ENERGY BARRIERS FOR EXOTHERMIC ION/RADICAL RECOMBINATIONS.

Unimolecular dissociations of molecules into pairs of radicals generally proceed such that the reverse exothermic radical/radical recombination does not require an activation energy. Similarly, the exothermic recombination of an ion and a free radical proceeds with zero or a very small activation energy: direct bond cleavage reactions are therefore often used to derive ionic heats of formation. We have generated in the gas phase 8 radical-cations which undergo a dissociation into an ion and a radical with a minimum energy requirement substantially above (c. leV) that calculated from the product enthalpies. These cations were generated by appropriate dissociative ionisations and identified by their collisional activation (CA) mass spectra, appearance energy (AE) measurements and by their metastable ion characteristics the structure of the daughter ions was determined by CA mass spectrometry.