1872
J . Org. Chem. 1998, 63, 1872-1877
Hom olytic Bon d Dissocia tion En th a lp ies of th e C-H Bon d s
Ad ja cen t to Ra d ica l Cen ter s
Xian-Man Zhang
Department of Chemistry, Wesleyan University, Hall-Atwater Laboratories, Middletown, Connecticut
06459, and Department of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
Received September 23, 1997
Homolytic bond dissociation enthalpies (BDEs) at 0 and 298 K of the C-H bonds adjacent to various
radical centers have been obtained from ab initio CBS-4 (complete basis set) model calculations
and experimental data available in the literature. The BDEs of the C-H bonds adjacent to the
radical centers derived from 11 saturated hydrocarbons were found to be 33.5 ( 3 kcal/mol at 298
K. The BDEs of the C-H bonds adjacent to nine allylic and benzylic radical centers were found to
be 48 ( 3 kcal/mol at 298 K, but the benzylic C-H BDE of the PhCH2CH2• radical was found to be
only 29.7 and 30.5 kcal/mol at 0 and 298 K, respectively. The BDEs of the vinylic C-H bonds
adjacent to four vinylic radical centers were found to be 35.5 ( 3.5 kcal/mol at 298 K. The BDEs
of the vinylic C-H bonds adjacent to three allylic radical centers were found to be 56.5 ( 3 kcal/
mol at 298 K. These results suggest that the radical centers weaken the adjacent C-H bond
strengths by about 50-70 kcal/mol. The calculated BDEs agree within (2 kcal/mol with most of
the available experimental results. Isomerization enthalpies of butenes and pentenes have been
obtained. Substituent effects on BDEs have also been examined.
In tr od u ction
equilibrium constants and redox potentials since BDE
is a state function that is independent of the reaction
pathway.7
Radicals are the chemical species with unpaired elec-
trons which are common and important reactive inter-
mediates for many chemical,1 biological,2 and technolog-
ical processes.3 For example, many organic reactions
long thought to proceed via polar mechanisms have been
reinvestigated and shown to proceed via radical mecha-
nisms.4
The quantitative information about the thermody-
namic stabilities of radicals is generally obtained from
indirect measurements since most radicals are highly
reactive and short-lived intermediates, which make the
corresponding equilibrium establishment and concentra-
tion measurements difficult. The homolytic bond dis-
sociation enthalpy (BDE) of the H-A bond has been
widely considered to provide the best reliable quantitative
information about the thermodynamic stability of the
radical (A•) formed by removal of one hydrogen atom.5
Lots of the BDEs are available in the literature from the
gas- or solution-phase experimental measurements.5,6
The BDE determination is generally based on the ap-
plication of the appropriate thermodynamic cycles by
using the experimentally measurable quantities such as
On the other hand, the quantitative information about
the effect of the radical center on the adjacent C-H bond
strength is scarce in the literature.5d,8,9 This is not
surprising since the determination of the C-H BDEs in
radicals is expected to be more difficult than those in
neutral molecules, primarily due to the extremely high
reactivities of the reactant radicals. In the present paper,
with the aid of the ab initio CBS-4 (complete basis set)
model calculation,10 we wish to report the BDEs of the
C-H bonds adjacent to 28 different radical centers.
Ca lcu la tion s
All of ab initio CBS-4 theoretical calculations were
carried out using Gaussian-92 or Gaussian-94.11
A
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