Consecutive radical ion pairs in the cycloreversion of a dimethylindene dimer: A case of cooperative spin polarization effects

Nuclear spin polarization effects observed during the electron transfer‐induced cleavage of the trans head‐to‐head dimer of 3,3‐dimethylindene are incompatible with the intermediacy of a single dimer radical cation. Instead, they are compatible with the ‘cooperative’ involvement of a localized, doubly linked and a delocalized, singly linked dimer radical cation. The mechanistic consequences for the cycloaddition and cycloreversion of radical cations are discussed.