The cyclic voltammetric investigation of a series of
α-substituted acetophenones allowed the identification
of the concerted and stepwise character of the dissociative electron
transfer reaction, and, in the stepwise cases, the
determination of the cleavage rate constants and the standard
potentials for the formation of the anion radical.
Analysis
of the data, using thermodynamical parameters derived from experiment
and from literature points to three mechanism
governing factors, the oxidability of the leaving group, the bond
dissociation energy of the bond being broken, and
the LUMO energy. The first of these factors appears to be largely
predominant in many cases in the control of the
concerted vs stepwise dichotomy. The fluoro substituent provides a
reverse example where the bond strength
overcomes the unfavorable effect of the leaving group oxidability.
It is also an exception, in terms of anion radical
cleavage reactivity, where the strength of the C−F bond significantly
contributes to slow down the cleavage as
opposed to the other substituents where solvent reorganization appears
as largely predominant. In the concerted
cases, the estimated lifetime of the anion radical is clearly larger
than the time of a vibration. The concerted character
of the reaction thus results from an energetic advantage rather than
from the “nonexistence” of the anion radical
intermediate.
Dissociative electron transfers in condensed phases occur in two
steps. The fragments are first
formed within a solvent cage from which they further diffuse. The
formation of caged, rather than free-moving, fragments is taken into account in an improved version of the
dissociative electron transfer theory
where entropic aspects are emphasized. A more detailed treatment
than previously available of the fragmentation
and solvent reorganization factors is given in terms of both energies
and free energies. The reason that the
bond dissociation energy, rather than the bond dissociation free
energy, represents the contribution of
fragmentation to the intrinsic barrier ensues. The resulting
equations that relate the activation free enthalpy
and entropy, as well as the symmetry factor, to the standard free
enthalpy and entropy of the reaction are
given for electrochemical, bimolecular, and intramolecular reactions.
Solvation radii change upon electron
transfer triggered bond cleavage. An iterative procedure is
proposed for adapting the estimation of the solvent
reorganization factor to the ensuing coupling of the fragmentation and
solvent reorganization coordinates.
Experimental examples illustrating applications of the theory are
discussed.
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