Analysis of Aromaticity in Planar Metal Systems using the Linear Response Kernel.

The linear response kernel is used to gain insight into the aromatic behavior of the less classical metal aromatic E-4(2-) and CE42- (E = Al, Ga) clusters. The effect of the systematic replacement of the aluminum atoms in Al-4(2-) and CAl42- by germanium atoms is studied using, Al3Ge-, Al2Ge2, AlGe3+, Ge-4(2+), CAl3Ge-, CAl2Ge2, CAlGe3+, and CGe42+. The results are compared with the values of the delocalization index (delta(1,3)) and nucleus independent chemical shifts (NICSzz). Unintegrated plots of the linear response, computed for the first time on molecules, are used to analyze the delocalization in these clusters. All aromaticity indices studied, the linear response, delta(1,3), and NICSzz, predict that the systems with a central carbon are less aromatic than the systems without a central carbon atom. Also, the linear response is more pronounced in the sigma-electron density than in the pi-density, pointing out that the systems are mainly sigma-aromatic.