Charged Dioxygen Species

Inspection of Table I shows that the dioxygen bond length becomes progressively larger on going from O2 to O2 and this increase is accompanied by a decrease in the dioxygen bond strength. These facts can be explained 

Enthalpy of Processes Involving Dioxygen Species in the Gas Phase  

Of the reactions listed in Table II, the only process that leads to a decrease of the energy of molecular oxygen is the formation of the free superoxide ion, Oj ( — 10.15 kcal/mol). The superoxide ion would therefore be expected to be the dioxygen species most commonly formed on oxide surfaces and in fact it is the species most studied, both in the bulk of various matrices and on surfaces. The other species (Oj and Oj ) are not stable in the gas phase, although they can be stabilized in the solid state (Table I) due to the additional coulombic stabilization from the lattice. 

Nearly all the data in the literature refers to the characterization of O2 on various surfaces and this is discussed in detail in the following sections. 

The usually accepted approach is to adopt an ionic model for the superoxide ion on the surface. In this model, an electron is transferred from the surface to the oxygen to form 02, and there is an electrostatic interaction between the cation at the adsorption site and the superoxide ion. A calculation of the g tensor based on this model (Section 111,A,1) accounts for nearly all the data from adsorbed 02 and is consistent with the evidence that the spin density on both oxygen nuclei is the same (Section III,A,2). However, there are examples of oxygen adsorbed on the surface where the g values do not fit the predictions of the ionic model (Section IV,E) and also a few cases where the spin density on the two oxygen nuclei is found to be different. In these situations it seems likely that a covalent model in which a a bond is formed between the cation and the adsorbed oxygen, is more relevant. 

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Table 14.4 Effect of electron configuration and charge on the bond properties of dioxygen species...

The IR data is consistent with a more complex model of the surface dioxygen rather than the clearly distinct species assumed above. In this paradigm a continuous gradation of species exists between O2 and Of and the species observed will depend on the specific environment at the adsorption site. Dioxygen species with fractional charges commonly occur on the surface. This, taken with the fact that nearly all spectroscopic measurements have been made below 100°C, lends support to the idea that some... 

Table 3 lists some relevant observed frequencies for diatomic molecules, ions and radicals the effect of the charge on the frequency in the series of the dioxygen species is worthy of note and also the vibrational frequencies characteristic of hydrogen halides which generally polymerize in the condensed phases. The HX stretching bands are shifted markedly to lower frequencies upon polymerization. 

If the equivalent of a negatively charged carbonyl group is added to another carbonyl, then a 1,2-dioxygenated species is produced. The reagents that can do this are called acyl anion equivalents—they are the synthons for [XC(=0)]. We have already met a number of such species, the cyanide ion, the anions of alkynes, and dithianes. 

In this view, the distinction between the various species becomes blurred and the actual form of oxygen present will reflect the coordinative environment and may correspond to dioxygen with a fractional charge. These ideas may be of great significance in understanding the mechanism of catalytic reactions. 

On photolysis of the reduced or mixed valence enzyme, the 590 nm band of the a3-CO species disappears and a new band appears at 612 nm, due to the generation of five-coordinate cytochrome a2+. If dioxygen is present, and at a temperature higher than about —100 °C, then a new species (A) is formed, which cannot be photolyzed, and which is spectrally similar to the a3 -CO compound. It is probable that this involves 02 bound to the cytochrome a3, and does not involve bridging a3 and CuB. This species would approximate to oxyhemoglobin, and would involve some charge transfer to bound dioxygen. 

The neutrally charged /x-nitrido dimer of TPPFe, [(TPPFe)2N] and its phthalocyanine " " and mixed-macrocycle counterparts, are isoelectronic with the TT-cation radical of the /x-oxo dimers discussed above, but they have a different electron configuration. " NMR shifts are more consistent with their description as delocalized system having two Fe + rather than a delocalized porphyrin TT-cation radical. This complex readily reacts reversibly with dioxygen to produce an EPR-active species that has... 

Close inspection of the solution phase redox peaks for the four species under analysis obtained in a thin layer cell in deaerated non-aqueous media (not shown here) yielded values for the onset of the reduction of the first Co site very similar to one another. Hence, it can only be surmised that the differences in specificity are related to the presence of the peripheral substituent and seemingly unrelated to the redox properties of the metal sites. Evidence that the differences in the specificity between the meso-substituted and non-meso-substituted materials are due to subtle electronic effects was provided by quantum mechanical calculations. These showed that upon addition of the weso-substituents, the electronic charge density associated with the HOMO (highest occupied molecular orbital) localized on the dioxygen moiety is markedly reduced, thereby weakening its Lewis acid character and hence its ability to coordinate a proton (see Figure 3.67). 

Simple redox solutes, ferrocene, N, N, N, JV-tetramethyl-l,4-phenylenediam-ine, decamethylferrocene, bis(i-propylcyclopentadienyl) iron(ll), [Ru(phen)j] (0104)2, [Fe(bpy)3](0104)2, [Co(bpy)3](0104)2, and iodine have been studied at electrodes modified with polymeric fullerene films. FuUerene-modified electrodes were prepared by electropolymerization of Cjq initiated by traces of dioxygen or by simultaneous electroreduction of fullerene and Pd(ll) acetate trimer. For the former films, the electrochemical activity decreases upon potential cycling. The electrochemical activity of the film is stabihzed by the redox solute added to the electropolimerization stage due to the catalytic oxidation of the fullerene film by the oxidized form of the redox system. Similarly, positively charged species can also be incorporated into the structure of the film. The reversible behavior of redox solutes decreases with the increase in the thickness of the Pd/C q film. This film also incorporates ferricinium ion, N, N, N, N-tetramethyl-l,4-phenylenediamine cation, decamethylferricinium ion, and to a smaller degree [Co(bpy)3]"+ [53]. 

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