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

Abstract

A few simple three-atom thermoneutral radical exchange reactions (i.e. A + BC→ AB + C) are examined by <i>ab initio </i>SCF methods. Emphasis is laid on the detailed analysis of density matrices rather than on energetics. Results reveal that the sum of the bond orders of the breaking and forming bonds is not conserved to unity, due to development of free valence on the migrating atom 'B' in the transition state. Bond orders, free valence and spin densities on the atoms are calculated. The present analysis shows that the bondcleavage

process is always more advanced than the bond-formation process in the transition state. Further analysis shows a development of the negative spin density on the migrating atom 'B' in the transition state. The depletion of the α-spin density on the radical site "A" in the reactant during the reaction lags behind the growth of the α-spin density on the terminal atom "C" of the reactant bond, 'B-C' in the transition state. But all these processes are completed simultaneously at the end of the reaction. Hence, the reactions are asynchronous but kinetically concerted in most cases.