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 α-proton hyperfine splitting constants (a(H_α)) with radical stabilization enthalpies (USE) 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_βMe)) 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_βMe)/ 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.