Theoretical Probes of Activated-Complex Structure and Properties: Substituent Effects in Carbonyl Addition

Structures, force fields, vibrational eigenvalues and eigenvectors, potential energies, and Gibbs free energies have been calculated by ab initio quantum-mechanical methods for the reactants, products, and activated complexes of five carbonyl-addition reactions, namely the addition of water, methanol, and ammonia to formaldehyde and the addition of water to acetaldehyde and formyl fluoride. All activated complexes have four-membered ring structures, and the reaction-coordinated eigenvectors show heavy-atom reorganization and proton transfer to be truly concerted in all cases. The structures of the activated complexes are essentially invariant to substitution, the Pauling bond order of the forming carbon-nucleophile bond being 0.42-0.45 and the Pauling bond order of the breaking OH or NH bond being 0.76-0.78 in all cases. Heavy-atom reorganization is thus more advanced than proton transfer in all activated complexes. Potential energies of reaction vary from -4.7 to -17.3 kcal mol^-1 and potential energies of activation from 35.6 to 46.2 kcal mol^-1. Gibbs free energies of reaction vary from -3.0 to +7.6 kcal mol^-1 and Gibbs free energies of activation from 46.4 to 57.6 kcal mol^-1. In neither case is there a systematic relationship between rate and equilibrium energetics. Reaction progress of proton transfer is ~25% and that for heavy-atom reorganization ~45% at the activated complex. Both of these features can be derived from the qualitative location of the activated complex on an MAR. This was arrrived at by deduction of the parallel effect from the exothermicity of the reactions and of the perpendicular effect from reactant frontier-orbital interactions.