A symmetry forbidden reaction and the BEBO model for locating the transition state

The reaction H-2+I-2=2HI has been studied using MO theory and the bond ener gy bond order (BEBO) model. Elementary considerations show that the reactio n path through the intuitive 4-center trapezoidal ransition state is symmet ry forbidden. A BEBO description of the process with the traditional active bond order conservation condition in force (n(H2) + n(I2)+ 2n(HI)=2, n(A2) being bond order of n(A2)), does not lead to a transition state (TS) for t he reaction! reflecting in a way the effect of symmetry forbiddenness. When we look for minimum energy paths (MEPs) under nonlinear BO constraints, on ly a specific choice of the constraint condition is seen to lead to a first order saddle point on the potential energy surface (PES), which is located by a stochastic as well as a deterministic method. The associated TS has z ero iodine-iodine bond order and appears to occur early in the n(H2) coordi nate but late in the n(HI) coordinate. Semiempirical (AM1) calculations as well as ab initio MO-SCF calculations asssuming a termolecular mechanism pr edict qualitatively similar TS. The implications of the findings are analyz ed.