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 may refer to: Science and technology Engineering * Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator *Helicopter rotor, the rotary wing(s) of a rotorcraft ...

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. Energy flows through a stator to or from the rotating component of the system. In an electric m ...

, 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 and angular velocity. It is a spinning wheel or disc in which the axis of rot ...

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 ...

triple bond A triple bond in chemistry is a chemical bond between two atoms involving six bonding electrons instead of the usual two in a covalent single bond. Triple bonds are stronger than the equivalent single bonds or double bonds, with a bond order o ...

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 the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pattern ...

with low packing density or by physically surrounding the rotor avoiding

steric Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...

contact. A qualitative distinction can be made based on whether the

activation energy In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules p ...

needed to overcome rotational barriers is higher than the available

thermal energy The term "thermal energy" is used loosely in various contexts in physics and engineering. It can refer to several different well-defined physical concepts. These include the internal energy or enthalpy of a body of matter and radiation; heat, de ...

. 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

inertia Inertia is the idea that an object will continue its current motion until some force causes its speed or direction to change. The term is properly understood as shorthand for "the principle of inertia" as described by Newton in his first law ...

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 Solid state, or solid matter, is one of the four fundamental states of matter. Solid state may also refer to: Electronics * Solid-state electronics, circuits built of solid materials * Solid state ionics, study of ionic conductors and their ...

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 x 10¹² revolutions per second.

References

{{Reflist, 30em Supramolecular chemistry Chemical physics