van der Waals complexes, intermediates in unimolecular reactions

Thermolysis of rac- and meso-1,1',2,2',3,3'-hexamethyl-1,1'biindenyl and (1E,5E)-1,3,4,6-tetraphenyl-1,5-hexadiene in supercritical CO2 in the presence of oxygen allows to identify three types of intermediates, the free radicals (2 and 5) and the equilibrating complexes I and II consisting of radical pairs. From the oxygen dependence of the trapping and rearrangement rates it is shown, that the complex I is the primary thermolysis product, which in contrast to the complex II does not react with oxygen. The temperature dependance of these rates allows to calculate the enthalpy well of the complex II, which agrees well with predictions of our reaction force field.