S= single excitation, D= double excitation, and Q= quadruple excitation. Reference 15. plex has to pass over a substantial energy barrier to complete the reaction, and this should be the rate-determining step for the overall reaction. The calculated SCF overall barriers of 11.7 (4- 31G) and 6.7 (6-31G**) kcal/mol are, as expected, too large, compared with a gas-phase experimental estimate (2 f 3 kcal/mol) 15 determined indirectly from an Arrhenius plot of ...