Grotthuss mechanism

Last updated
Protons tunnel across a series of hydrogen bonds between hydronium ions and water molecules. Proton Zundel.gif
Protons tunnel across a series of hydrogen bonds between hydronium ions and water molecules.

The Grotthuss mechanism (also known as proton jumping) is a model for the process by which an 'excess' proton or proton defect diffuses through the hydrogen bond network of water molecules or other hydrogen-bonded liquids through the formation and concomitant cleavage of covalent bonds involving neighboring molecules.

Contents

In his 1806 publication “Theory of decomposition of liquids by electrical currents”, Theodor Grotthuss proposed a theory of water conductivity. [1] Grotthuss envisioned the electrolytic reaction as a sort of ‘bucket line’ where each oxygen atom simultaneously passes and receives a single hydrogen ion. It was an astonishing theory to propose at the time, since the water molecule was thought to be OH not H2O and the existence of ions was not fully understood. On its 200th anniversary, his article was reviewed by Cukierman. [2]

Although Grotthuss was using an incorrect empirical formula of water, his description of the passing of protons through the cooperation of neighboring water molecules proved prescient.

Lemont Kier suggested that proton hopping may be an important mechanism for nerve transduction. [3]

Proton transport mechanism and proton-hopping mechanism

The Grotthuss mechanism is now a general name for the proton-hopping mechanism. In liquid water the solvation of the excess proton is idealized by two forms: the H9O4+ (Eigen cation) or H5O2+ (Zundel cation). While the transport mechanism is believed to involve the inter-conversion between these two solvation structures, the details of the hopping and transport mechanism is still debated. Currently there are two plausible mechanisms:

  1. Eigen to Zundel to Eigen (E–Z–E), on the basis of experimental NMR data, [4]
  2. Zundel to Zundel (Z–Z), on the basis of molecular dynamics simulation.

The calculated energetics of the hydronium solvation shells were reported in 2007 and it was suggested that the activation energies of the two proposed mechanisms do not agree with their calculated hydrogen bond strengths, but mechanism 1 might be the better candidate of the two. [5]

By use of conditional and time-dependent radial distribution functions (RDF), it was shown that the hydronium RDF can be decomposed into contributions from two distinct structures, Eigen and Zundel. The first peak in g(r) (the RDF) of the Eigen structure is similar to the equilibrium, standard RDF, only slightly more ordered, while the first peak of the Zundel structure is actually split into two peaks. The actual proton transfer (PT) event was then traced (after synchronizing all PT events so that t=0 is the actual event time), revealing that the hydronium indeed starts from an Eigen state, and quickly transforms into the Zundel state as the proton is being transferred, with the first peak of g(r) splitting into two. [6]

For a number of important gas phase reactions, like the hydration of carbon dioxide, a Grotthuss-like mechanism involving concerted proton hopping over several water molecules at the same time has been shown to describe the reaction kinetics. [7] [8] This Grotthuss-like concerted proton transfer seems to be especially important for atmospheric chemistry reactions, like the hydration of sulfur oxides, [9] [10] the hydrolysis of chlorine nitrate [11] and other reactions important for ozone depletion. [12] [13] [14]

The anomalous diffusion of protons

The Grotthuss mechanism, along with the relative lightness and small size (ionic radius) of the proton, explains the unusually high diffusion rate of the proton in an electric field, relative to that of other common cations (Table 1) whose movement is due simply to acceleration by the field. Random thermal motion opposes the movement of both protons and other cations. Quantum tunnelling becomes more probable the smaller the mass of the cation is, and the proton is the lightest possible stable cation. Thus there is a minor effect from quantum tunnelling also, although it dominates at low temperatures only.

Table 1
Cation Mobility / cm2 V−1 s−1
NH4+0.763×10−3
Na+0.519×10−3
K+0.762×10−3
H+3.62×10−3

Possible alternative mechanism

Some evidence from theoretical calculations, supported by recent X-ray absorption spectroscopy findings, has suggested an alternative mechanism in which the proton is attached to a "train" of three water molecules as it moves through the liquid. [15]

Related Research Articles

<span class="mw-page-title-main">Hydrogen bond</span> Intermolecular attraction between a hydrogen-donor pair and an acceptor

In chemistry, a hydrogen bond is primarily an electrostatic force of attraction between a hydrogen (H) atom which is covalently bonded to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a lone pair of electrons—the hydrogen bond acceptor (Ac). Such an interacting system is generally denoted Dn−H···Ac, where the solid line denotes a polar covalent bond, and the dotted or dashed line indicates the hydrogen bond. The most frequent donor and acceptor atoms are the period 2 elements nitrogen (N), oxygen (O), and fluorine (F).

