HOMOCONJUGATION IN RADICAL CATIONS OF 7-SUBSTITUTED QUADRICYCLANES AND NORBORNADIENES

Radical cations derived from norbornadiene (N) and quadricyclane (Q) s ystems bearing an exo-methylene function (M) or a spirocyclopropane gr oup (Sp) in the 7-position have been generated by photoinduced electro n transfer to tetrachlorobenzoquinone. CIDNP effects observed during t hese reactions illuminate the spin density distributions in these inte rmediates. Significant positive spin density is indicated for the exo- methylene carbon (C-8) of MN(.+); this can be ascribed to a homoconjug ative interaction between the N SOMO and the M FMO because of their co mpatible symmetries. The strong negative spin density indicated for th e exo-methylene carbon (C-8) of MQ(.+) is tentatively ascribed to an e xchange interaction between the Q SOMO and the M FMO, which are aligne d orthogonal to each other. These assignments are born out by ab initi o molecular orbital calculations on the radical cations MN(.+) and MQ( .+) (at the MP2/6-31G level). The effects for the spirocyclopropane s ignals of SpN and SpQ are weaker than those for MN and MQ; the homocon jugative and exchange interactions between the N or Q SOMOs, respectiv ely, and the Sp FMO must be relatively weak.