Claudio Carra, Francesco Fiussello, and Glauco Tonachini*
Dipartimento di Chimica Generale ed Organica Applicata, Universita` di Torino,
Corso Massimo D?Azeglio 48, 10125 Torino, Italy

Received October 19, 1998

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 C_s 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 C_s 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 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
Cs saddle point is lower in energy than the dissociation limit in the carbon case, it is higher for
silicon (ca. -7 and +2 kcal mol^-1, respectively). It has to be pointed out, however, that even in the
more endoergic radical anion fragmentations the process is easier than homolysis in the neutral
parent molecules. The calculations carried out on C_s radical anions show the possible occurrence,
in rigid systems, of real surface crossings, which open in principle the possibility of obtaining excited
fragment products. However, it is clear that for more flexible systems a deformation of the structure
along the dissociation pathway could generate a conical intersection. In this case the radical anions
could certainly follow a lower-energy C₁ pathway in correspondence of an avoided crossing and
bypass the real crossing.