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

Abstract

The reaction H₂+I₂ =2HI has been studied using MO theory and the bond energy bond order (BEBO) model. Elementary considerations show that the reaction path through the intuitive 4-center trapezoidal ransition state is symmetry forbidden. A BEBO desccription 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 the reaction, reflecting in a way the effect of symmetry forbiddenness. When we look for minimum energy paths (MEPs) under nonlinear BO constraints, <i>only a specific choice of the constraint condition </i>is seen to lead to a first order saddle point on the potential energy surface (PES), which is located by a <i>stochastic </i>as well as a <i>deterministic </i>method. The associated TS has zero iodine-iodine bond order and appears to occur early in the n_H2, coordinate but late in the n_HI coordinate. Semiempirical (AM1) calculations as well as <i>ab initio </i>MO-SCF calculations asssuming a termolecular mechanism predict qualitatively similar TS. The implications of the findings are analyzed.