Nature of methyl and silyl mesolytic dissociations in substituted cyclopropenyl radical cations and anions. A CAS-MCSCF and CCSD(T) theoretical study

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 produce d. CAS-MCSCF C-s energy profiles show the presence of C-X bond cleavage sad dle points in all four cases, separated from the resulting products by ener gy minima corresponding to electrostatic complexes. These features are reta ined in the coupled cluster C, energy profiles, obtained by series of singl e-point calculations on CAS-MCSCF geometries, optimized at fixed C-X distan ces. However, at this theory level, the radical cation reactions are signif icantly more endoergic. The methyl system has a less unfavorable reaction e nergy than the silyl(16 vs 20 kcal mol(-1)), and both saddle points prove t o be slightly lower in energy than the dissociation limits (by ca. -4 and - 2.5 kcal mol(-1), respectively). For the radical anions, a more pronounced endoergicity in the carbon case and a less unfavorable process for silicon are found (54 vs 39 kcal mol(-1)). Moreover, while the C-s saddle point is lower in energy than the dissociation limit in the carbon case, it is highe r for silicon (ca. -7 and +2 kcal mol(-1), respectively). It has to be poin ted out, however, that even in the more endoergic radical anion fragmentati ons the process is easier than homolysis in the neutral parent molecules. T he calculations carried out on C-s radical anions show the possible occurre nce, in rigid systems, of real surface crossings, which open in principle t he possibility of obtaining excited fragment products. However, it is clear that for more flexible systems a deformation of the structure along the di ssociation pathway could generate a conical intersection. In this case the radical anions could certainly follow a lower-energy C-1 pathway in corresp ondence of an avoided crossing and bypass the real crossing.