Potential-energy surfaces for chemical reactions. Dimerization of CH2 and SiH2, the SN2 reaction in gas-phase clusters and CH activation in transition-metal complexes

Abstract

We present the results of three applications of the molecular-orbital method to problems of potential-energy surfaces that control chemical reactions. In the first application the dimerization of CH₂ and SiH₂ in both their singlet and triplet states is investigated in connection with the path of least motion as against the path of non-least motion. Two ground-state (³B₁) methylenes in the path of non-least motion give ground-state ethylene with no barrier. The ground-state (¹A₁) silylenes give a ground-state disilene with a barrier in the path of least motion and no barrier in the path of non-least motion. In the second problem potential-energy surfaces are calculated for an S_N2 reaction (H₂O)_nOH^–+ CH₃Cl → HOCH₃+ Cl^–+nH₂O, where the reactants are complexed with one or two water molecules. When the hydroxide ion is solvated by two water molecules, the reaction takes place through the first step of reactant complex formation, followed by inversion of the methyl group. The transition state for methyl inversion has an energy comparable to that of the reactants. The migration of water molecules from the hydroxide side to the chloride side is not involved in the rate-determining process. The last problem is concerned with the activation of an inert CH bond in transition-metal complexes. In six-coordinate Ti d⁰ complexes the optimized geometry shows theoretical evidence that the distortion of the ethyl or methyl ligand, represented by a short M⋯H distance, a small M⋯C—C (or H) angle and a long CH bond, is of electronic origin. Electronegative axial ligands are essential for the existence of an agostic interaction, which is stabilized by CH σ→ Ti d_xy charge transfer. A similarly distorted ethyl group has been found in the calculation of a three-coordinate Pd complex. The low-energy transition state for β-elimination lies along a smooth extension of the ethyl distortion.