fully in Chapter 4, there are other mechanisms by which substituents affect the energy of reactants and products. The detailed understanding of substituent effects will require that we separate polar effects fiom these other factors. 

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