Molecular gyroscopes are

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

s or supramolecular complexes containing a

rotor ROTOR was an elaborate air defence radar system built by the British Government in the early 1950s to counter possible attack by Soviet bombers. To get it operational as quickly as possible, it was initially made up primarily of WWII-era syst ...

that moves freely relative to a

stator The stator is the stationary part of a rotary system, found in electric generators, electric motors, sirens, mud motors, or biological rotors (such as bacterial flagella or ATP synthase). Energy flows through a stator to or from the rotat ...

, and therefore act as

gyroscope A gyroscope (from Ancient Greek γῦρος ''gŷros'', "round" and σκοπέω ''skopéō'', "to look") is a device used for measuring or maintaining Orientation (geometry), orientation and angular velocity. It is a spinning wheel or disc in ...

s. Though any

single bond In chemistry, a single bond is a chemical bond between two atoms involving two valence electrons. That is, the atoms share one pair of electrons where the bond forms. Therefore, a single bond is a type of covalent bond. When shared, each of th ...

triple bond A triple bond in chemistry is a chemical bond between two atoms involving six Electron pair bond, bonding electrons instead of the usual two in a covalent bond, covalent single bond. Triple bonds are stronger than the equivalent covalent bond, sin ...

permits a chemical group to freely rotate, the compounds described as gyroscopes may protect the rotor from interactions, such as in a

crystal structure In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that repeat ...

with low

packing density A packing density or packing fraction of a packing in some space is the fraction of the space filled by the figures making up the packing. In simplest terms, this is the ratio of the volume of bodies in a space to the volume of the space itself. ...

or by physically surrounding the rotor avoiding steric contact. A qualitative distinction can be made based on whether the

activation energy In the Arrhenius model of reaction rates, activation energy is the minimum amount of energy that must be available to reactants for a chemical reaction to occur. The activation energy (''E''a) of a reaction is measured in kilojoules per mole (k ...

needed to overcome rotational barriers is higher than the available

thermal energy The term "thermal energy" is often used ambiguously in physics and engineering. It can denote several different physical concepts, including: * Internal energy: The energy contained within a body of matter or radiation, excluding the potential en ...

. 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 A potential energy surface (PES) or energy landscape describes the energy of a Physical system, system, especially a collection of atoms, in terms of certain Parameter, parameters, normally the positions of the atoms. The Surface (mathematics), ...

. 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 The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying optical instruments. It is the opposite of microscopic. Overview When applied to physical phenome ...

freely rotating

inertia Inertia is the natural tendency of objects in motion to stay in motion and objects at rest to stay at rest, unless a force causes the velocity to change. It is one of the fundamental principles in classical physics, and described by Isaac Newto ...

l mass.

For example, several studies in 2002 with a ''p''-

phenylene In organic chemistry, the phenylene group () is based on a di-substituted benzene ring ( arylene). For example, poly(''p''-phenylene) is a polymer built up from ''para''-phenylene repeating units.p. C-9, Section 11.6, Handbook of Chemistry and ...

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.

References

{{Reflist, 30em Supramolecular chemistry Chemical physics