Size-dependent binding energies of methane to small gold clusters.

The reactions of small gold cluster cations Au(x)(+) (x=2-6) with CH(4) were studied by joint gas-phase kinetics and first-principles density functional theory calculations. The experimentally obtained temperature-dependent low pressure rate constants were analyzed by employing the Lindemann energy transfer model for association reactions in conjunction with statistical RRKM theory. In this way cluster-size-dependent binding energies of methane to the gold cluster cations were determined from the experimental data for two different transition-state models. The experimental binding energies obtained by employing a "loose" transition-state model are in good agreement with the theoretical values at the optimal adsorption geometries, while a "tight" transition-state model clearly gives a lower limit for the binding energies. Additionally, Kohn-Sham molecular orbitals of Au(x)-CH(4)(+) are presented to gain detailed insight into the cluster-methane bonding mechanism.