Hydronium

Hydronium

Overview
In chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

, a hydronium ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

 is the cation , a type of oxonium ion
Oxonium ion
The oxonium ion in chemistry is any oxygen cation with three bonds. The simplest oxonium ion is the hydronium ion H3O+. Another oxonium ion frequently encountered in organic chemistry is obtained by protonation or alkylation of a carbonyl group e.g...

 produced by protonation
Protonation
In chemistry, protonation is the addition of a proton to an atom, molecule, or ion. Some classic examples include*the protonation of water by sulfuric acid:*the protonation of isobutene in the formation of a carbocation:2C=CH2 + HBF4 → 3C+ + BF4−*the protonation of ammonia in the...

 of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

. This cation is often used to represent the nature of the proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

 in aqueous solution, where the proton is highly solvated (bound to a solvent). The reality is far more complicated, and a proton is bound to several molecules of water, such that other descriptions such as H5O2+, H7O3+ and H9O4+ are increasingly accurate description of the environment of a proton in water.
Discussion
Ask a question about 'Hydronium'
Start a new discussion about 'Hydronium'
Answer questions from other users
Full Discussion Forum
 
Unanswered Questions
Encyclopedia
In chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

, a hydronium ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

 is the cation , a type of oxonium ion
Oxonium ion
The oxonium ion in chemistry is any oxygen cation with three bonds. The simplest oxonium ion is the hydronium ion H3O+. Another oxonium ion frequently encountered in organic chemistry is obtained by protonation or alkylation of a carbonyl group e.g...

 produced by protonation
Protonation
In chemistry, protonation is the addition of a proton to an atom, molecule, or ion. Some classic examples include*the protonation of water by sulfuric acid:*the protonation of isobutene in the formation of a carbocation:2C=CH2 + HBF4 → 3C+ + BF4−*the protonation of ammonia in the...

 of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

. This cation is often used to represent the nature of the proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

 in aqueous solution, where the proton is highly solvated (bound to a solvent). The reality is far more complicated, and a proton is bound to several molecules of water, such that other descriptions such as H5O2+, H7O3+ and H9O4+ are increasingly accurate description of the environment of a proton in water. The ion has been detected in the gas phase.

Determination of pH


It is the presence of hydronium ion relative to hydroxide
Hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...

 that determines a solution's pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

. The molecules in pure water auto-dissociate
Self-ionization of water
The self-ionization of water is the chemical reaction in which a proton is transferred from one water molecule to another, in pure water or an aqueous solution, to create the two ions, hydronium, H3O+ and hydroxide, OH−...

 into hydronium and hydroxide ions in the following equilibrium:
2 +


In pure water, there is an equal number of hydroxide and hydronium ions so it has a neutral pH of 7. A pH value less than 7 indicates an acidic solution, and a pH value more than 7 indicates a basic solution. (Note that this is only true at 25°C as the equilibrium is temperature dependent).

Nomenclature


According to IUPAC nomenclature of organic chemistry, the hydronium ion should be referred to as oxonium. Hydroxonium may also be used unambiguously to identify it. A draft IUPAC proposal also recommends the use of oxonium and oxidanium in organic and inorganic chemistry contexts, respectively.

An oxonium ion
Oxonium ion
The oxonium ion in chemistry is any oxygen cation with three bonds. The simplest oxonium ion is the hydronium ion H3O+. Another oxonium ion frequently encountered in organic chemistry is obtained by protonation or alkylation of a carbonyl group e.g...

 is any ion with a trivalent oxygen cation. For example, a protonated hydroxyl group is an oxonium ion, but not a hydronium.

Structure


Since and N have the same number of electrons, is isoelectronic with ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

. As shown in the images above, has a trigonal pyramid geometry with the oxygen atom at its apex. The H-O-H bond angle is approximately 113°, and the center of mass is very close to the oxygen atom. Because the base of the pyramid is made up of three identical hydrogen atoms, the molecule's symmetric top configuration is such that it belongs to the C3v point group
Point groups in three dimensions
In geometry, a point group in three dimensions is an isometry group in three dimensions that leaves the origin fixed, or correspondingly, an isometry group of a sphere. It is a subgroup of the orthogonal group O, the group of all isometries that leave the origin fixed, or correspondingly, the group...

. Because of this symmetry and the fact that it has a dipole moment, the rotational selection rules are ΔJ = ±1 and ΔK = 0. The transition dipole lies along the c axis and, because the negative charge is localized near the oxygen atom, the dipole moment points to the apex, perpendicular to the base plane.

