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Hydronium
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In chemistry, hydronium is the common name for the aqueous cation H3O+ derived from protonation of water. It is the simplest type of an oxonium ion.
rding 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 is any ion with a trivalent oxygen cation.
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Encyclopedia
In chemistry, hydronium is the common name for the aqueous cation H3O+ derived from protonation of water. It is the simplest type of an oxonium ion.
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 is any ion with a trivalent oxygen cation. For example, a protonated hydroxyl group is an oxonium ion, but not a hydronium.
Chemical Structure
Since O+ and N have the same number of electrons, H3O+ is isoelectronic with NH3. As shown in the images above, H3O+ has a trigonal pyramid geometry with the oxygen atom at its apex. The H-O-H bond angle is approximately 107o, and the center of mass is very close to the O atom. Because the base of the pyramid is made up of three identical hydrogen atoms, the H3O+ molecule's symmetric top configuration is such that it belongs to the C3v point group. Because of this symmetry and the fact that it has a dipole moment, the rotational selection rules are J = 1 and K = 0. The transition dipole lies along the c axis and, because the negative charge is localized near the O 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 ions. These hydrons do not exist in a free state: they are extremely reactive and are solvated by water. An acid is generally the source of these hydrons; however, since water can behave as both an acid and a base, hydroniums exist even in pure water. This special case of water reacting with water to produce hydronium (and hydroxide) ions is commonly known as the self-ionization of water. The resulting hydronium ions are few and short-lived. Despite their short life they form the basis for determining the pH of basic aqueous solutions, since the less there are of these autoionized hydroniums, the more basic the solution is.
Hydronium is very acidic: at 25°C, its pKa is -1.7. It is also the most acidic species that can exist in water (assuming sufficient water for dissolution): 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 acids must be better proton donors than hydronium, otherwise a significant portion of acid will exist in a non-ionized state. Unlike the hydronium that results from water's autodissociation, these hydronium ions are long-lasting and concentrated, in proportion to the strength of the dissolved acid.
The pH of a solution is a measure of its hydrogen ion concentration. As free protons react with water to form hydronium, the acidity of an aqueous solution is determined by its hydronium concentration.
In Organic syntheses, such as acid catalyzed reactions, the hydronium ion (H3O+) 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 of hydronium ion in water, in part because many different meanings of solvation exist. A freezing-point depression study determined that the mean hydration ion in cold water is approximately H3O+(H2O)6 : 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 H3O+(H2O)20 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 molecular dynamics simulations have shown that, on average, the hydrated proton resides on the surface of the H3O+(H2O)20 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 H9O4+ complex in which the hydronium is strongly hydrogen-bonded to 3 neighbouring water molecules . In the Zundel H5O2+ complex the proton is shared equally by two water molecules in a symmetric hydrogen bond. 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 solution (HF-SbF5SO2). The ion was characterized by high resolution O-17 nuclear magnetic resonance..
A 2007 calculation of the enthalpies and free energies 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 showed that the hydrogen-bonds around the hydronium ion (formed with the three water ligands in the first solvation shell of the hydronium) are quite strong compared to those of bulk water.
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 H3O+ salts. For example, hydrochloric acid has an ionization constant of 107, and mixtures with water at all proportions are liquid at room temperature. However, perchloric acid 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 H(CB11H(CH3)5Br6) . X-ray crystallography shows a C3v symmetry for the hydronium ion with each proton interacting with a bromine atom each from three carborane anions 320 pm apart on average. The [H3O][H(CB11HCl11)] salt is also soluble in benzene. In crystals grown from a benzene solution the solvent co-crystallizes and a H3O.(benzene)3 cation is completely separated from the anion. In the cation three benzene molecules surround hydronium forming pi-cation interactions with the hydrogen atoms. The closest (nonbonding) approach of the anion at chlorine to the cation at oxygen is 348 pm.
There are also many examples of hydrated hydronium ions known, for example the H5O2+ ion in HCl.2H2O, the H7O3+ and H9O4+ both found In HBr.4H2O.
Interstellar H3O+
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 H2 into H2+ by cosmic radiation . H3O+ can produce either OH or H2O through dissociative recombination 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 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 deteurated version of water) could potentially be used for estimating H2O abundances, the ratio of HDO to H2O 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 H3O+ dissociative recombination reactions to provide what are believed to be relatively accurate OH and H2O abundances without requiring direct observation of these species , .
Interstellar Chemistry
As mentioned previously, H3O+ is found in both diffuse and dense molecular clouds. By applying the the reaction rate constants (α, β, and γ) from udfa.net corresponding to all of the currently available characterized reactions involving H3O+, 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 [H2]2. 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 H3O+'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 reactions.
| Reaction | Type | Rel. Rate (cm3-s-1) at 10 K | Rel. Rate (cm3-s-1) at 50 K |
|---|
| H2 + H2O+ ? H3O+ + H | Formation | 2.97 x 10-22 | 2.97 x 10-22 | | H2O + HCO+ ? CO + H3O+ | Formation | 4.52 x 10-23 | 4.52 x 10-23 | | H3+ + H2O ? H3O+ + H2 | Formation | 3.75 x 10-23 | 3.75 x 10-23 | | H3O+ + e- ? OH + H + H | Destruction | 2.27 x 10-22 | 1.02 x 10-22 | | H3O+ + e- ? H2O + H | Destruction | 9.52 x 10-23 | 4.26 x 10-23 | | H3O+ + e- ? OH + H2 | Destruction | 5.31 x 10-23 | 2.37 x 10-23 |
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 and constants of 0, resulting in k=\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 m (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 m (2.98073 THz), 11- - 11+ at 181.054 m (1.65582 THz) and 20- - 10+ at 100.869 m (2.9721 THz).
See also
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