Isotope effect of methane adsorbed on fcc metal (1 1 1) surfaces

Abstract

By means of the van der Waals density functional and quasi-harmonic approximation, we predict that the adsorption potential energies of CD4 on various fcc metal (111) surfaces are shallower than those of CH4, whereas the equilibrium distances from the surface are larger. This isotope effect originates from the softening of the CH bond pointing toward the surface, leading to the significant lowering of its vibrational frequency and hence the large zero-point energy difference between CH4 and CD4. The CH bond softening is attributed to the hybridization of the lowest unoccupied molecular orbital with the substrate state.