Acids and acidity


Hydronium is the cation that forms from water in the presence of hydrogen ion
Hydrogen ion
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....

s. These hydrons do not exist in a free state: they are extremely reactive and are solvated
Solvation
Solvation, also sometimes called dissolution, is the process of attraction and association of molecules of a solvent with molecules or ions of a solute...

 by water. An acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

ic solute is generally the source of these hydrons; however, hydroniums exist even in pure water. This special case of water reacting with water to produce hydronium (and hydroxide
Hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...

) ions is commonly known as the self-ionization of water
Self-ionization of water
The self-ionization of water is the chemical reaction in which a proton is transferred from one water molecule to another, in pure water or an aqueous solution, to create the two ions, hydronium, H3O+ and hydroxide, OH−...

. The resulting hydronium ions are few and short-lived. pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 is a measure of the relative activity of hydronium and hydroxide ions in aqueous solutions. In acidic solutions, hydronium is the more active, its excess proton being readily available for reaction with basic species.

Hydronium is very acidic: at 25°C, its pKa
PKA
PKA, pKa, or other similar variations may stand for:* pKa, the symbol for the acid dissociation constant at logarithmic scale* Protein kinase A, a class of cAMP-dependent enzymes* Pi Kappa Alpha, the North-American social fraternity...

 is -1.74 . It is also the most acidic species that can exist in water (assuming sufficient water for dissolution)(see leveling effect): any stronger acid will ionize and protonate a water molecule to form hydronium. The acidity of hydronium is the implicit standard used to judge the strength of an acid in water: strong acid
Strong acid
A strong acid is an acid that ionizes completely in an aqueous solution by losing one proton, according to the equationFor sulfuric acid which is diprotic, the "strong acid" designation refers only to dissociation of the first protonMore precisely, the acid must be stronger in aqueous solution than...

s must be better proton donors than hydronium, otherwise a significant portion of acid will exist in a non-ionized state. Unlike hydronium in neutral solutions that result from water's autodissociation, hydronium ions in acidic solutions are long-lasting and concentrated, in proportion to the strength of the dissolved acid.

pH was originally conceived to be a measure of the hydrogen ion
Hydrogen ion
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....

 concentration of aqueous solution. We now know that virtually all such free protons quickly react with water to form hydronium; acidity of an aqueous solution is therefore more accurately characterized by its hydronium concentration. In organic syntheses, such as acid catalyzed reactions, the hydronium ion can be used interchangeably with the H+ ion; choosing one over the other has no significant effect on the mechanism of reaction.

Solvation


Researchers have yet to fully characterize the solvation
Solvation
Solvation, also sometimes called dissolution, is the process of attraction and association of molecules of a solvent with molecules or ions of a solute...

 of hydronium ion in water, in part because many different meanings of solvation exist. A freezing-point depression
Freezing-point depression
Freezing-point depression describes the phenomenon in which the freezing point of a liquid is depressed when another compound is added, meaning that a solution has a lower freezing point than a pure solvent. This happens whenever a non-volatile solute is added to a pure solvent, such as water...

 study determined that the mean hydration ion in cold water is approximately : on average, each hydronium ion is solvated by 6 water molecules which are unable to solvate other solute molecules.

Some hydration structures are quite large: the magic ion number structure (called magic because of its increased stability with respect to hydration structures involving a comparable number of water molecules) might place the hydronium inside a dodecahedral cage. However, more recent ab initio method
Ab initio quantum chemistry methods
Ab initio quantum chemistry methods are computational chemistry methods based on quantum chemistry. The term ab initiowas first used in quantum chemistry by Robert Parr and coworkers, including David Craig in a semiempirical study on the excited states of benzene.The background is described by Parr...

 molecular dynamics simulations have shown that, on average, the hydrated proton resides on the surface of the cluster. Further, several disparate features of these simulations agree with their experimental counterparts suggesting an alternative interpretation of the experimental results.

Two other well-known structures are the Zundel cations and Eigen cations. The Eigen solvation structure has the hydronium ion at the center of an complex in which the hydronium is strongly hydrogen-bonded
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...

 to three neighbouring water molecules. In the Zundel complex the proton is shared equally by two water molecules in a symmetric hydrogen bond
Symmetric hydrogen bond
A symmetric hydrogen bond is a special type of hydrogen bond in which the proton is spaced exactly halfway between two identical atoms. The strength of the bond to each of those atoms is equal. It is an example of a 3-center 4-electron bond. This type of bond is much stronger than "normal" hydrogen...

. Recent work indicates that both of these complexes represent ideal structures in a more general hydrogen bond network defect.

