A universal mechanism of the planar boron rotors B 11 - , B 13 + , B 15 + , and B 19 - : inner wheels rotating in pseudo-rotating outer bearings.

Planar boron clusters B ₁₁ ^- , B ₁₃ ⁺ , B ₁₅ ⁺ , and B ₁₉ ^- have been introduced recently as molecular Wankel motors or tank treads. Here we present a universal mechanism for these dynamically fluxional clusters; that is, they are molecular rotors with inner wheels that rotate almost freely in pseudo-rotating outer bearings, analogous to rotating molecules trapped in pseudo-rotating cages. This mechanism has significant quantum mechanical consequences: the global-minimum structures of the clusters have C _2v symmetry, whereas the wheels rotating in pseudo-rotating bearings generate rosette-type shapes with D _9h , D _10h , D _11h , and D _13h symmetries. The related rotational/pseudo-rotational energies appear with characteristic band structures, effecting the dynamics.