NMR investigation of the binding of CO on supported Pd clusters

The authors report an analysis of nuclear relaxation rates and Knight shifts of ${}_{}{}^{13}{}_{}{}^{}\mathrm{CO}$ molecules adsorbed on supported Pd clusters. They find that the relaxation rates for on-top (linear) and bridge-bonded CO are proportional to the temperature, an indication of the mixing of molecular orbitals with conduction-band levels. They establish that the relaxation rate of the bridge-bonded CO arises from the Fermi contact interaction. The relaxation rate for the linear-bonded CO is determined, in addition to the Fermi contact interaction, by the interaction of nuclear spins with electron spins in non-s states and with the orbital moments of the electrons. These results indicate that at the Fermi surface bridge-bonded CO exhibits an antibonding combination of the 5σ orbital of CO with the metal d band, while linear-bonded CO presents admixture also with the CO 2${\mathrm{\pi }}^{\mathrm{*}}$ orbitals.