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.
Recently there has been some controversy about how CO and benzene adsorb on platinum. With CO, there is disagreement over whether there is back donation of electrons from the metal to the CO 2π* orbital. With benzene, there is disagreement, over whether benzene adsorbs in a distorted state. In this article we use a standard high resolution electron energy loss (HREELS) spectrometer with modified electronics to measure the equivalent of a ultraviolet (UV) spectrum for two different systems: CO on Pt(110) and benzene on Pt(110) and benzene on Pt(110). In the case of CO, the UV spectrum shows peaks at 5.6 and 8.2 eV that may shift slightly with coverage. By comparison, gas-phase CO shows peaks at 6.2 and 8.3 eV. The difference between the gas-phase peak positions and those on the surface are indicative of the antibonding orbitals being stabilized, as one would expect from the Blyholder model. With benzene we observe two different spectra: a first monolayer spectrum with a broad peak centered at 4.9 eV, and a multilayer spectrum with peaks at 0.75, 3.82, 4.71, 6.20, and 6.84 eV. The multilayer spectrum matches the spectrum of condensed benzene, while the first monolayer spectrum is quite different and resembles that of a diene. Together, these results show that UV/HREELS spectroscopy provides useful information about adsorbates on surfaces.
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