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Singlet molecular oxygen, 1O2

Singlet molecular oxygen, purple = -ve, green= +ve

Oxygen and water

V Oxygen

V Peroxide and oxygen radicals

V Redox reactions of O2

Oxygen

The greater solubility of O2 over N2, in spite of its lesser clathrate forming ability [1168] has been proposed due to its formation of weak hydrogen bonds to water [1168]. The formation of O=O···H-OH hydrogen bonds may be seen as the first stage in the natural low-level formation of oxygen redox products (for example, H2O2) in water. As the ratio of O2/N2 solubilities has a maximum at 290 K, there is an indication that partial clathrate cages may be responsible for the polarization that encourages the hydrogen bond formation.

 

Water stores and transmits information, concerning solutes, by means of its hydrogen-bonded network. Changes to this clustering network brought about by gaseous solutes may take some time to re-equilibrate. It has been shown that a high magnetic field has an insignificant effect on the equilibrium content of dissolved oxygen (< 0.3 mM at 20 °C under atmospheric conditions) but does significantly enhance its dissolution rate [176]. There is one report that magnetically treated water (also from the same laboratory, electromagnetically treated water) retains a significantly changed effect on fungal spore germination for at least 24 hours [174]; however other parameters (for example, reduced dissolved oxygen levels) may be responsible for such effects. Mechanically-induced hydrogen bond breakage, caused by shaking, has been reported to last for weeks [336].

 

High electric fields (E ~109 V m-1) reduce water's permittivity [616], which will increase the solubility of gases. Water may be supersaturated with oxygen (~3-6 mM; equivalent to less than a breath of air in each liter of supersaturated water) under pressure. It should be noted that, left by itself, degassed water may take days to re-equilibrate with atmospheric gases (except for CO2 that dissolves much faster) and as even small amounts of dissolved gases are reported to have relatively large effects on the structuring of water [560], it is not unreasonable to suppose that artificially induced metastable conditions with higher gas content may last for some time. Drinking of oxygenated water does give a transient moderate increase in serum ascorbyl radicals (with unknown consequences), an effect that disappears with regular consumption [422]. It will not, however, significantly add to the body's oxygen intake and has no apparent harmful or health-promoting effects [772].

 

Triplet oxygen may convert to singlet oxygen under near-infrared irradiation in solution [2275]., in spite of this transition being 'forbidden' in isolated molecules

λ = 1264 nm

3g               3O2 -> 1O2               1Δg

 

followed by,

slow  

  1O2 + H2O -> HO2· + ·OH

 

Production of singlet oxygen (1O2;1Δg+, electrons paired in their π-antibonding molecular orbitals, compare 3O2, normal triplet oxygen, 3Σg-, where two electrons are in equivalent but separate π-antibonding orbitals with the same unpaired spin) during processing may cause the dissolved peroxide concentration to increase via the water-catalyzed H2O-oxidation reaction;

 

                                   x.1O2+2H2O -> (1 –x ).3O2+2H2O2        [1199]

 

with possible consequential pharmacological effects. Interestingly singlet oxygen takes part in antibody-catalyzed water oxidation similarly producing triplet oxygen and hydrogen peroxide [624]. However, as the lifetime of the singlet oxygen is expected to be in the μs range when dissolved upwards towards 45 min in the (low pressure) gas phase, singlet oxygen molecules are not expected to remain in the processed bottled water. 

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Lewis electron configurations

 

Lewis electron configurations

Peroxide and oxygen radicals

Water can break down in a number of ways

 

H2O -> e-(aq) + H· + ·OH + H3O+ + OH- + H2O2 + HO2· + H2 + O2·- + 1O2

 

Free radicals (for example, hydroxyl radicals) and free (hydrated) electrons can be introduced into water by techniques such as electrochemistry, ultrasonics, by direct water photolysis by ultraviolet radiation (~150 nm, [497], 253.7 nm, [3102] disinfection or simply by agitation. Such processes are catalyzed by the trace amounts of Fe2+/Fe3+ present even in purified water. Radicals can initiate chain reactions involving cascades of reactions; for example, a single ·OH hydroxyl radical may result in the formation of 34 peroxide molecules [3102]. Such hydroxyl radicals may also be created by 190-300 nm radiation exciting an electron from the hydroxide ion [3022]. Water may also be split by electrolysis or mechanical methods such as ultrasonics or stirring with a catalyst [739], to give H2 and O2 and some associated free radicals such as the highly reactive hydroxyl radical. In particular, low concentrations of hydrogen peroxide (H2O2) may be produced from water (H2O) by any process that moves clusters of water relative to each other such as mechanical vibration and stirring [1066].

