Structures and electronic properties of Al₇X^0,- and Al₁₃X_1,2,12^- clusters with X=F, Cl, and Br

The structures, binding energies, and electronic properties for Al ₇X, Al₇X^- , Al₁₃X^-, Al₁₃X₂^-, and Al₁₃X₁₂ ^- (X = F, Cl, Br) were studied at the B3LYP/6-311+G(2d,p) level. Among the systems studied, Al₇ and Al₁₃ clusters in Al₇X and Al₁₃X^- reveal alkali-like and halogen-like superatom characters, respectively. Al₇ can bind with one halogen atom to form a salt-like compound as Al₇ ^+δ-X^-δ. Al₁₃^- can combine with one halogen atom to form a diatomic halogen anion Al ₁₃X^-. However, when adding more halogens, the superatom structure would be destroyed, resulting in low-symmetry compounds with the center Al atom moving toward the cluster surface. The structures of Al ₁₃X_1,2,12^- (X = F, Cl, Br) are similar to those of X = I; however, their binding energies and electron structures are much different. In addition, the analyses of the calculated NBO charges show that Cl and Br have similar properties, but much different from F, when interacting with the Al clusters. The Al-Cl and Al-Br bonds have more covalent character in Al₇X and Al₁₃X_2,12^-, in contrast to the corresponding Al-F bond, which has prominent ionic character.