Details

Autor(en) / Beteiligte

• Brocks, Jochen J
• Beckhaus, Hans-Dieter
• Beckwith, Athelstan L. J
• Rüchardt, Christoph

Titel

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

Ist Teil von

• Journal of organic chemistry, 1998-03, Vol.63 (6), p.1935-1943

Ort / Verlag

American Chemical Society

Erscheinungsjahr

1998

Link zum Volltext

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• 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 α-proton hyperfine splitting constants (a(Hα)) 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(Hα) with BDE(C−H) determined by Bordwell's electrochemical and acidity function method. In all cases the correlations fail for nonplanar radicals. As expected, β-proton hyperfine splitting constants (a(HβMe)) 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(HβMe)/a(Hα) 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.