Binding of ammonia to small copper and silver clusters

We report equilibrium geometries, harmonic frequencies, and thermochemical data for the metal cluster–ammonia complexes Agn(NH3) and Cun(NH3) (n=1,2,3,4), Ag4(NH3)2, and Cu4(NH3)2 calculated by a density functional method. The calculated shifts in ammonia umbrella mode frequency correlate with the observed shifts and the calculated enthalpies of complexation. The preferred site for NH3 adsorption and the calculated bond enthalpies can be rationalized by considering atomic charges obtained from a natural population analysis and polarization of the metal electron density.