B11(-): a moving subnanoscale tank tread.

We present a concept that an elongated, planar boron cluster can serve as a "tank tread" at the sub-nanometer scale, a novel propulsion system for potential nanomachines. Density functional calculations at the PBE0/6-311+G* level for the global-minimum B11(-)C2v ((1)A1) and B11C2v ((2)B2) structures along the soft in-plane rotational mode allow the identification of their corresponding B11(-)C2v and B11C2v transition states, with small rotational energy barriers of 0.42 and 0.55 kcal mol(-1), respectively. The energy barriers are refined to 0.35 and 0.60 kcal mol(-1) at the single-point CCSD(T) level, suggesting that the clusters are structurally fluxional at room temperature. Molecular dynamics simulations show that B11(-) and B11 behave exactly like a tank tread, in which the peripheral B9 ring rotates almost freely around the B2 core. A full turn of rotation may be accomplished in around 2 ps. In contrast to molecular wheels or Wankel motors, the peripheral boron atoms in the tank tread behave as a flexible chain gliding around, rather than as a rigid wheel rotation. This finding is beyond imagination, which expands the concepts of molecular wheels and Wankel motors.