(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.