Isolation of the hydronium ion monomer in liquid phase was achieved in a nonaqueous, low nucleophilicity superacid
Superacid
According to the classical definition superacid is an acid with an acidity greater than that of 100% pure sulfuric acid, which has a Hammett acidity function of −12. According to the modern definition, superacid is a medium, in which the chemical potential of the proton is higher than in pure...

 solution (HF-SbF5SO2). The ion was characterized by high resolution O-17 nuclear magnetic resonance
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...

.

A 2007 calculation of the enthalpies
Enthalpy
Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a...

 and free energies
Thermodynamic free energy
The thermodynamic free energy is the amount of work that a thermodynamic system can perform. The concept is useful in the thermodynamics of chemical or thermal processes in engineering and science. The free energy is the internal energy of a system less the amount of energy that cannot be used to...

 of the various hydrogen bonds around the hydronium cation in liquid protonated water at room temperature and a study of the proton hopping mechanism using molecular dynamics
Molecular dynamics
Molecular dynamics is a computer simulation of physical movements of atoms and molecules. The atoms and molecules are allowed to interact for a period of time, giving a view of the motion of the atoms...

 showed that the hydrogen-bonds around the hydronium ion (formed with the three water ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

s in the first solvation shell of the hydronium) are quite strong compared to those of bulk water.

A new model was proposed by Stoyanov based on infrared spectroscopy
Infrared spectroscopy
Infrared spectroscopy is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic...

 in which the proton exists as an ion. The positive charge is thus delocalized over 6 water molecules.

Solid hydronium salts


For many strong acids, it is possible to form crystals of their hydronium salt that are relatively stable. Sometimes these salts are called acid monohydrates. As a rule, any acid with an ionization constant of 109 or higher may do this. Acids whose ionization constant is below 109 generally cannot form stable salts. For example, hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....

 has an ionization constant of 107, and mixtures with water at all proportions are liquid at room temperature. However, perchloric acid
Perchloric acid
Perchloric acid is the inorganic compound with the formula HClO4. Usually encountered as an aqueous solution, this colourless compound is a strong acid comparable in strength to sulfuric and nitric acids. It is a powerful oxidizer, but its aqueous solutions up to appr. 70% are remarkably inert,...

 has an ionization constant of 1010, and if liquid anhydrous perchloric acid and water are combined in a 1:1 molar ratio, solid hydronium perchlorate forms.

The hydronium ion also forms stable compounds with the carborane superacid . X-ray crystallography
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...

 shows a C3v symmetry
Point group
In geometry, a point group is a group of geometric symmetries that keep at least one point fixed. Point groups can exist in a Euclidean space with any dimension, and every point group in dimension d is a subgroup of the orthogonal group O...

 for the hydronium ion with each proton interacting with a bromine atom each from three carborane anions 320 pm apart on average. The salt is also soluble in benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....

. In crystals grown from a benzene solution the solvent co-crystallizes and a ·(benzene)3 cation is completely separated from the anion. In the cation three benzene molecules surround hydronium forming pi
Pi bond
In chemistry, pi bonds are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital...

-cation interactions with the hydrogen atoms. The closest (non-bonding) approach of the anion at chlorine to the cation at oxygen is 348 pm.

There are also many examples of hydrated hydronium ions known, such as the ion in , the and ions both found in .

Motivation for study


Hydronium is an abundant molecular ion in the interstellar medium and is found in diffuse and dense molecular clouds as well as the plasma tails of comets. Interstellar sources of hydronium observations include the regions of Sagittarius B2, Orion OMC-1, Orion BN–IRc2, Orion KL, and the comet Hale-Bopp.

Interstellar hydronium is formed by a chain of reactions started by the ionization of into by cosmic radiation. can produce either or through dissociative recombination
Dissociative recombination
Dissociative recombination is a process where a positive molecular ion recombines with an electron, and as a result, the neutral molecule dissociates. This reaction is important for extraterrestrial and atmospheric chemistry. On Earth, dissociative recombination is practically non-existent, as...

 reactions, which occur very quickly even at the low (≥10 K) temperatures of dense clouds. This leads to hydronium playing a very important role in interstellar ion-neutral chemistry.

Astronomers are especially interested in determining the abundance of water in various interstellar climates due to its key role in the cooling of dense molecular gases through radiative processes. However, H2O does not have many favorable transitions for ground based observations. Although observations of HDO (the deuterated version of water
Heavy water
Heavy water is water highly enriched in the hydrogen isotope deuterium; e.g., heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction...

) could potentially be used for estimating H2O abundances, the ratio of HDO to is not known very accurately.

Hydronium, on the other hand, has several transitions that make it a superior candidate for detection and identification in a variety of situations. This information has been used in conjunction with laboratory measurements of the branching ratios of the various dissociative recombination reactions to provide what are believed to be relatively accurate and H2O abundances without requiring direct observation of these species.

