Quasi-Barrierless Submolecular Motion in Mechanically Interlocked Carbon Nanotubes

The motion of molecular fragments in close contact with atomically flat surfaces is still not fully understood. Does a more favorable interaction imply a larger barrier toward motion even if there are no obvious minima? Here, we use mechanically interlocked rotaxane-type derivatives (MINTs) of single-walled carbon nanotubes (SWNTs) featuring four different types of macrocycles with significantly different affinities for the SWNT thread as models to study this problem. Using molecular dynamics, we find that there is no direct correlation between the interaction energy of the macrocycle with the SWNT and its ability to move along or around it. Density functional tight-binding calculations reveal small (<2.5 kcal-mol(-1)) activation barriers, the height of which correlates with the commensurability of the aromatic moieties in the macrocycle with the SWNT. Our results show that macrocycles in MINTs rotate and translate freely around and along SWNTs at room temperature, with an energetic cost lower than that for the rotation around the C-C bond in ethane.