Promoters electrostatic model

The simplest interpretation of the promotional effect is based upon an electrostatic model.As reflected by the strong decrease in the work function upon potassium adsorption, there is a pronounced transfer of electronic charge to the substrate, leading to the formation of a dipole. A nitrogen molecule... 

From the electrostatic model for the promotional action of adsorbed one would expect a destabilizing effect to the adsorption of NH3—in contrast to N2—since the bond with the surface is in this case dominated by electron donation to the substrate. Recent TPD measurements have indeed demonstrated a decrease in the adsorption energy induced by the presence of coadsorbed potassium, from which an additional promoter effect due to reduced site-blocking by the reaction product was concluded. ... 

The parameter a in Equation (11.6) is positive for electrophobic reactions (5r/5O>0, A>1) and negative for electrophilic ones (3r/0Oelectrochemical promotion behaviour is frequently encountered, leading to volcano-type or inverted volcano-type behaviour. However, even then equation (11.6) is satisfied over relatively wide (0.2-0.3 eV) AO regions, so we limit the present analysis to this type of promotional kinetics. It should be remembered thatEq. (11.6), originally found as an experimental observation, can be rationalized by rigorous mathematical models which account explicitly for the electrostatic dipole interactions between the adsorbates and the backspillover-formed effective double layer, as discussed in Chapter 6. 

In view of the approximate nature of the model, and the complete neglect of nonelectrostatic factors, no great weight should be placed on the absolute values of AW. In particular, comparisons between the anti and syn-cyclic transition states are the most suspect, since the assumptions that have to be made about them are different and are likely to introduce errors which do not cancel. Nevertheless, it appears that the crude figures do indicate that an electrostatic factor could accelerate some eliminations, e.g. 103, and retard others, e.g. in 102. Of course, whatever one might decide about the electrostatic influence of an electronegative substituent, just the opposite conclusions would apply if base-promoted elimination from onium species such as 103 were considered. Ion aggregation would, of course, favor syn elimination. 

The goal of this section is to describe a semiempirical model of nanoscale solvation that captures the dielectric modulation brought about by the approach of a hydrophobe to a protein hydrogen bond. In essence, the model captures the solvent-ordering effect promoted by the hydrophobe and quantifies the effect of this induced organization on the electrostatics of a pre-formed amide-carbonyl hydrogen bond. This model reproduces the crossover point in hydrogen bond dehydration... 

Fig. 3.7 Basic tenets of the nanoscale solvation theory. The solvent ordering promoted by the approaching hydrophobe enhances the electrostatics, an effect that must be captured by the model. The induced organization decreases the polarizability of the environment, preventing water dipoles from aligning with the electrostatic field lines. The thicker lines represent a stronger field. By contrast, the region exposed to bulk water facilitates dipole organization along the field lines, weakening the electrostatic field (thin lines). Reprinted from [35], with permission from Elsevier...

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