A universal mechanism of the planar boron rotors B11-, B13+, B15+, and B19-: inner wheels rotating in pseudo-rotating outer bearings.

Planar boron clusters B11-, B13+, B15+, and B19- 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 C2v symmetry, whereas the wheels rotating in pseudo-rotating bearings generate rosette-type shapes with D9h, D10h, D11h, and D13h symmetries. The related rotational/pseudo-rotational energies appear with characteristic band structures, effecting the dynamics.