Ring opening of the cyclopropylcarbinyl radical and its N- and O-substituted analogues.: A theoretical examination of very fast unimolecular reactions

High level ab initio molecular orbital calculations have been used to examine the ring opening of the cyclopropylcarbinyl radical and its heterosubstituted analogues. The applicability of various theoretical techniques to this ring-opening reaction has been investigated. A variant of the recently introduced CBS-RAD procedure is found to give good agreement with the experimental thermochemistry. The hybrid density functional method B3-LYP is found to perform well for various geometry- and frequency-dependent quantities and to provide a possible economical alternative for the reliable prediction of the energetics. We fmd that heterosubstitution by nitrogen at the 1-position has very little effect on the kinetics of ring opening. On the other hand, heterosubstitution by nitrogen or oxygen at the 2-position results in a significant rate enhancement for an already rapid reaction. In both these:latter cases, the kinetically preferred ring-opening pathway is predicted not to lead to the thermodynamically preferred products.