In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the aqueous cation H3O+, the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid molecules in solution give up a proton (a positive hydrogen ion, H+) to the surrounding water molecules (H2O). In fact, acids must be surrounded by more than a single water molecule in order to ionize, yielding aqueous H+ and conjugate base. Three main structures for the aqueous proton have garnered experimental support: the Eigen cation, which is a tetrahydrate, H3O+(H2O)3, the Zundel cation, which is a symmetric dihydrate, H+(H2O)2, and the Stoyanov cation, an expanded Zundel cation, which is a hexahydrate: H+(H2O)2(H2O)4. Spectroscopic evidence from well-defined IR spectra overwhelmingly supports the Stoyanov cation as the predominant form. For this reason, it has been suggested that wherever possible, the symbol H+(aq) should be used instead of the hydronium ion.

Carbonic acid is a chemical compound with the chemical formula H2CO3. The molecule rapidly converts to water and carbon dioxide in the presence of water. However, in the absence of water, it is quite stable at room temperature. The interconversion of carbon dioxide and carbonic acid is related to the breathing cycle of animals and the acidification of natural waters.

In chemistry, an acid dissociation constant is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction

In chemistry, an amphoteric compound is a molecule or ion that can react both as an acid and as a base. What exactly this can mean depends on which definitions of acids and bases are being used.

In chemistry, a hydration reaction is a chemical reaction in which a substance combines with water. In organic chemistry, water is added to an unsaturated substrate, which is usually an alkene or an alkyne. This type of reaction is employed industrially to produce ethanol, isopropanol, and butan-2-ol.

The self-ionization of water (also autoionization of water, and autodissociation of water, or simply dissociation of water) is an ionization reaction in pure water or in an aqueous solution, in which a water molecule, H2O, deprotonates (loses the nucleus of one of its hydrogen atoms) to become a hydroxide ion, OH−. The hydrogen nucleus, H+, immediately protonates another water molecule to form a hydronium cation, H3O+. It is an example of autoprotolysis, and exemplifies the amphoteric nature of water.

In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carries a partial positive charge, or have an atom that does not have an octet of electrons.

A hydrogen ion is created when a hydrogen atom loses an electron. A positively charged hydrogen ion (or proton) can readily combine with other particles and therefore is only seen isolated when it is in a gaseous state or a nearly particle-free space. Due to its extremely high charge density of approximately 2×1010 times that of a sodium ion, the bare hydrogen ion cannot exist freely in solution as it readily hydrates, i.e., bonds quickly. The hydrogen ion is recommended by IUPAC as a general term for all ions of hydrogen and its isotopes. Depending on the charge of the ion, two different classes can be distinguished: positively charged ions and negatively charged ions.

Sulfur trioxide (alternative spelling sulphur trioxide, also known as nisso sulfan) is the chemical compound with the formula SO3. It has been described as "unquestionably the most important economically" sulfur oxide. It is prepared on an industrial scale as a precursor to sulfuric acid.

The Brønsted–Lowry theory (also called proton theory of acids and bases) is an acid–base reaction theory which was first developed by Johannes Nicolaus Brønsted and Thomas Martin Lowry independently in 1923. The basic concept of this theory is that when an acid and a base react with each other, the acid forms its conjugate base, and the base forms its conjugate acid by exchange of a proton (the hydrogen cation, or H+). This theory generalises the Arrhenius theory.

<span class="mw-page-title-main">Matrix isolation</span> Experimental chemistry technique

Matrix isolation is an experimental technique used in chemistry and physics. It generally involves a material being trapped within an unreactive matrix. A host matrix is a continuous solid phase in which guest particles are embedded. The guest is said to be isolated within the host matrix. Initially the term matrix-isolation was used to describe the placing of a chemical species in any unreactive material, often polymers or resins, but more recently has referred specifically to gases in low-temperature solids. A typical matrix isolation experiment involves a guest sample being diluted in the gas phase with the host material, usually a noble gas or nitrogen. This mixture is then deposited on a window that is cooled to below the melting point of the host gas. The sample may then be studied using various spectroscopic procedures.

