Correlation between the energy levels of occupied molecular orbitals in hydrocarbons and their chemisorption activity toward metals

Abstract

The perturbation theory approach employed by Fukui et al. for gas phase reactions is applied to gas-metal systems with the aim to estimate which bonds in various hydrocarbon molecules will be attacked first during their chemisorption on metal surfaces. A model for gas-metal interaction is formulated in the framework of the mentioned approach, and expressions are derived which make it feasible to estimate the reactivity of individual bonds in a molecule and differences in the overall reactivity of particular compounds at the initial stage of their interaction with a metal surface upon arriving from the gas phase. Analysis of these expressions shows that in the first step of chemisorption of compounds on the same metal, the reactivity of the particular intramolecular bonds is determined by the extent of perturbation of the corresponding molecular orbitals which in turn primarily depends on their energy. The required energies of the molecular orbitals in methane, ethane, propane, cyclopropane, ethylene, propylene, allene, acetylene, and methylacetylene, were obtained by quantum chemical calculations. The energy gap between the Fermi level of molybdenum and the energies of the occupied molecular orbitals located at particular bonds provides an explanation for the observed behavior of the mentioned hydrocarbons on polycrystalline molybdenum, and gives a rationale for the formation of adsorbed species suggested from adsorption experiments.