Dioxygen triplet state

The dioxygen molecule exists in two forms a triplet or ground state in which it is a stable biradical and a singlet or excited state in which it is not a radical. Reactions of carotenoids with singlet oxygen have already been presented in this chapter and we now focus on the reactions of carotenoids and oxygen in the ground or triplet state. 

The study of the mechanism of photoinitiation is complicated by the quenching action of dioxygen (see page 122). The values of the triplet state of selected compounds used as photosensitizers are given in Table 3.17. 

Such compounds with conugated ir-bonds are excited by light to the triplet state, and then such a triplet molecule reacts with dioxygen with the formation of peroxide. 

This chapter s discussion does not treat inorganic and organic free radicals and triplet states (such as dioxygen, O2), which produce EPR spectra. Rather, the focus here will be on EPR behavior of transition metal centers that occur in biological species. An excellent presentation of the subject, written by Graham Palmer, is found in Chapter 3 of reference l.16 The discussion here is summarized mostly from that source. 

For a more in-depth interpretation of the inertness of dioxygen, the fact that 02 is a triplet state bi-radical, i.e. it has two unpaired electrons in the 2jig orbitals, needs to be considered. It follows that the oxidation of singlet state substrates by the triplet 02 to form singlet products is spin-forbidden and, as a consequence, relatively slow. 

This chapter s discussion does not treat inorganic and organic free radicals and triplet states (such as dioxygen, O2), which produce EPR spectra. Rather,... 

Fig. 27. A plot of the v00 bond frequency for dioxygen species in the solid state as a function of the bond order. The value for 02 refers to the gas phase value for oxygen in the triplet state (21).

Normally the term radical (or free radical ) is confined to molecules or ions with one unpaired electron (called doublet states because the electron has two magnetic quantum numbers +1/2). In the non-metal field the most common paramagnetic species other than radicals are those with two unpaired electrons, called triplet states (magnetic quantum numbers 0, 1). Quite the most important triplet-state molecule is dioxygen and it is a great pity that ESR spectroscopy can, for various reasons, only be used to detect O2 in the gas-phase or certain crystalline solids. Other important triplet-states are sometimes obtained on photo-excitation of ordinary (singlet-state) molecules or ions, and these have reactions in some ways typical of di-radicals (i.e.,... 

Dioxygen in its ground (triplet) state reacts as a rather stable diradical. One of its most important radical reactions is [4] in which radicals R are converted into peroxyl radicals R-OO which generally have quite different reactivities from those of the parent R species. This reaction is, in general, reversible, and stable radicals such as nitrox-ides fail to undergo reaction [1.1] to any measurable extent. 

Ground-State dioxygen has two unpaired electrons ( 02 ), which makes it a biradical with a triplet electronic state (see Table 4). Its radical character is limited because the H O bond is weak (—AGbf, 51 kcalmol" ), and the triplet state precludes direct reaction with singlet-state substrate molecules with saturated a bonding. 

Triplet states are generally molecules in which one electron is promoted to a higher electronic orbital (e.g. by the absorption of light) under spin reversal, i.e. it is no longer paired with its partner in the orbital it has left and the two electrons have a combined magnetic moment what is twice that of a radical. Some molecules, e.g. dioxygen, have a triplet ground state. 

Figure 11.2. Molecular orbital diagrams for O2 and O2. The ground state normal O2 (dioxygen) = triplet ground state) has two unpaired ir-electrons (it is a biradical) and is paramagnetic. Two excited forms of singlet oxygen O2 ( A and E ) can be generated (often by light irradiation in the presence of a sensitizer). The first product in a one-electron reduction of oxygen is the superoxide ) anion. (Adapted from Sawyer, 1991.)...

Equation 2.52 Quenching of singlet and triplet states by dioxygen... 

First-order decay rate constants of transients that are also subject to second-order reactions cannot be determined reliably, even when attempts are made to correct for the observable second-order contributions. A typical example is the triplet state half-life of anthracene in degassed solution, which appears to be somewhere in the range 20 (ts 1 ms on most instruments (the values are reproducible on a given instrument but vary widely between different setups and solvent purification methods). The lifetime of triplet anthracene in solution rises to 25 ms when measures are taken to reduce quenching by diffusional encounters with the parent molecule (self-quenching) with other molecules in the triplet state (triplet triplet annihilation) and with impurity quenchers such as dioxygen.200 Hence lifetimes of transients that are prone to second-order decay contributions, such as long-lived triplet states or radicals, should always be considered as lower limits. 

The highest rate constant observed for intramolecular ET in AO is 1100 s , which still is considerably smaller than the turnover number of about 14,000 s . Thus, interaction between dioxygen and the trinuclear site is not sufficient to ensure maximal enzymatic activity. Under optimal conditions, the concentration of reducing substrate (e.g., ascorbate) is sufficiently high to maintain a steady state of fully reduced copper sites. Thus, an antithetical approach was very recently taken by Tollin and co-workers studying the reoxidation of fully reduced AO by a laser-generated triplet state of 5-deazariboflavin 41). Subsequent to the assumed one-electron oxidation of the reduced trinuclear cluster, a rapid, biphasic intramolecular ET occurs from Tl[Cu(I)] (and presumably) to the oxidized trinuclear center. The faster of the two observed rate constants (9500 and 1400 s, respectively) is comparable to the turnover number determined for AO under steady-state conditions and renders it likely that this is the rate-limiting step in catalysis. 

Beyond the relevance in this particular study, the peroxy-like defects can be reaction intermediates in other zeolites-based oxidations in industrial applications. On the basis of present results, it can be argued that more effective oxidizing media can be obtained by modifying zeolites and mesoporous aluminosilicates in order to allow an easier formation of peroxy-like structures. In this respect, the presented data may suggest a possible activation mechanism of the inert triplet state of dioxygen in the cavities of nitrite sodalite to the more reactive singlet O2 [26]. 

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