Energy Well of Diradicals, VII. – Conjugative‐Stabilized Trimethylenemethane Derivatives. Geometry Dependence of the Singlet‐Triplet Splitting For the three 2,1′‐bis‐allyl diradicals 3–5 the singlet‐triplet splitting has been determined by the oxygen‐trapping technique. In agreement with theory the value for the planar diradical is large (>14 kcal · mol−1) whereas for the orthogonal geometry the energy gap is small (6.3 kcal · mol−1). In all cases a triplet groundstate is observed. From 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 formation of the analogous orthogonal diradical 29 from homofulvene 17 is a concerted process. This difference is an important observation with respect to the fundamental understanding of concerted and non‐concerted reactions.

Die in der Gasphase zwischen 160 und 280°C ermittelten Geschwindigkeitskonstanten erster Ordnung zeigen eine Temperaturabhangigkeit, die sich durch die Arrhenius-Gleichungen (l), (2) und (3) beschreiben lassen, wobei die Feh-