Activation Barriers in the Homolytic Cleavage of Radicals and Ion Radicals

Abstract

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 therefore 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 pi*--> sigma* 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.

Electroreduction of aliphatic chlorides at silver cathodes in water

“…ClH 2 CACH 2 Cl does not exhibit at GC any reduction peak before the onset of the supporting electrolyte reduction (as previously observed by Saveant and coworkers [39]) while an irreversible peak at about À2.5 V vs. SCE was observed at silver. Cl 2 HCACHCl 2 and Cl 2 C@CHCl show two successive irreversible two-electrons peaks at GC.…”

“…Hence, presumably, also at silver cathode a concerted and a stepwise dissociative electron transfer mechanism, are involved for Cl 2 HCACHCl 2 and Cl 2 C@CHCl, respectively, according to considerations previously developed by Saveant for GC cathodes [39]. It follows that the same reaction pathway illustrated by Scheme 1 is probably followed also at silver cathodes for these substrates.…”

“…(5)) [39]. When the cyclic voltammetry of Cl 2 HCACHCl 2 and Cl 2 C@CHCl were repeated at silver electrode, a similar behavior was observed even if the peak potentials shifted to significantly less negative potentials.…”

“…The second peak is located at the same potential of 1,2-dichloroethene and chloroacetylene for Cl 2 HCACHCl 2 and Cl 2 C@CHCl, respectively, according to the studies previously performed by Saveant and coworkers. in DMF [39]. These authors found that a-b-poly-chloroalkanes reduction (values of transfer coefficient a between 0.3 and 0.4) is concerted with the expulsion of the first chloride ion (Eq.…”

Passage from Stepwise to Concerted Dissociative Electron Transfer through Modulation of Electronic States Coupling

“…[1a, 8] In the heterolytic case, [1d, 7, 9] but also in the homolytic case, [10] it can be viewed as an intramolecular dissociative electron-transfer reaction.…”