C. Carra, G. Ghigo, G. Tonachini
Dipartimento di Chimica Generale ed Organica Applicata, Universita` di Torino,
Corso Massimo D Azeglio 48, 10125 Torino, Italy

Abstract: Methyl or silyl dissociation in the CH₂-CHCH₂-XH₃ (a-XH₃^.+ ) and CH₂-CHCH-CHCH₂-XH₃
(p-XH₃^.+ ) radical cations (X = C, Si) yields a⁺ or p⁺ and XH₃^. . Similarly, the radical anions a-CH₃^.- and
p-CH₃^.- give the pi-delocalized anion and CH₃^. preferentially. In contrast, a-SiH₃^.- and p-SiH3^.- prefer to
dissociate into the pi-delocalized radical and silide. All reactions are endoergic: by 43-50 kcal mol^-1 in the
radical cations, and easier to some extent in the radical anions, that require 29-33¹ (X = C) and 13-14
kcal mol^-1 (X = Si). The fragmentation energy profiles do not present significant barriers for the backward
process in the case of the radical cations. All radical anions exhibit an energy maximum along the dissociation
pathway, but the barrier is lower than the dissociation limit. Fragmentation is  activated  more in the anions
than in the cations with respect to homolysis in the corresponding neutrals (that requires 72-81 kcal mol^-1 ).
Wave function analysis indicates that the C-X bond cleavage in the hydrocarbon radical ions, although formally
comparable to a homolytic process, is at variance with this model, due to the spin recoupling of one of the two
C-X bond electrons with the originally unpaired electron. This is basically true also for the silyl-substituted
radical anions, in which the initial more delocalized charge distribution might suggest some heterolytic
character of the bond cleavage.