Theoretical studies of Na(H2O)19–21+ and K(H2O)19–21+ clusters: explaining the absence of magic peak for Na(H2O)20+

The M(H2O)19–21+ clusters (M = Na or K in cavity) consisting of broken and distorted dodecahedral cages are studied by optimizing geometry at the B3LYP/6-311++G** level. The stabilization energy (relative to separated H2O and M+) per monomer (SEP) exhibits a maximum for K(H2O)20+ and no such maximum for Na(H2O)20+ cluster. While K in dodecahedral cavity carries a +1 charge, Na remains as a neutral atom, and suggests that the electron affinity (EA) of Na+ > EA of (H2O)20+ dodecahedral cage > EA of K+. On the basis of above trends in the SEP values and the charge on the metal, one can explain the absence of a magic number peak for Na(H2O)20+ and the presence of the magic peak for K(H2O)20+ cluster.