jo0343681_si_001.xls (88.5 kB)
Methyl and Silyl Mesolytic Dissociations in the Radical Cations and Radical Anions of But-1-ene, Allylsilane, Hexa-1,3-diene, and Penta-2,4-dienylsilane. CAS−MCSCF and Coupled Cluster Theoretical Study
dataset
posted on 2003-07-11, 00:00 authored by Claudio Carra, Giovanni Ghigo, Glauco TonachiniMethyl or silyl dissociation in the CH2CHCH2−XH3 (a-XH3•+) and CH2CHCHCHCH2-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 π-delocalized anion and CH3• preferentially. In contrast, a-SiH3•- and p-SiH3•-
prefer to dissociate into the π-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
29−33 (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.