On the Unusual Reactivity Patterns in Copper doped Aluminum Cluster Anions*

We have measured the relative reactivity of Al$_{n}$Cu$^{-}$ clusters (n=11-34) with O$_{2}$. An odd-even alternation is observed that is in accordance with spin-dependant etching, and CuAl$_{22}^{-}$ is observed as a ``magic peak.'' As CuAl$_{22}^{-}$ has 68 valence electrons, it has an open shell within a spherical jellium model and would be expected to be reactive. First principles electronic structure studies indicate that the results from an unusually large splitting of the 2D$^{10}$ sub-shell in the jellium model leading to a HOMO-LIMO gap of 1.24 eV. We show that the splitting results from a geometric distortion of the cluster that may also be understood as a crystal field splitting of the superatomic orbitals.