Publication
591
J.
Phys. Chem. A 109 (18), 4125 -4132, 2005
DOI: 10.1021/jp050017+ S1089-5639(05)00017-4
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Origin of Activation Barriers in the Dimerization of Neutral
Radicals: A "Nonperfect Synchronization" Effect? |
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Cyrille Costentin and Jean-Michel Savéant
Laboratoire d'Electrochimie Moléculaire, Unité Mixte de Recherche
Université - CNRS No. 7591, Université de Paris 7 - Denis Diderot, 2 place
Jussieu, 75251 Paris Cedex 05, France
Dimerizations of delocalized neutral radicals may be endowed with quite
significant activation barriers. The origin of these barriers is discussed in
terms of a model that emphasizes the role of localization of the unpaired
radical upon bond formation. Several examples are given in which the model is
compared with the results of quantum chemical calculations including the
coupling of allyl radicals and of benzyl radicals at various possible carbon
sites. The dimerization behavior of radicals in the NADH family is also
examined. The connection between the reasons that underlay the existence of the
activation barrier and the principle of "nonperfect synchronization" is
discussed. The dimerization of conjugated radicals indeed offers a precious
example that can be used to decipher the reasons behind these behaviors, being
devoid of the ambiguities arising from the simultaneous involvement of ionic and
covalent states, significant solvent reorganization, and the contribution of
extensive proton tunneling, in the mostly discussed case of proton transfer at
carbon. |
