RING-OPENING OF THE CYCLOPROPYLCARBINYL RADICAL AND ITS N-SUBSTITUTEDAND O-SUBSTITUTED ANALOGS - 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 he terosubstituted analogues. The applicability of various theoretical te chniques to this ring-opening reaction has been investigated. A varian t of the recently introduced CBS-RAD procedure is found to give good a greement with the experimental thermochemistry. The hybrid density fun ctional method B3-LYP is found to perform well for various geometry- a nd frequency-dependent quantities and to provide a possible economical alternative for the reliable prediction of the energetics. We fmd tha t heterosubstitution by nitrogen at the 1-position has very little eff ect on the kinetics of ring opening. On the other hand, heterosubstitu tion by nitrogen or oxygen at the 2-position results in a significant rate enhancement for an already rapid reaction. In both these:latter c ases, the kinetically preferred ring-opening pathway is predicted not to lead to the thermodynamically preferred products.