Estimation of bond dissociation energies and radical stabilization energies by ESR spectroscopy

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