Nature of Methyl and Silyl Mesolytic Dissociations in Substituted Cyclopropenyl Radical Cations and Anions. A CAS-MCSCF and CCSD(T) Theoretical Study

Abstract

Methyl- and silyl-cyclopropenyl radical charged systems are chosen to model the dissociative behavior of rigid and symmetric species. Dissociation of the radical cations in two fragments yields c-C3H3+ and XH3• moieties (X = C, Si), while, in the radical anions c-C3H3• and XH3- fragments are produced. CAS-MCSCF Cs energy profiles show the presence of C−X bond cleavage saddle points in all four cases, separated from the resulting products by energy minima corresponding to electrostatic complexes. These features are retained in the coupled cluster Cs energy profiles, obtained by series of single-point calculations on CAS-MCSCF geometries, optimized at fixed C−X distances. However, at this theory level, the radical cation reactions are significantly more endoergic. The methyl system has a less unfavorable reaction energy than the silyl (16 vs 20 kcal mol-1), and both saddle points prove to be slightly lower in energy than the dissociation limits (by ca. −4 and −2.5 kcal mol-1, respectively). For the radical ...