Fig. 2. Two alternative reaction pathways for isophorone on different metal surfaces.

(A and B) Structure and molecular orbitals for the isolated isophorone molecule. (C) Structures of isophorone adsorbed on transition metal surfaces. (D) Molecular orbital density of states (MODOS) projected on the free isophorone HOMO−1, HOMO, and LUMO orbitals for isophorone on the Pd(111) surface (top) and Au(111) surface (bottom). The zero of energy corresponds to the Fermi level, and the d-band center of the clean Pd(111) surface is located at −2.11 eV. (E) Side view of the electron density difference upon isophorone adsorption on Pd(111) and Au(111) at their equilibrium adsorption structures. The values of the isosurface 0.04 and 0.015 Å⁻³ were used for the former and the latter. Cyan and purple indicate electron depletion and accumulation, respectively. The computed projected occupation of selected molecular orbitals near the Fermi level is also shown in this plot.