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.
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.
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Theoretical calculation of rotation-vibration energy levels of polyatomic molecules is a topic with a long history, characterized by a close, symbiotic relationship with molecular spectroscopy on one side and quantum chem­ istry on the other. What brings them together is the notion of the potential energy surface, which plays a central role in our understanding of the molecular structure and dynamics. In the case of polyatomic molecules, the experimental spectra cannot be inverted directly to yield potential surfaces [see Ref. (1) for some recent efforts within the semiclassical SCF approach], but they do provide a stringent test for the theoretically obtained potential surfaces and observables derived from them. These surfaces, usually from ab initio calculations, seldom meet the standards of spectroscopic accuracy, especially if more extended, high-energy regions are of interest. The only practical way available to test and improve them is by comparing the calculated and the experimental spectra, and minimizing the difference between the two. The subject of the theoretical treatment of coupled molecular vibrations has undergone a real renaissance in the past decade. Significant conceptual advances have been made, particularly concerning highly vibrationally and