Spin delocalization, polar effects

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... 

For both species the unpaired spin density resides in four equivalent carbons nuclei attached to these centers (N0, Q0) have sizeable hfcs due to the familiar n, a spin polarization (cf. Fig. 4a). The CIDNP effects of the bridgehead (p) protons indicate major differences a sizeable positive hfc of Q-+ can be ascribed to hyperconjugation (it, cr spin delocalization cf. Fig. 4b) in contrast, N-+ shows only very weak and negative ( ) hfcs. Since the p protons of N,+ lie in the nodal plane of its SOMO, the hyperconjugative interaction is inefficient the observed sign of the p protons was therefore ascribed to residual ic, a polarization [324], This type of interaction is usually obscured by the typically much stronger hyperconjugative interaction. 

The chromophores of sulfonyl-substituted pyrroles display maxima falling below 402 nm in dioxane, with A -methyl derivatives absorbing at slightly higher wavelengths compared to their parent pyrroles <1999TL2157>. Correlation analysis of K-band values for several l-methyl-2-formyl-5-substituted pyrroles and their phenylhydrazones indicates concurrent polar and spin delocalization effects for most members of the series whereas for the 4-nitrophenylhydrazones polar effects are dominant <1999JP0392>. 

As a last open shell system, we have chosen the H2C0 radical, which has been well characterized both at the theoretical and experimental level [77-80]. In this radical the symmetry of the singly occupied molecular orbital (a rt-in plane orbital located mainly on the oxygen center) [79] determines that only spin polarization effects contribute to isotropic hcc s of C and O, whereas spin densities at hydrogens have also a direct delocalization contribution. The results of table 5 show that, as expected, isotropic hcc s at the H atoms are well reproduced by aU the functionals, whereas the results for heavy atoms are much more scattered. In particular, the hyperfine constant of the carbon atom is -35 G at the PBEO level, in better agreement with the experimental value (-39 G)[80] than the B3LYP prediction (-34 G). It is noteworthy that the PBEO functional provides values for the H and C atoms which are sufficiently accurate. Unfortunately, no experimental value is available for the oxygen atom. 

The present results clearly establish the D parameter of localized triplet diradicals as a reliable spectral tool to probe for electronic factors that control spin delocalization and radical stabilization in the benzyl-type radicals 14. Equation (8) offers us the opportunity to interpret the experimental results through semiempirical MO calculations in terms of the theoretically accessible a-spin-density variations. The spectroscopic AD scale does not suffer from the limitations (polar vs. radical contributions) inherent in the kinetic chemical scales and provides us with over 40 aryl substituents, which is the most comprehensive collection of electronic effects on radicals. Its extension to heteroaryl-substituted diradicals (12) provides for the first time a quantitative experimental measure of delocalization in aromatic n systems. The salient features of this novel method are... 

In Table 6 are listed the calculated APSE and ARSE, together with computed and predicted ABDEs for the substituted phenols. According to the derived stabilization energies, the ABDEs of the phenols with electron-withdrawing substituents are mainly determined by the polar stabilization of the parent molecules. The polar effect is less important for the phenols with electron-donating substituents, but is in all cases destabilizing. For these substituents, it is instead the spin delocalization that has the greatest effect on... 

Cp atom than at the Ca atom of styrene, consistent with a non-concerted mechanism. The rate-limiting generation of a benzylic radical intermediate has been proposed and it has been suggested that the spin delocalization effect is more important than the polar effect in the epoxidation reactions. The head-on approach model rather than a side-on approach model is implicated in the epoxidation.The dioxoruthenium(VI) complex (5) (Figure 2) catalyses the enantioselective epoxidation of alkenes with enantiomeric excess (ee) up to 11%. Kinetic studies on the epoxidation of para-substituted styrenes suggest the formation of a radical intermediate. ... 

One may first discuss the amplitude of the impact of the spin polarization on the energies. As seen from Tables 14.1 and 14.5 this effect is not extremely sensitive to the extent of the spin delocalization. It only increases slightly when going from la to 3a. One may notice that this energy is almost constant in the series la, 2c, 3c, which is in line with the localized character of the SOMO in the series, as predicted by Hiickel calculation. 

Marked contrasts in the effects of molecular structure on hydrogen-atom transfer reactions (equation 6) and the corresponding proton-transfer reactions (equation 2) have been found. The former reaction involves no change in charge (all species are univalent cations). Consequently polarization effects of hydrocarbon substituents are minimal, but large effects of delocalization of charge and spin densities in the cation radicals are observed. 

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