A computationally-efficient method for evaluating friction in molecular rotary bearings is presented. This method estimates drag from fluctuations in molecular dynamics simulations via the fluctuation–dissipation theorem. This is effective even for simulation times short compared to a bearing's energy damping time and for rotation speeds comparable to or below typical thermal values. We apply this method to two molecular rotary bearings of similar size at 300 K: previously studied nested (9,9) (14,14) double-walled carbon nanotubes and a hypothetical rotary joint consisting of single acetylenic bonds in a rigid diamondoid housing. The acetylenic joint has a rotational frictional drag coefficient of 2 × 10−35 kg m2 s−1. The friction for the nested nanotubes is 120 times larger, comparable to values reported by previous studies. This fluctuation-based method could evaluate dissipation in a variety of molecular systems with similarly rigid and symmetric bearings.
@cite_65: Using a noncontact atomic-force and scanning-tunneling microscope in ultrahigh vacuum, we have measured the switching energy of a single molecule switch based on the rotation of a di-butyl-phenyl leg in a Cu-tetra-3,5 di-tertiary-butyl-phenyl porphyrin. The mechanics and intramolecular conformation of the switched leg is controlled by the tip apex of the noncontact atomic-force microscope. The comparison between experimental and calculated force curves shows that the rotation of the leg requires an energy less than 100x10(-21) J, which is 4 orders of magnitude lower than state-of-the-art transistors.
An experimental and MD study measured the energy required to force rotation of a sterically-congested molecular bond in a surface-bound molecule @cite_65 . The work to rotate one bond was @math . The energy barrier involved in rotating the bond in this molecule is much larger than that of the rotary joint presented in this paper. All of this energy is not necessarily dissipated since much could be recovered by a properly designed mechanism. Nevertheless, this study provides an upper bound on the energy dissipated, and also indicates the work that would need to be supplied and, ideally, recovered for a single rotation.