 

Hydrogen peroxide

Hydrogen peroxide, purple = +ve, green= -ve

 

        (H2O)n(H2O pull leftH-OHpull right OH2)(H2O)m ->

                (H2O)n(H2O + H· + ·OH + OH2)(H2O)m

 

2 ·OH -> H2O2

 

without the need for molecular oxygen but increased by it [1066], for example,

 

(normal triplet oxygen)          3O2 + ·H -> HO2·                                 

     HO2·+ ·H -> H2O2

           HO2·+HO2· -> H2O2 + 1O2

(highly reactive singlet oxygen)

 

H2O2 torsional energies

H2O2 torsional energies, calculated using the Restricted Hartree-Fock wave function (RHF) and the 6-31G** basis set

 

 

 

 

The hydrogen peroxide structure (see above right has been calculated using the Restricted Hartree-Fock wave function (RHF) and the 6-31G** basis set. Torsional rotation around the central O-O bond gives two positions of minimum energy (116.26° and 243.74°, see left) and a high rotational barrier due to repulsion between the lone pairs of the oxygen atoms (cis, at 0°). These two minima are connected by a low-energy saddle-point (trans, at 180°) of approximately thermal energy (shown orange), so that the H2O2 molecule rapidly changes its torsional angle between these minima but does not fully rotate.

 

H2O2 is a weak acid with pKa = 11.65 (25 °C). A second pKa has not been found but is likely to be > 22. The hydrogen atoms carry a charge of +0.365, similar to those on water, with the O–H bonds of similar length to those in H2O. The minimum energy structure gives a slightly greater molecular dipole moment (2.26 D) than has water (1.85 D). H2O2 is an oxidizing agent stronger than chlorine but weaker than ozone,

 

H2O2 + MH2 -> 2H2O + M

 

where M represents part of the oxidized material. H2O2 can also be a reducing agent (for example, of sodium hypochlorite in the preparation of oxygen),

NaOCl + H2O2 -> NaCl + O2+ H2O

Hydrogen peroxide is unstable and slowly breaks down to water and triplet oxygen (normal oxygen),

 

2H2O2 -> 2H2O + 3O2       ΔG° = -77 kJ ˣ mol-1

 

This decomposition can be rapidly accelerated by catalysts such as platinum metal and the enzyme catalase. Other materials (e.g. Fe2+) can catalyze its breakdown to free radicals such as hydroxyl HO· and hydroperoxyl HOO·. Hydrogen peroxide is more stable at slightly acid pH and at lower temperatures. Hydrogen peroxide may also break down under UV irradiation to form strongly oxidizing hydroxyl (HO·) and hydroperoxyl (HO2·) radicals

 

λ = 250~420 nm 

+ H2O2 -> 2 ·OH     

   ·OH + H2O2 -> HO2· + H2O

 

Hydrated superoxide anion, O2·-

superoxide with 4 water molecules

The hydroperoxyl radical (HO2·) is a weak acid (pKa = 4.8) and ionizes to give the superoxide radical anion (O2·-). The superoxide radical anion (O2·-) hydrates in a planar manner (see right) to H-O protons from four water molecules [2007]. It decays by reaction with its conjugate acid HO2·,

 

O2·- + HO2· + H2O -> H2O2 + O2 + OH-

 

in water at a rate constant of about 108 M-1 ˣ s-1 which gives a half life-period of about a second, when both O2·- and HO2· are at micromolar concentration.

 

The hydroxyl radical, ·OH, is a highly reactive, if short-lived (< 1 ns in biological systems), radical. It is also a strong oxidizing agent where it picks up an electron to become the hydroxyl ion (OH-). It removes hydrogen atoms from organic molecules, leaving the organic radicals to further react,

·OH + RH -> H2O + R·

R· + O2 -> RO2·    

 

Other reactions can occur with other materials such as bicarbonate

 

 ·OH + HCO3- -> ·CO3- + H2O
·CO3- + ·OH -> CO2 + HO2-
  ·CO3- + H2O2 -> HCO3- + HO2·

 

Other more exotic compounds of oxygen and hydrogen have been examined; for example, H2O3 [2498] made from H2O2+ O3, H2O4 [2500] the (HO2)2 dimer, and planar trans-hydridotrioxygen (HO3·) formed from ·OH and O2 [2499].

 

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Oxidation state (Frost) diagram for O2 at pH 7,

from [3334].

 

Oxidation state (Frost) diagram for O2, from [3334]

Redox reactions of O2

The redox reactions of water and oxygen are shown in a Frost diagram (see right) [3334]. It shows nE0 plotted against the oxidation number n, where, E0 is the standard reduction potential for the couple, and n is the number of electrons transferred in the conversion. The vertical axis shows the reaction free energy as nE0 = -ΔG/F, so the higher the potential the more reactive the material. The standard oxidation potentials are the negative slopes of the lines joining any two states that form a redox couple.

 

As examples, the long red line represents,

 

½O2 + 2H3O++ 2e- -> 3H2O           E°' = +0.815 V

 

the lightest blue line represents,

 

H2O2 + e- + H3O+ -> ·OH + 2H2O  E°' = +0.39 V

 

and the long purple line represents,

 

·OH + e- + H3O+ -> 2H2O           E°' = +2.31 V

 

See also water electrolysis.

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This page was established in 2001 and last updated by Martin Chaplin on 26 July, 2018


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