in a nonconcerted fashion, from the bicyclicmethylenecyclopropanes 26, 30, 33, and 40, through the planar diradicals 28, 31, 35, and 50, respectively. The different reaction modes are viewed as being caused by the different coupling probabilities of the underlying vibrations. The orthogonal or planar geometries of the diradicals were derived from stereochemical and kinetic arguments. Additional thermodynamic