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Publication 563
J. Am.
Chem. Soc., 125 (1), 105 -112, 2003
DOI: 10.1021/ja027287f S0002-7863(02)07287-6 |
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Activation Barriers in the Homolytic Cleavage of Radicals and
Ion Radicals
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Cyrille
Costentin, Marc Robert and Jean-Michel Savéant
Contribution from the Laboratoire d'Electrochimie
Moléculaire, Université de Paris 7 -
Denis Diderot, Case Courrier 7107, 2 place Jussieu,
75251 Paris Cedex 05, France. E-mail: saveant@paris7.jussieu.fr
As revealed by several experimental examples, radicals and ion radicals
may, in contrast with closed-shell molecules, undergo exothermic homolytic
cleavages (·A··B  A:
+ ·B) with substantial activation barriers. A two-state semiclassical
model is proposed for explaining the existence of the barrier and estimating
its magnitude. It is based on the intersection of the potential energy
surfaces characterizing the dissociation of a bonding state, ·A··B ·A· + ·B,
on one hand, and the approach to bonding distance of a repulsive state,
A: + ·B A B,
on the other. After inclusion of the bond cleavage and formation as Morse
curves in the normal-mode analysis, a simple activation driving force
relationship is obtained, the two main ingredients of the intrinsic barrier
being the triplet excitation energy of the A moiety and the  *  *
excitation energy in ·A-B. The model is then tested by quantum
chemical calculations, first on a simplified system to evaluate the calculation
techniques and then on a real system. A comparison of the model predictions
with experiment is finally performed using the rate data recently gathered
for the cleavage of 4-cyanophenyl alkyl ether anion radicals, which cover
a respectable range of driving forces, showing satisfactory agreement
between theoretical predictions and experimental data. |
