Boron-substituted aluminum clusters B3Al n-3 +/0/- (n = 3-21): stability, halogen properties and ring current characters.

Boron-substituted aluminum clusters B3Al n-3 +/0/- with n = 3-21 in three charge states are systematically investigated using density functional theory (DFT) computations using the TPSSh functional and 6-311+G(d) basis set. Low-energy structures of B3Al n-3 +/0/- clusters are constructed around a stable boron trimer, while Al atoms are progressively built surrounding the framework. An exohedral-to-endohedral transition occurs when the framework reaches thirteen Al atoms, and a triangle-to-line rearrangement of the B3 unit is observed at eighteen Al atoms for the neutral and cationic series. Such a transformation is suppressed for the anions due to the exceptional stabilization of the cyclic B3 - anion. Thermodynamic analysis shows that B3 incorporation tends to enhance the stability of the Al frameworks in all charge states, with the B3Al n-3 - anions being the most stable overall. Pronounced electron affinity EAa maxima are observed for B3Al10, B3@Al13 and B3@Al16 (∼3.0-3.1 eV), indicating that a quasi-halogen behaviour is retained even with slightly smaller EAa. These EAa reflect a strong tendency to accept additional electron(s) to reach closed-shell configurations in some magic clusters, namely [(1S)2(1P)6(2S)2(1D)10(2P)6(1F)14] for B3Al10 -, [(1S)2(1P)6(1D)10(1F)14(2S)2(2P)6(2D)10(3S)2(3P)6] for B3@Al16 -, and [(1S)2(1P)6(1D)10(1F)14(2S)2(2P)6(2D)10] for B3@Al13 2-. Polarized Al δ+-B δ- interactions are identified with electron transfer from Al to the B3 dopant, and multicenter delocalized bonding supports an aromatic stabilization across the framework due to the isovalent character of both B and Al elements. Coherent electron dynamics calculations predict a pronounced circulating ring current for the linear-B3 unit, which is strongly enhanced when the full p-orbital set (p x , p y and p z ) is included.