<span class="mw-page-title-main">Coordination sphere</span> All ligands directly bound to the central metal atom of a chemical complex

In coordination chemistry, the first coordination sphere refers to the array of molecules and ions directly attached to the central metal atom. The second coordination sphere consists of molecules and ions that attached in various ways to the first coordination sphere.

In chemistry, the hydron, informally called proton, is the cationic form of atomic hydrogen, represented with the symbol H+
. The general term "hydron", endorsed by the IUPAC, encompasses cations of hydrogen regardless of their isotopic composition: thus it refers collectively to protons (1H+) for the protium isotope, deuterons (2H+ or D+) for the deuterium isotope, and tritons (3H+ or T+) for the tritium isotope.

<span class="mw-page-title-main">Proton-transfer-reaction mass spectrometry</span>

Proton-transfer-reaction mass spectrometry (PTR-MS) is an analytical chemistry technique that uses gas phase hydronium reagent ions which are produced in an ion source. PTR-MS is used for online monitoring of volatile organic compounds (VOCs) in ambient air and was developed in 1995 by scientists at the Institut für Ionenphysik at the Leopold-Franzens University in Innsbruck, Austria. A PTR-MS instrument consists of an ion source that is directly connected to a drift tube and an analyzing system. Commercially available PTR-MS instruments have a response time of about 100 ms and reach a detection limit in the single digit pptv or even ppqv region. Established fields of application are environmental research, food and flavor science, biological research, medicine, security, cleanroom monitoring, etc.

<span class="mw-page-title-main">Properties of water</span> Physical and chemical properties of pure water

Water is a polar inorganic compound that is at room temperature a tasteless and odorless liquid, which is nearly colorless apart from an inherent hint of blue. It is by far the most studied chemical compound and is described as the "universal solvent" and the "solvent of life". It is the most abundant substance on the surface of Earth and the only common substance to exist as a solid, liquid, and gas on Earth's surface. It is also the third most abundant molecule in the universe.

In chemistry, metal aquo complexes are coordination compounds containing metal ions with only water as a ligand. These complexes are the predominant species in aqueous solutions of many metal salts, such as metal nitrates, sulfates, and perchlorates. They have the general stoichiometry [M(H2O)n]z+. Their behavior underpins many aspects of environmental, biological, and industrial chemistry. This article focuses on complexes where water is the only ligand, but of course many complexes are known to consist of a mix of aquo and other ligands.

A metal ion in aqueous solution or aqua ion is a cation, dissolved in water, of chemical formula [M(H2O)n]z+. The solvation number, n, determined by a variety of experimental methods is 4 for Li+ and Be2+ and 6 for most elements in periods 3 and 4 of the periodic table. Lanthanide and actinide aqua ions have higher solvation numbers (often 8 to 9), with the highest known being 11 for Ac3+. The strength of the bonds between the metal ion and water molecules in the primary solvation shell increases with the electrical charge, z, on the metal ion and decreases as its ionic radius, r, increases. Aqua ions are subject to hydrolysis. The logarithm of the first hydrolysis constant is proportional to z2/r for most aqua ions.

<span class="mw-page-title-main">Copper(II) glycinate</span> Chemical compound

Copper(II) glycinate (IUPAC suggested name: bis(glycinato)copper(II)) refers to the coordination complex of copper(II) with two equivalents of glycinate, with the formula [Cu(glycinate)2(H2O)x] where x = 1 (monohydrate) or 0 (anhydrous form). The complex was first reported in 1841, and its chemistry has been revisited many times, particularly in relation to the isomerisation reaction between the cis and trans forms which was first reported in 1890.

<span class="mw-page-title-main">Thomas Loerting</span> Austrian chemist

Thomas Loerting is an Austrian chemist and associate professor at the University of Innsbruck. His research focuses on amorphous systems, the physics and chemistry of ice and chemistry at low temperatures.

