Association polarization effects

The values associated with the contours in Fig. 8.1 correspond to the interaction energies of a proton with the unperturbed charge distribution of the molecule. It must, of course, be recognized that the latter will not remain unperturbed as the proton approaches. (There have been several attempts to take such polarization effects into account, for instance by means of perturbation theory [7, 10, 27, 28].) Nevertheless, the Vmin can be quite effective in ranking protonation sites if these are chemically similar, for example the nitrogens in a series of azines [8, 29, 30]. Problems can arise, however, when the charge-transfer capabilities of the sites inherently differ significantly, e.g. NH3 compared with PH3 [31, 32]. 

For co-oxidation of nuclear-substituted styrenes, Table I shows a consistently decreasing rarh product as the polarity difference increases in pairs of substituted styrenes, the minimum value being about 0.4. This effect cannot be steric and must be polar. The polar effects could be caused by transmission of substituent effects through the O—O link, to some tendency of different styrenes to associate in solution—i.e., actual... 

Theoretical appraisal of polar effects [2] has indicated a charge alternation associated with back-donation of lone pair electrons of donors. Coulombic interactions seem to be sufficient to stabilize certain acyclic systems and therefore it is not necessary to invoke resonance effects [3]. 

Activation polarization effect, which is associated with the kinetics of the electrochemical oxidation-reduction or charge-transfer reactions occurring at the electrode/electrolyte interfaces of the anode and the cathode. 

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