An inversion of the ground-state spin multiplicity of trimethylenemethane (TMM) by electron- withdrawing groups was investigated by EPR spectroscopy, for a series of TMMs 2b-g generated photochemically from methylenequadricyclanes lb-g. Curie law analyses between 4 and 50 K proved the triplet ground states of the monophenyl derivative 2b and the monocyano derivatives 2c-d as well as that of the diphenyl derivative 2a. The ...