Radical cations derived from norbornadiene (N) and quadricyclane (Q) systems bearing an exo-methylene function (M) or a spirocyclopropane group (Sp) in the 7-position have been generated by photoinduced electron transfer to tetrachlorobenzoquinone. CIDNP effects observed during these reactions illuminate the spin density distributions in these intermediates. Significant positive spin density is indicated for the exo-methylene carbon (C-8) of MN(.+); this can be ascribed to a homoconjugative interaction between the N SOMO and the M FMO because of their compatible symmetries. The strong negative spin density indicated for the exo-methylene carbon (C-8) of MQ(.+) is tentatively ascribed to an exchange interaction between the Q SOMO and the M FMO, which are aligned orthogonal to each other. These assignments are born out by ab initio molecular orbital calculations on the radical cations MN(.+) and MQ(.+) (at the MP2/6-31G* level). The effects for the spirocyclopropane signals of SpN and SpQ are weaker than those for MN and MQ; the homoconjugative and exchange interactions between the N or Q SOMOs, respectively, and the Sp FMO must be relatively weak.