Aromaticity in a Surface Deposited Cluster: Pd$_4$ on TiO$_2$ (110)
  We report the presence of \sigma-aromaticity in a surface deposited cluster, Pd$_4$ on TiO$_2$ (110). In the gas phase, Pd$_4$ adopts a tetrahedral structure. However, surface binding promotes a flat, \sigma-aromatic cluster. This is the first time aromaticity is found in surface deposited clusters. Systems of this type emerge as a promising class of catalyst, and so realization of aromaticity in them may help to rationalize their reactivity and catalytic properties, as a function of cluster size and composition. 
Jin Zhang and Anastassia N. Alexandrova
cond-mat.mtrl-sci