L'Évaluation Semi‐Empirique des Densités de Spin π dans les Radicaux Carbones

Leroy, Georges; Peeters, Daniel; Sana, M.; Wilante, Claude

doi:10.1002/bscb.19880971126

Bulletin des Soc Chimique

1988

DOI: 10.1002/bscb.19880971126

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L'Évaluation Semi‐Empirique des Densités de Spin π dans les Radicaux Carbones

Georges Leroy

Daniel Peeters

M. Sana

et al.

Abstract: A critical analysis of the well known Mc Connell relations a = QpaS(Cu) leads us to propose a new value for the parameter Qp and an additive rule to calculate the spin densities, pas(C,). Other values of substituent parameters AX^ are also proposed. The difference between a(Hp)/Qp and the spin densities calculated by use of the additive relation is generally negative, in captodative-substituted ethyl radicals. Furtheron, it is found that the observed values of prs(Cq) are pretty well correlated to the correspo… Show more

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Cited by 3 publications

References 12 publications

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Influence of polar substituents on the Antioxidant Properties of para-Vinyl Phenols and on the stabilization of the corresponding phenoxy radicals

Borghese¹,

Merényi²,

Penelle³

et al. 1992

J. Prakt. Chem.

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Phenols 1 substituted in the para position by vinyl groups bearing different combinations of thioether and cyano substituents in the β positions were synthesized. The determination of their efficiency as antioxidants led to the following order of reactivity: RS,RS ≫ RS,CN > CN,CN. The analysis of the ESR spectra of analogous phenoxy radicals 4 indicates however a different order for the radical stabilization: RS,CN > RS,RS ≫ CN,CN. This behavior is explained by a charge or electron transfer in the transition state as the main factor governing the reactivity. Relative stabilizations of the phenoxy radicals are easily explained according to the captodative concept.

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Influence of polar substituents on the Antioxidant Properties of para-Vinyl Phenols and on the stabilization of the corresponding phenoxy radicals

Borghese¹,

Merényi²,

Penelle³

et al. 1992

J. Prakt. Chem.

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Thermochemical Stability of α-Amino-α-carbonylmethyl Radicals and Their Resonance As Measured by ESR

1997

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ESR spectra of the captodative alpha-amino-alpha-carbonylmethyl radicals 8 have been recorded. No coalescence temperature for the rotation of the two NMe groups was found at temperatures below the decomposition temperature of the radicals. From known coalescence temperatures and rotational barriers of substituted methyl radicals the rotational barrier of >/=17 kcal mol(-)(1) was estimated for the (*)C-N bond in the radicals 8. Enthalpies DeltaH(diss) and entropies DeltaS(diss) of the homolytic dissociation of 7a,c,d into 8a,c,d have been obtained from equilibrium measurements by ESR. By correcting for substituent interaction enthalpies in 7 (steric and geminal), a radical stabilization enthalpy RSE = -20.7 +/- 1.0 kcal mol(-)(1) was obtained for 8. By addition of the known RSEs of dialkylamino- and carbonyl groups, a RSE = -9.9 kcal mol(-)(1) is predicted for 8. The difference between the experimental and predicted values of 10.8 kcal mol(-)(1) is attributed to a synergistic captodative substituent effect. A linear correlation between the radical stabilization enthalpies of the radicals 8 and of other mono- and disubstituted alkyl radicals and their ESR aH(alpha) coupling constants was found. According to this correlation the reduction of aH(alpha) by 1 G corresponds to an increase in RSE of 1.57 kcal mol(-)(1). The large resonance of the captodative alpha-amino-alpha-carbonylmethyl radicals 3, expressed by their high RSE, their small aH(alpha) coupling constant, and their high rotational barrier, can be rationalized by a strong interaction between the alpha-amino and the alpha-carbonyl groups similar to that in amides and expressed in the resonance structures 6.

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Estimation of Bond Dissociation Energies and Radical Stabilization Energies by ESR Spectroscopy

et al. 1998

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Correlations of various indices of the stability and reactivity of carbon-centered radicals with ESR hyperfine splitting constants have been examined. For a large number of mono-and disubstituted radicals there is a moderately good linear correlation of a-proton hyperfine splitting constants (a(Ha)) with radical stabilization enthalpies (RSE) and with BDE(C-H), the C-H bond dissociation energies for the corresponding parent compounds determined from thermodynamic and kinetic studies of C-C homolysis reactions. There is a similarly satisfactory linear correlation of a(H a) with BDE-(C-H) determined by Bordwell's electrochemical and acidity function method. In all cases the correlations fail for nonplanar radicals. As expected, j3-proton hyperfine splitting constants (a(H,Me)) for radicals with a freely rotating methyl substituent are less sensitive to deviations from planarity and give better linear correlations with RSE and BDE(C-H). The correlations cover a range of more than 20 kcal/mol and are reliable predictors of RSE and BDE(C-H) for a variety of radicals including captodative species. However, the correlations fail for significantly nonplanar radicals and for radicals with cyclic delocalized systems, e.g., cyclopentadienyl. The ratio a(HfiMe)/ a(Ha) for suitably substituted radicals provides an index of pyramidalization and allows one to decide for which compounds values of RSE and BDE(C-H) can be confidently estimated.

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L'Évaluation Semi‐Empirique des Densités de Spin π dans les Radicaux Carbones

Cited by 3 publications

References 12 publications

Influence of polar substituents on the Antioxidant Properties of para-Vinyl Phenols and on the stabilization of the corresponding phenoxy radicals

Influence of polar substituents on the Antioxidant Properties of para-Vinyl Phenols and on the stabilization of the corresponding phenoxy radicals

Thermochemical Stability of α-Amino-α-carbonylmethyl Radicals and Their Resonance As Measured by ESR

Estimation of Bond Dissociation Energies and Radical Stabilization Energies by ESR Spectroscopy

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