(111) vs. (100) (effect of lattice plane)
Adsorbates on the (100) surface are more strongly bonded than on the
(111) surface, except for NH3, where the bonding
energies are comparable. The hollow site on (100) is not a preferred
binding site for any of the intermediates. The spacing between Ir atoms
is too great in this region for a minimum energy to be found in the
overlap between the orbitals from N and those from the four Ir nearest
neighbors in the hollow position. As a result, N and NH adsorb at the
bridge position on the (100) surface (forming two bonds each), whereas
they adsorb at an FCC site on (111) (forming three bonds and two bonds
respectively). Despite the increased binding energy, Ir-adsorbate bond
lengths are greater on the (100) surface, except for
NH2, where they are slightly shorter. Frequency
calculations result in a ~5-150 cm-1difference between vibrations of the same mode on different lattice
planes.