B 3 Al 4 + : A Three-Dimensional Molecular Reuleaux Triangle.

We systematically explore the potential energy surface of the B 3 Al 4 + combination of atoms. The putative global minimum corresponds to a structure formed by an Al 4 square facing a B 3 triangle. Interestingly, the dynamical behavior can be described as a Reuleaux molecular triangle since it involves the rotation of the B 3 triangle at the top of the Al 4 square. The molecular dynamics simulations, corroborating with the very small rotational barriers of the B 3 triangle, show its nearly free rotation on the Al 4 ring, confirming the fluxional character of the cluster. Moreover, while the chemical bonding analysis suggests that the multicenter interaction between the two fragments determines its fluxionality, the magnetic response analysis reveals this cluster as a true and fully three-dimensional aromatic system.