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

Abstract

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 (H α )) 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 α ) 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 (H β 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 (H β Me)/ a (H α ) 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.