VAN-DER-WAALS COMPLEXES, INTERMEDIATES IN UNIMOLECULAR REACTIONS

Thermolysis of rac- and meso-1,1',2,2',3,3'-hexamethyl-1,1'biindenyl a nd (1E,5E)-1,3,4,6-tetraphenyl-1,5-hexadiene in supercritical CO2 in t he 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 trappin g and rearrangement rates it is shown, that the complex I is the prima ry thermolysis product, which in contrast to the complex II does not r eact 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.