Ab initio calculations of the reactions of hydrogen with methanol: A comparison of the role of bond distortions and pauli repulsions on the intrinsic barriers for chemical reactions

People often view barriers to reaction as being associated with either bond stretching and distortion or with curve-crossings on a potential energy surface. However, another important contribution to barriers to reaction comes from the energy required to push the reactants together. In this paper we used ab initio methods at various levels including G2.MP2/6-31G* and QCISD(T)/6-311g** to assess the contributions from bond distortions, the curve-crossing, and the energies to move the reactants together for the following reactions: H′ + CH₃OH → HH′ + CH₂OH; H′ + CH₃OH → HH′ + CH₃O; H′ + CH₃OH → H + CH₂H′OH; H′ + CH3OH → H + CH₃OH′; H′ + CH₃OH → CH₃H′ + OH; H′ + CH₃OH → CH₃ + OHH. We find that the activation barriers correlate very well with the energy to move the reactants together. However, there is little correlation between the activation barriers and either the energy of the curve-crossing or the bond distortion energy. Physically, orbitals distort when the reactants come together. These distorted orbitals have contributions from many states which are not occupied in either the reactants or products. As a result, the physical picture of the reaction as a curve-crossing does not work. We provide a new physical picture in this paper, where the main barrier to reaction is associated with bringing the reactants together and populating the states which are not occupied in either the reactants or products. In this picture, bond distortion lowers the barriers to reaction by reducing the stresses associated with orbital overlap between the reactants. At this point, we do not know if these are general results or results specific to these reactions. However, if they are general, then the ideas we use to think about a reaction, or a reaction coordinate, will need to be rethought.