References

  1. ↑ de Grotthuss, C.J.T. (1806). "Sur la décomposition de l'eau et des corps qu'elle tient en dissolution à l'aide de l'électricité galvanique". Ann. Chim. 58: 54–73.
  2. ↑ Cukierman, Samuel (2006). "Et tu Grotthuss!". Biochimica et Biophysica Acta (BBA) - Bioenergetics . 1757 (8): 876–8. doi: 10.1016/j.bbabio.2005.12.001 . PMID   16414007.
  3. ↑ Kier, Lemont B. (2016). "Proton Hopping as the Nerve Conduction Message". Current Computer-Aided Drug Design. 12 (4): 255–258. doi:10.2174/1573409912666160808092011. ISSN   1875-6697. PMID   27503744.
  4. ↑ Agmon, Noam (1995). "The Grotthuss mechanism". Chem. Phys. Lett. 244 (5–6): 456–462. Bibcode:1995CPL...244..456A. doi:10.1016/0009-2614(95)00905-J. Archived from the original on 2011-07-19. Retrieved 2007-04-10.
  5. ↑ Markovitch, Omer; Agmon, Noam (2007). "Structure and energetics of the hydronium hydration shells". J. Phys. Chem. A. 111 (12): 2253–6. Bibcode:2007JPCA..111.2253M. CiteSeerX   10.1.1.76.9448 . doi:10.1021/jp068960g. PMID   17388314.
  6. ↑ Markovitch, Omer; et al. (2008). "Special Pair Dance and Partner Selection: Elementary Steps in Proton Transport in Liquid Water". J. Phys. Chem. B. 112 (31): 9456–9466. doi:10.1021/jp804018y. PMID   18630857.
  7. ↑ Loerting, Thomas; Tautermann, Christofer; Kroemer, Romano T.; Kohl, Ingrid; Hallbrucker, Andreas; Mayer, Erwin; Liedl, Klaus R. (2000). "On the Surprising Kinetic Stability of Carbonic Acid (H2CO3)". Angewandte Chemie International Edition. 39 (5): 891–894. doi:10.1002/(SICI)1521-3773(20000303)39:5<891::AID-ANIE891>3.0.CO;2-E. PMID   10760883.
  8. ↑ Tautermann, Christofer S.; Voegele, Andreas F.; Loerting, Thomas; Kohl, Ingrid; Hallbrucker, Andreas; Mayer, Erwin; Liedl, Klaus R. (2002). "Towards the Experimental Decomposition Rate of Carbonic Acid (H2CO3) in Aqueous Solution". Chemistry - A European Journal. 8 (1): 66–73. doi:10.1002/1521-3765(20020104)8:1<66::AID-CHEM66>3.0.CO;2-F. PMID   11822465.
  9. ↑ Loerting, Thomas; Kroemer, Romano T.; Liedl, Klaus R. (2000). "On the competing hydrations of sulfur dioxide and sulfur trioxide in our atmosphere". Chemical Communications (12): 999–1000. doi:10.1039/b002602f.
  10. ↑ Loerting, Thomas; Liedl, Klaus R. (2000). "Toward elimination of discrepancies between theory and experiment: The rate constant of the atmospheric conversion of SO3 to H2SO4". Proceedings of the National Academy of Sciences. 97 (16): 8874–8878. Bibcode:2000PNAS...97.8874L. doi: 10.1073/pnas.97.16.8874 . PMC   16788 . PMID   10922048.
  11. ↑ Loerting, Thomas; Liedl, Klaus R. (2001). "The reaction rate constant of chlorine nitrate hydrolysis". Chemistry - A European Journal. 7 (8): 1662–1669. doi:10.1002/1521-3765(20010417)7:8<1662::AID-CHEM16620>3.0.CO;2-P. PMID   11349907.
  12. ↑ Voegele, Andreas F.; Tautermann, Christofer S.; Loerting, Thomas; Liedl, Klaus R. (2003). "Toward elimination of discrepancies between theory and experiment: The gas-phase reaction of N2O5 with H2O". Physical Chemistry Chemical Physics. 5 (3): 487–495. Bibcode:2003PCCP....5..487V. doi:10.1039/b208936j.
  13. ↑ Voegele, Andreas F.; Tautermann, Christofer S.; Loerting, Thomas; Liedl, Klaus R. (2002). "Reactions of HOCl + HCl + nH2O and HOCl + HBr + nH2O". Journal of Physical Chemistry A. 106 (34): 7850–7857. Bibcode:2002JPCA..106.7850V. doi:10.1021/jp0255583.
  14. ↑ Voegele, Andreas F.; Tautermann, Christofer S.; Loerting, Thomas; Liedl, Klaus R. (2003). "Reactions of HOBr+ HCl+ nH2O and HOBr+ HBr+ nH2O". Chemical Physics Letters. 372 (3–4): 569–576. Bibcode:2003CPL...372..569V. doi:10.1016/S0009-2614(03)00447-0.
  15. ↑ "Scientist resolves one of the holy grails of physical chemistry after 17 years of research". Phys.Org. Ben-Gurion University of the Negev. 29 September 2022. Retrieved 12 November 2022.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.