ENERGY WELL OF DIRADICALS .7. CONJUGATIVE -STABILIZED TRIMETHYLENEMETHANE DERIVATIVES - GEOMETRY DEPENDENCE OF THE SINGLET-TRIPLET SPLITTING

For the three 2,1'-bis-allyl diradicals 3-5 the singlet-triplet splitt ing has been determined by the oxygen-trapping technique. In agreement with theory the value for the planar diradical is large (>14 kcal . m ol(-1)) whereas for the orthogonal geometry the energy gap is small (6 .3 kcal . mol(-1)). In all cases a triplet groundstate is observed. Fr om the rotational barrier of the exo methylene groups in 6 it is shown that the interconversion of the planar and orthogonal singlet states have activation barriers (6-9 kcal mol(-1)) which are responsible for their kinetic stability. In contrast to 6, where the formation of the orthogonal diradical proceeds by way of the planar diradical 3, the fo rmation of the analogous orthogonal diradical 29 from homofulvene 17 i s a concerted process. This difference is an important observation wit h respect to the fundamental understanding of concerted and non-concer ted reactions.