The quantum tunneling of water occurs when water molecules in nanochannels exhibit

quantum tunneling In physics, a quantum (: quanta) is the minimum amount of any physical entity (physical property) involved in an interaction. The fundamental notion that a property can be "quantized" is referred to as "the hypothesis of quantization". This me ...

behavior that smears out the positions of the

hydrogen atom A hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral hydrogen atom contains a single positively charged proton in the nucleus, and a single negatively charged electron bound to the nucleus by the Coulomb for ...

s into a pair of correlated rings. In that state, the water molecules become delocalized around a ring and assume an unusual double top-like shape. At low temperatures, the phenomenon showcases the quantum motion of water through the separating potential walls, which is forbidden in

classical mechanics Classical mechanics is a Theoretical physics, physical theory describing the motion of objects such as projectiles, parts of Machine (mechanical), machinery, spacecraft, planets, stars, and galaxies. The development of classical mechanics inv ...

, but allowed in

quantum mechanics Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Addison-Wesley, 1989, It is ...

. The quantum tunneling of water occurs under ultraconfinement in rocks, soil and cell walls. The phenomenon is predicted to help scientists better understand the thermodynamic properties and behavior of water in confined environments such as water

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

, transport in the channels of

cell membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

s and in

carbon nanotube A carbon nanotube (CNT) is a tube made of carbon with a diameter in the nanometre range ( nanoscale). They are one of the allotropes of carbon. Two broad classes of carbon nanotubes are recognized: * ''Single-walled carbon nanotubes'' (''S ...

History

Quantum tunneling in water was reported as early as 1992. At that time it was known that motions can destroy and regenerate the weak

hydrogen bond In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...

by internal rotations of the substituent water

monomers A monomer ( ; ''wikt:mono-, mono-'', "one" + ''wikt:-mer, -mer'', "part") is a molecule that can chemical reaction, react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called ...

. On 18 March 2016, it was reported that the hydrogen bond can be broken by quantum tunneling in the water

hexamer In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomer, monomers.Quote: ''Oligomer molecule: A molecule of intermediate ...

. Unlike previously reported tunneling motions in water, this involved the concerted breaking of two hydrogen bonds. On 22 April 2016, the journal ''

Physical Review Letters ''Physical Review Letters'' (''PRL''), established in 1958, is a peer-reviewed, scientific journal that is published 52 times per year by the American Physical Society. The journal is considered one of the most prestigious in the field of physics ...

'' reported the quantum tunneling of water molecules as demonstrated at the Spallation Neutron Source and

Rutherford Appleton Laboratory The Rutherford Appleton Laboratory (RAL) is one of the national scientific research laboratories in the UK operated by the Science and Technology Facilities Council (STFC). It began as the Rutherford High Energy Laboratory, merged with the At ...

. First indications of this phenomenon were seen by scientists from Russia and Germany in 2013 based on the splitting of terahertz absorption lines of a water molecule captured in five-

ångström The angstrom (; ) is a unit of length equal to m; that is, one ten- billionth of a metre, a hundred-millionth of a centimetre, 0.1 nanometre, or 100 picometres. The unit is named after the Swedish physicist Anders Jonas Ångström (1814� ...

channels in

beryl Beryl ( ) is a mineral composed of beryllium aluminium Silicate minerals#Cyclosilicates, silicate with the chemical formula Be3Al2(SiO3)6. Well-known varieties of beryl include emerald and Aquamarine (gem), aquamarine. Naturally occurring Hex ...

. Subsequently it was directly observed using

neutron scattering Neutron scattering, the irregular dispersal of free neutrons by matter, can refer to either the naturally occurring physical process itself or to the man-made experimental techniques that use the natural process for investigating materials. Th ...

and analyzed by

ab initio ( ) is a Latin term meaning "from the beginning" and is derived from the Latin ("from") + , ablative singular of ("beginning"). Etymology , from Latin, literally "from the beginning", from ablative case of "entrance", "beginning", related t ...

simulations. In a beryl channel, the water molecule can occupy six symmetrical orientations, in agreement with the known

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 ...

. A single orientation has the oxygen atom approximately in the center of the channel, with the two hydrogens pointing to the same side toward one of the channel’s six hexagonal faces. Other orientations point to other faces, but are separated from each other by

energy barrier 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 ...

s of around 50

meV In physics, an electronvolt (symbol eV), also written electron-volt and electron volt, is the measure of an amount of kinetic energy gained by a single electron accelerating through an electric potential difference of one volt in vacuum. When us ...

. These barriers, however, do not stop the hydrogens from tunneling among the six orientations and thus split the

ground state The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state ...

energy into multiple levels.

References

{{reflist Quantum chemistry Water physics