Interstellar chemistry


As mentioned previously, is found in both diffuse and dense molecular clouds. By applying the reaction rate constants (α, β, and γ) from udfa.net corresponding to all of the currently available characterized reactions involving , it is possible to calculate k(T) for each of these reactions. By multiplying these k(T) by the relative abundances of the products (also from udfa.net), the relative rates (cm3·s−1) for each reaction at a given temperature can be determined. These relative rates can be made in absolute rates by multiplying them by the . By assuming T = 10 K for a dense cloud and T = 50 K for a diffuse could, the results indicate that most dominant formation and destruction mechanisms were the same for both cases. It should be mentioned that the relative abundances used in these calculations correspond to TMC-1, a dense molecular cloud, and that the calculated relative rates are therefore expected to be more accurate at T = 10 K. The three fastest formation and destruction mechanisms are listed in the table below, along with their relative rates. Note that the rates of these six reactions are such that they make up approximately 99% of 's chemical interactions under these conditions. More about these reactions can be found in. Finally, it should also be noted that all three destruction mechanisms in the table below are classified as dissociative recombination
Dissociative recombination
Dissociative recombination is a process where a positive molecular ion recombines with an electron, and as a result, the neutral molecule dissociates. This reaction is important for extraterrestrial and atmospheric chemistry. On Earth, dissociative recombination is practically non-existent, as...

 reactions.
Reaction Type Rel. Rate (cm3·s−1) at 10 K Rel. Rate (cm3·s−1) at 50 K
H2 + H2O+ → + H Formation 2.97 2.97
H2O + HCO+ → CO + Formation 4.52 4.52
+ H2O → + H2 Formation 3.75 3.75
+ e- → OH + H + H Destruction 2.27 1.02
+ e- → H2O + H Destruction 9.52 4.26
+ e- → OH + H2 Destruction 5.31 2.37


It is also worth noting that the relative rates for the formation reactions in the table above are the same for a given reaction at both temperatures. This is due to the reaction rate constants for these reactions having β and γ constants of 0, resulting in k=α\alpha$ which is independent of temperature.

Since all three of these reactions produce either H2O or OH, these results reinforce the strong connection between their relative abundances and that of H3O+. The rates of these six reactions are such that they make up approximately 99% of H3O+'s chemical interactions under these conditions.

Astronomical detections


As early as 1973 and before the first interstellar detection, chemical models of the interstellar medium (the first corresponding to a dense cloud) predicted that hydronium was an abundant molecular ion and that it played an important role in ion-neutral chemistry. However, before an astronomical search could be underway there was still the matter of determining hydronium's spectroscopic features in the gas phase, which at this point were unknown. The first studies of these characteristics came in 1977, which was followed by other, higher resolution spectroscopy experiments. Once several lines had been identified in the laboratory, the first interstellar detection of H3O+ was made by two groups almost simultaneously in 1986. The first, published in June 1986, reported observation of the JKvt = 11- - 21+ transition at 307192.41 MHz in OMC-1 and Sgr B2. The second, published in August, reported observation of the same transition toward the Orion-KL nebula.

These first detections have been followed by observations of a number of additional H3O+ transitions. The first observations of each subsequent transition detection are given below in chronological order:

In 1991, the 32+ - 22- transition at 364797.427 MHz was observed in OMC-1 and Sgr B2. One year later, the 30+ - 20- transition at 396272.412 MHz was observed in several regions, the clearest of which was the W3 IRS 5 cloud.

The first far-IR 43- - 33+ transition at 69.524 µm (4.3121 THz) was made in 1996 near Orion BN-IRc2. In 2001, three additional transitions of H3O+ in were observed in the far infrared in Sgr B2; 21- - 11+ transition at 100.577 µm (2.98073 THz), 11- - 11+ at 181.054 µm (1.65582 THz) and 20- - 10+ at 100.869 µm (2.9721 THz).

See also

  • Hydron
    Hydron (chemistry)
    In chemistry, a hydron is the general name for a cationic form of atomic hydrogen : most commonly a "proton". However, hydron includes cations of hydrogen regardless of their isotopic composition: thus it refers collectively to protons , deuterons , and tritons...

     (hydrogen cation)
  • Hydride
    Hydride
    In chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...

  • Hydrogen anion
    Hydrogen anion
    The hydrogen anion is a negative hydrogen ion, H−. It is an important constituent of the atmosphere of stars, such as the Sun, where it is the dominant absorber of photons with energies in the range 0.75-4.0 eV, ranging from the infrared into the visible spectrum...

  • Hydrogen ion
    Hydrogen ion
    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....

  • Proton
    Proton
    The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....


External references