AB-INITIO CALCULATIONS ON THE BETA-CLEAVAGE REACTIONS OF POLYCYCLIC RADICALS - WHY DOES CUBYLCARBINYL REACT MUCH FASTER THAN EITHER HOMOCUBYL OR 1-BICYCLO[1.1.1]PENTYL

In order to understand the very different rates of beta-scission react ions in cubylcarbinyl (1) and homocubyl (3) radicals, ab initio calcul ations have been performed on these and other polycyclic radicals in w hich the scissile bond is part of a four-membered ring. The finding of a Bell-Evans-Polanyi relationship between the calculated values of De lta H and E(a) for the beta-cleavage reactions of 1, 3, bicyclo[2.2.0] hexylcarbinyl (7), bicyclo[1.1.1]pentylcarbinyl (9), and basketyl (11) radicals with r(2) = 0.997 indicates that the rates of these reaction s parallel their exothermicities. However, this parallel is not found in the beta-scission reaction of either 1-bicyclo[1.1.1]pentyl (13) or 2-bicyclo[1.1.1]pentyl (15) radicals. Despite the high exothermicitie s calculated for both reactions, the activation energies are also comp uted to be very high. Population analyses support the hypothesis that the transition states for beta-cleavage in 13 and 15 are destabilized by strong antibonding interactions between the two AOs to which the SO MO is largely confined. Comparison of the transition state geometries calculated for the eight beta-scission reactions studied indicates tha t only the transition states for beta-cleavage in 13 and 15 are substa ntially destabilized by overlap between the AOs of the SOMO.