Molecular gyroscope

Supramolecular complex of a chloride ion (in yellow), cucurbit[5]uril (rotor, in red), and cucurbit[10]uril (stator, in purple), one of the first reported molecular gyroscopes

Molecular gyroscopes are chemical compounds or supramolecular complexes containing a rotor that moves freely relative to a stator, and therefore act as gyroscopes. Though any single bond or triple bond permits a chemical group to freely rotate, the compounds described as gyroscopes may protect the rotor from interactions, such as in a crystal structure with low packing density ^[2] or by physically surrounding the rotor avoiding steric contact. ^[3] A qualitative distinction can be made based on whether the activation energy needed to overcome rotational barriers is higher than the available thermal energy. If the activation energy required is higher than the available thermal energy, the rotor undergoes "site exchange", jumping in discrete steps between local energy minima on the potential energy surface. If there is thermal energy sufficiently higher than that needed to overcome the barrier to rotation, the molecular rotor can behave more like a macroscopic freely rotating inertial mass. ^[2]

A p-Phenylene rotor (red) rotates on two acetylene (black) axles between two m-methoxy-substituted trityl stators (blue).

For example, several studies in 2002 with a p-phenylene rotor found that some structures using variable-temperature (VT) solid-state ¹³C CPMAS and quadrupolar echo ²H NMR were able to detect a two-site exchange rate of 1.6 MHz (over 10⁶/second at 65 °C), described as "remarkably fast for a phenylene group in a crystalline solid", with steric barriers of 12–14  kcal/mol. However, tert-butyl modification of the rotor increased the exchange rate to over 10⁸ per second at room temperature, and the rate for inertially rotating p-phenylene without barriers is estimated to be approximately 2.4 trillion revolutions per second. ^[2]

Year of publication	Rotor	Stator	Linkage	Reference
2002	cucurbit[5]uril	cucurbit[10]uril	noncovalent	[1]
2007	p-phenylene	two m-methoxy-substituted trityl groups	triple bonds	[2]
2007	p-phenylene	triply bridged trityl cage	triple bonds	[4]
2010	halogen-substituted p-phenylene	silaalkane chains	single bonds	[3]
2014	p-phenylene	trityl groups bridged by photoactive azobenzene bridge	triple bonds	[5]
2015	H–Pt–H	two tri-tert-butylphosphine groups	Pt–P bonds	[6]

References

1. Day, Anthony I.; Blanch, Rodney J.; Arnold, Alan P.; Lorenzo, Susan; Lewis, Gareth R.; Dance, Ian (2002). "A Cucurbituril-Based Gyroscane: A New Supramolecular Form". Angew. Chem. Int. Ed. 41 (2): 275–277. doi:10.1002/1521-3773(20020118)41:2<275::AID-ANIE275>3.0.CO;2-M. PMID   12491407.
2. Tinh-Alfredo V. Khuong; Hung Dang; Peter D. Jarowski; Emily F. Maverick & Miguel A. Garcia-Garibay (2007). "Rotational Dynamics in a Crystalline Molecular Gyroscope by Variable-Temperature ¹³C NMR, ²H NMR, X-Ray Diffraction, and Force Field Calculations" (PDF). J. Am. Chem. Soc. 129 (4): 839–845. Bibcode:2007JAChS.129..839K. doi:10.1021/ja064325c. PMID   17243820. Archived from the original (PDF) on 2014-10-06. Retrieved 2014-10-04.
3. Wataru Setaka; Soichiro Ohmizu; Mitsuo Kira (2010). "Molecular Gyroscope Having a Halogen-substituted p-Phenylene Rotator and Silaalkane Chain Stators". Chemistry Letters. 39 (5): 468–469. doi:10.1246/cl.2010.468.
4. Jose E. Nuez; Arunkumar Natarajan; Saeed I. Khan & Miguel A. Garcia-Garibay (2007). "Synthesis of a Triply-Bridged Molecular Gyroscope by a Directed Meridional Cyclization Strategy" (PDF). Org. Lett. 9 (18): 3559–3561. doi:10.1021/ol071379y. Archived from the original (PDF) on 2014-10-06. Retrieved 2014-10-04.
5. Patrick Commins & Miguel A. Garcia-Garibay (2014). "Photochromic Molecular Gyroscope with Solid State Rotational States Determined by an Azobenzene Bridge". J. Org. Chem. 79 (4): 1611–1619. doi:10.1021/jo402516n.
6. Ernest Prack; Christopher A. O’Keefe; Jeremy K. Moore; Angel Lai; Alan J. Lough; Peter M. Macdonald; Mark S. Conradi; Robert W. Schurko; Ulrich Fekl (2015). "A Molecular Rotor Possessing an H–M–H "Spoke" on a P–M–P "Axle": A Platinum(II) trans-Dihydride Spins Rapidly Even at 75 K". J. Am. Chem. Soc. 137 (42): 13464–13467. Bibcode:2015JAChS.13713464P. doi:10.1021/jacs.5b08213. PMID   26448538.