Size dependence of mobility on the surface of metal clusters: Ammonia on Ag10 and Ag16

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Abstract
Equilibrium constants for the reactions of NH3 with the silver clusters, Ag10 and Ag16, have been measured in the gas phase as a function of temperature using a cluster flow reactor. The binding enthalpies, ΔH300°=−9.8±1.0 and −15.9±1.0 kcal mol−1, and entropies, ΔS300°=−10.3±3.0 and −23.5±3.0 cal mol−1 K−1, have been obtained for the complexes Ag10NH3 and Ag16NH3, respectively. Statistical mechanics modeling indicates that NH3 is mobile on the surface of Ag10NH3. In contrast, it is locally bound on Ag16. The implication is that the binding mechanism involves only electrostatic interaction on Ag10 and that Ag16 provides a site favorable to a stronger classical chemical lone‐pair bond. Thus ammonia may diffuse on Ag10 and be localized on Ag16.