Structure and electron delocalization in Al₄^2- and Al₄^4-

Structure, dynamics, and electron derealization of Al₄ ^2- and Al₄^4- based clusters are investigated. Gradient-corrected Density-Functional Bom-Oppenheimer Molecular Dynamics simulations indicate that Al₄^2- based clusters have a rigid planar Al framework, while the Al₄^4- based moieties show large distortions from planarity. The induced magnetic field analysis of these species indicates that both systems have diatropic σ-systems, while the π-system is diatropic for Al₄^2- and paratropic for Al₄^4-. The total magnetic response is diatropic for Al₄^2-, while Al₄^4- is "bitropic": it has typical antiaromatic long-range cones, while the magnetic field in the Al₄^4- ring plane is similar to that of aromatic annulenes.