Hydronium

Hydronium
3D diagram showing the pyramidal structure of the hydroxonium ion
Ball-and-stick model of the hydronium ion
Hydroxonium-3D-elpot.png
Van der Waals radius of Hydronium
IUPAC name oxonium
Other names hydronium ion
Properties
Molecular formula H3O+
Molar mass 19.02 g/mol
Acidity (pKa) −1.7
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)
Infobox references

In chemistry, hydronium is the common name for the cation H3O+ derived from protonation of water. It is the simplest type of an oxonium ion.

Contents

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.

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. Since free protons react with water to form hydronium, the acidity of an aqueous solution is determined by its hydronium concentration.

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 [1]: 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 [2]. 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[3]. Further, several disparate features of these simulations agree with their experimental counterparts suggesting an alternative interpretation of the experimental results.

Zundel cation

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 [4]. In the Zundel H5O2+ complex the proton is shared equally by two water molecules in a symmetric hydrogen bond[5]. Recent work indicates that both of these complexes represent ideal structures in a more general hydrogen bond network defect [6].

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.[7].

A 2007 calculation of the enthalpies and free energies of the various hydrogen bonds around the hydronium cation in liquid protonated water[8] 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) [9]. 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.

Interstellar H3O+

Hydronium is an abundant molecular ion in the interstellar medium and is found in diffuse[10] and dense [11] molecular clouds as well as the plasma tails of comets [12]. 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 [13]. 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 [14]. 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 [15]. However, H2O does not have many favorable transitions for ground based observations [16]. 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 [17].

Hydronium, on the other hand, has several transitions that make it a superior candidate for detection and identification in a variety of situations [18]. This information has been used in conjunction with laboratory measurements of the branching ratios of the various H3O+ dissociative recombination reactions [19] to provide what are believed to be relatively accurate OH and H2O abundances without requiring direct observation of these species [20], [21].

References

  1. ↑ Zavitsas, A. A. (2001) Properties of water solutions of electrolytes and nonelectrolytes. J. Phys. Chem. B 105 7805-7815.
  2. ↑ Hulthe, G.; Stenhagen, G.; Wennerström, O. & C-H. Ottosson, C-H. (1997) Water cluster studied by electrospray mass spectrometry. J. Chromatogr. A 512 155-165.
  3. ↑ Iyengar, S. S. ;Petersen, M. K.; Burnham, C. J.; Day, T. J. F.; Voth, G. A. (2005) The Properties of Ion-Water Clusters. I. The Protonated 21-Water Cluster. J. Chem. Phys. 123 084309.
  4. ↑ Zundel, G. & Metzger, H. (1968) Energiebänder der tunnelnden Überschuß-Protonen in flüssigen Säuren. Eine IR-spektroskopische Untersuchung der Natur der Gruppierungen H5O2+ Z. Phys. Chem. 58 225-245.
  5. ↑ Wicke, E.; Eigen, M. & Ackermann, Th. (1954) Über den Zustand des Protons (Hydroniumions) in wäßriger Lösung. Z. Phys. Chem. 1 340-364.
  6. ↑ Marx, D.; Tuckerman, M. E.; Hutter, J. & Parrinello, M. (1999) The nature of the hydrated excess proton in water. Nature 397 601-604.
  7. ↑ Mateescu, Gheorghe D. & Benedikt, George M. (1979) Water and related systems. 1. The hydronium ion (H3O+). Preparation and characterization by high resolution oxygen-17 nuclear magnetic resonance. Journal of the American Chemical Society , 101(14), 3959-60.
  8. ↑ Structure and energetics of the hydronium hydration shells. Omer Markovitch and Noam Agmon J. Phys. Chem. A; 2007; 111(12) pp 2253 - 2256; [1]
  9. ↑ The Nature of the H3O+ Hydronium Ion in Benzene and Chlorinated Hydrocarbon Solvents. Conditions of Existence and Reinterpretation of Infrared Data Evgenii S. Stoyanov, Kee-Chan Kim, and Christopher A. Reed J. Am. Chem. Soc.; 2006; 128(6) pp 1948 - 1958; Abstract
  10. ↑ Faure A. & Tennyson, J. (2003) Rate coefficients for electron-impact rotational excitation of H3+ and H3O+. Mon. Not. R. Astron. Soc. 340 468-472.
  11. ↑ Hollis, J.M.; Churchwell, E.B.; Herbst, E. & De Lucia, F.C. (1986) An interstellar line coincident with the P(2,l) transition of hydronium (H3O+). Nature 322 524-526.
  12. ↑ Rauer, H. (1997) Ion composition and solar wind interaction: Observations of comet C/1995 O1 (Hale-Bopp). Earth, Moon, and Planets 79 161-178.
  13. ↑ Vejby-Christensen, L.; Andersen, L.H.; Heber, O.; Kella, D.; Pedersen, H.B.; Schmidt, H.T. & Zajfman, D. (1997) Complete Branching Ratios for the Dissociative Recombination of H2O+, H3O+, and CH3+. Ap. J. 483 531-540.
  14. ↑ Neau, A.; Khalili, A.A.; Rosen, S.; Le Padellec, A.; Derkatch, A.M.; Shi, W.; Vikor, L.; Larsson, M.; Semaniak, J.; & Thomas, R. (2000) Dissociative recombination of D3O+ and H3O+: Absolute cross sections and branching ratios. J. Chem. Phys. 113 1762-1770.
  15. ↑ Neufeld, D.A.; Lepp, S.; & Melnick, G.J. (1995) Thermal Balance in Dense Molecular Clouds: Radiative Cooling Rates and Emission-Line Luminosities. Ap. J. Supplement V 132-147.
  16. ↑ Wootten, A.; Boulanger, F.; Bogey, M.; Combes, F.; Encrenaz, P.J.; Gerin, M. & Ziurys, L.(1986) A search for interstellar H3O+. A&A 166 L15-L18.
  17. ↑ Wootten, A.; Boulanger, F.; Bogey, M.; Combes, F.; Encrenaz, P.J.; Gerin, M. & Ziurys, L.(1986) A search for interstellar H3O+. A&A 166 L15-L18.
  18. ↑ Wootten, A.; Boulanger, F.; Bogey, M.; Combes, F.; Encrenaz, P.J.; Gerin, M. & Ziurys, L.(1986) A search for interstellar H3O+. A&A 166 L15-L18.
  19. ↑ Neau, A.; Khalili, A.A.; Rosen, S.; Le Padellec, A.; Derkatch, A.M.; Shi, W.; Vikor, L.; Larsson, M.; Semaniak, J.; & Thomas, R. (2000) Dissociative recombination of D3O+ and H3O+: Absolute cross sections and branching ratios. J. Chem. Phys. 113 1762-1770.
  20. ↑ Herbst, E.; Green, S.; Thaddeus, P. & Klemperer, W. (1977) Indirect observation of unobservable interstellar molecules. Ap. J. 215 503-510.
  21. ↑ Phillips, T.G.; Dishoeck, E.F. & Keene, J.B. (1992) Interstellar H3O+ and its Relation to the O2 and H2O Abundances. Ap. J. 399 533-550.