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 B-11(-) C-2v ((1)A(1)) and B-11 C-2v (B-2(2)) structures along the soft in-plane rotational mode allow the identification of their corresponding B-11(-) C-2v and B-11 C-2v 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 B-11(-) and B-11 behave exactly like a tank tread, in which the peripheral B-9 ring rotates almost freely around the B-2 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.