Acid strength field effects

For discussions of field effects on acid and base strength and on reactivity, see Chapters 8 and 9, respectively. 

It must be considered that (i) the IR spectra reported in the cited references have typical resolutions of about 2 cm-1 (ii) calorimetric measurements are made at a well-defined temperature (typically, 303 K), whereas IR spectra are only nominally collected at room temperature since the heating effect of the IR beam on the sample is not precisely known and (iii) in the presence of different adspecies, the estimated heat values are not likely to correspond to the 0 = 0 limit. These results clearly demonstrate that the spectroscopic method for the establishment of reasonable Lewis acid strengths of oxides is based on solid ground. We emphasize here that the electric field sensed by the probe molecule at a given site is always the result of the sum of contributions from the cation and from the surrounding anions consequently, what is probed is never a property of a single ion. The same consideration holds for calorimetric measurements. 

With its determination carried out at constant ionic strength the pH, value, so obtained, provides an assessment of the acidity of the amphoteric ion exchanger. This pH, relatable to the average value of pKj and pK2 with Eq. (16), is very close to the value of the isoelectric point 0 )- Paries [105] has pointed out that the relationship between the lEP of a solid surface and the valency-effective ionic radii, when corrected for crystal field effects, coordination, hydration, and other factors, is quite good. He also has indicated that the broad probable lEP range characteristic of a cation oxidation state may be selected from the data in Table 2 as shown below. It is known that the lEP for amphoteric oxides is affected by the presence of impurities, crystallinity and the chemical species under investigation. 

The attempt to correct experimental data to zero ionic strength is fundamental to the treatment, even though often this can be done only approximately. The model of non-conjugative substituent effects which is used is a combination of Lewis s model of the inductive effect as a through-bonds displacement of electrons, and the electrostatic model of the field effect as devised by Bjerrum in his treatment of the first and second ionization constants of aliphatic dicarboxylic acids in water. As Wepster acknowledges, these models have their limitations, but he claims that their combination has nevertheless led to a very successful treatment. 

The second crystal field effect leads to stabilizaton of the states (I) and (II) by reference to water and acid molecules in vacuo and therefore of the H bond strength E, . The competition between states (I) and (II) arises from the relative basicities of A and H2O towards H. ... 

In addition to inductive and field effects, resonance effects can also play an important role in determining the strength of organic acids and bases. For example, the pX of a simple alkyl alcohol is 15, but that of the hydroxyl of tyrosine is 9.11. By analogy, the pX of the hydroxyl of phenol is 9.8 (see Table 2.1). Such may be understood by the realization that once the ionization has occured, the phenoxide anion may be stabilized by electron resonance ... 

The dissociation constants of eight 6-substituted spiro[3,3]heptane-2-carboxylic acids in 50% aqueous ethanol at 25 " C have been measured and used to correlate non-conjugative substituent effects. Detailed analysis of the results suggests that a field effect rather than a a-inductive effect is the more reliable model. The first and second dissociation constants of squaric acid (3,4-dihydroxycyclobut-3-ene-l,2-dione) in aqueous 3M-NaC104 have been measured by e.m.f. methods.Results were in agreement with earlier ones after correction for ionic strength. 

Klapper 1, R Hagstrom, RFine, K Sharp and B Honig 1986. Focusing of Electric Fields in tire Actir e Sit of CuZn Superoxide Dismutase Effects of Ionic Strength and Amino-Acid Substitution. Proteins Structure, Function and Genetics 1 47-59. 

In both Navanax neurons (65) and an artificial phospholipid bilayer membrane (66). salicylic acid (1-30 mM) increased K" " permeability but decreased Cl- permeability resulting in a net Increase in membrane conductance. To account for the selective effect of salicylic acid (and other benzoic acids) on the two permeabilities, it was proposed that the anions of the organic acids adsorb to membranes to produce either a negative surface potential (66) or an increase in the anionic field strength of the membrane (47, 48). 

Morris and Haskin (1974) observed an increase in the Eu /Eu ratio with increasing (Al+Si)/0 and high field-strength ions in the melt. This effect may be ascribed to the different acid-base properties of EU2O3 and EuO (Eraser, 1975a,b). Equilibrium 10.45 may in fact be rewritten in the form... 

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