Symmetrical transition states in thermoneutral reactions: An AM1 SCF-MO study

Abstract

This semiempirical molecular orbital study seeks to locate transition states for thermoneutral reactions where reactant(s) and product(s) are identical. Reactions studied include the S_N2 attack of a nucleophile X^-on a substrate CH₃X, as well the degenerate [11 ,5] sigmatropic shift of hydrogen within a 1,3-diene. By imposing the appropriate symmetry constraints, a pseudoreaction pathway may be described whose vector is orthogonal to the actual reaction pathway itself. The minimum of the pseudoreaction pathway corresponds to the transition state of the actual reaction. This symmetry-constrained method to locate the transition state is expected to be much more efficient than the usual SADDLE method of the semiempirical molecular orbital MOPAC package employing a reverse search strategy. These easily computed transition states can serve as good models for transition states of the general case of the asymmetric reactions, which would be more difficult to locate otherwise.