Asynchronous behaviour in some three-atom linear concerted radical-exchange reactions

A few simple three-atom thermoneutral radical exchange reactions (i.e. A BC --> AB + C) are examined by ab initio SCF methods. Emphasis is laid on t he detailed analysis of density matrices rather than on energetics. Results reveal that the sum of the bond orders of the breaking and forming bonds i s not conserved to unity, due to development of free valence on the migrati ng atom 'B' in the transition state. Bond orders, free valence and spin den sities on the atoms are calculated. The present analysis shows that the bon d-cleavage process is always more advanced than the bond-formation process in the transition state. Further analysis shows a development of the negati ve spin density on the migrating atom 'B' in the transition state. The depl etion of the alpha-spin density on the radical site "A" in the reactant dur ing the reaction lags behind the growth of the alpha-spin density on the te rminal atom "C" of the reactant bond, 'B-C' in the transition state. But al l these processes are completed simultaneously at the end of the reaction. Hence, the reactions are asynchronous but kinetically concerted in most cas es.