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

Paul Blowers, Laura Ford, Rich Masel

Research output: Contribution to journalArticle

36 Citations (Scopus)

Abstract

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.

Original languageEnglish (US)
Pages (from-to)9267-9277
Number of pages11
JournalJournal of Physical Chemistry A
Volume102
Issue number46
StatePublished - Nov 12 1998
Externally publishedYes

Fingerprint

Methanol
Chemical reactions
Hydrogen
chemical reactions
methyl alcohol
hydrogen
Chemical activation
Potential energy surfaces
curves
Stretching
orbitals
energy
activation
products
potential energy

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

Cite this

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title = "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",
abstract = "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.",
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