Structure and Electron Delocalization in Al4(2-) and Al4(4.).

Structure, dynamics, and electron delocalization of Al4(2-) and Al4(4-) based clusters are investigated. Gradient-corrected Density-Functional Born-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.