Spin delocalization substituent constant

These observations clearly support the naive idea that the para substituents exert almost no effect at remote carbenic center of brominated DPCs, at least in terms of steric congestion. Furthermore, electronic effects do not seem important enough to change the reactivity of brominated DPCs so significantly. For example there is rather small distribution in the Hammett and spin-delocalizing substituent constants. 

The differential activation parameters of p-OMe and p-OPh show considerable negative deviations from the linear relationship (A in Figure 1). These substituents are so strongly electron-releasing that jr-bond formation by factor b becomes large enough to unusually reduce the effect exerted by factor a. /7-NO2 is a notorious spin delocalizer , and carries a spin delocalization constant of = 0.57 . The plot of vs. a+ also... 

The rates of bromine atom abstraction by tris(trimethylsilyl)silyl radicals from a range of /Jara-substitutcd benzyl bromides has indicated that the silyl radical is nucleophilic. In addition both the polar and spin-delocalization effects of the substituents play a role in the abstraction reaction with the latter effect greater than for H-atom abstractions.166 The perfluoroalkylation of aromatics and alkenes has been investigated using C4F9I as the source of C,. Measurement of rate constants indicated that perfluoroalkyl radicals were 2-3 orders of magnitude more reactive than the corresponding alkyl radicals. This was attributed primarily to the reaction enthalpy and far less to the electrophilic nature of the radicals.167... 

The AD parameter is a spectroscopic measure of radical stabilization through spin delocalization by aryl substituents. A priori there is no reason to justify their use as radical substituent constants (ffrad) in linear free energy relationships, since for the latter the correlation of kinetic (rate constants) or thermodynamic (equilibrium constants) and not of spectroscopic data is of primary interest. Nevertheless, the linear correlation with the calculated RSE (Fig. 13) and other established substituent constants (cf. Section III.D) strongly suggests that the AD parameter may be appropriately employed to assess electronic effects in benzyl-type radicals. 

The INDO and GAUSSIAN 76 calculations of ESR constants and molecular geometry values in the radical (2) showed a fair agreement with experiment. The very low substituent effect on constants implies that there is no significant delocalization of an unpaired spin density from the heterocycle to substituent <83JCS(F1)925>. 

A recent study [93JCS(D)967] of aryl-l,3,2,4-dithiadiazoIylium salts has shown that they typically show a half-wave reduction potential in the region +0.29 Em < +0.4 V (with reference to the standard calomel electrode). As found for the 1,2,3,5-isomers, these salts show a Hammett relationship between 1/2 and the a value for the para substituent on the aryl group. The greater reaction constant p, for the 3+/3 redox system, compared with 2+/2, is in agreement with MO calculations—which indicate some unpaired spin density on the heterocyclic carbon, allowing delocalization onto the aryl substituent. (The nodal plane associated with 2 prohibits, to a first-order approximation, delocalization with the aryl functionality.)... 

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