Jj. Brocks et al., ESTIMATION OF BOND-DISSOCIATION ENERGIES AND RADICAL STABILIZATION ENERGIES BY ESR SPECTROSCOPY, Journal of organic chemistry, 63(6), 1998, pp. 1935-1943
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
.