ESTIMATION OF BOND-DISSOCIATION ENERGIES AND RADICAL STABILIZATION ENERGIES BY ESR SPECTROSCOPY

Correlations of various indices of the stability and reactivity of car bon-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 spl itting constants (a(H-alpha) with radical stabilization enthalpies (RS E) and with BDE(C-H), the C-H bond-dissociation energies for the corre sponding parent compounds determined from thermodynamic and kinetic st udies of C-C homolysis reactions. There is a similarly satisfactory li near correlation of a(H-alpha) with BDE(C-H) determined by Bordwell's electrochemical and acidity function method. In all cases the correlat ions fail for nonplanar radicals. As expected, beta-proton hyperfine s plitting constants (a(HbetaMe)) for radicals with a freely rotating me thyl substituent are less sensitive to deviations from planarity and g ive 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 spec ies. However, the correlations fail for significantly nonplanar radica ls and for radicals with cyclic delocalized systems, e.g., cyclopentad ienyl. The ratio a(HbetaMe)/a(H-alpha for suitably substituted radical s provides an index of pyramidalization and allows one to decide for w hich compounds values of RSE and BDE(C-H) can be confidently estimated .