Dioxygen paramagnetism

Representations showing electrons in molecules seem to suggest localisation of the valence electrons, but there are problematic issues in this regard. For example, we might ask if dioxygen has a double bond and two lone pairs on each O atom (as in Table 1.1) - a stmcture that does not reconcile with the paramagnetic nature of the substance - or a single bond and an odd number of electrons localised on each atom, as shown here ... 

The failures of CeOj alone or RhjOj alone to yield RO or 01-type paramagnetic centers, when subjected to the same experimental strategy as that which did yield such centers over RhOj/CeOj after T > 573 K, pointed to synergism between the RhO, and CeO components of the latter in producing at 300 K the dioxygen species required for 01 and for RO-type signals, possibly through the intermediacy of Rh-O-o-Ce and Rh-O-O-Ce interfacial sites. 

As can be seen from its ground-state molecular orbital diagram in Figure 4.11, dioxygen has a paramagnetic ground state. It is the only stable homonuclear diatomic molecule with this property. 

The common element of Schemes 1-3 is that they each postulate direct interaction between the metal center and dioxygen. Although it is not stated explicitly, Eqs. (3) and (11) most likely proceed via an inner-sphere mechanism. Thus, the metal-dioxygen interaction implies spin pairing between the reactants when the metal ion is paramagnetic. As a consequence, the formation of the M-O2 type intermediates circumvents the restriction posed by the triplet to singlet transition which seems to be the major kinetic barrier of autoxidation reactions (5). 

On adding dioxygen to the fully reduced laccase of the lacquer tree Rhus vemicifera, the type-1 Cu and the type-3 Cu-pair were oxidized in the ms range and an optical intermediate was observed at 360 nm At liquid helium temperatures an EPR signal was observed, which was tentatively interpreted as due to O ", as a result of its very short relaxation time and of the increase of its linewidth when the reduced laccase of the fungus Polyporus versicolor was treated with 0 A similar paramagnetic oxygen intermediate was also observed with the laccase of another lacquer tree Rhus succedanea and with ceruloplasmin. The decay of the intermediate at 25 °C (tj = 1 s at pH 5.5 with R. succedanea laccase) was accompanied by the reoxidation of the type-2 Cu >. One would expect, however, such an intermediate to be extremely reactive (See Sect. 3.3), while it was stable in tree laccase depleted of type-2 Cu(II)... 

Oxygen is the most abundant element in the earth s crust. Dioxygen (O2) can be prepared in the laboratory by electrolysis of water, by catalytic decomposition of hydrogen peroxide, or by thermal decomposition of KCIO3. Oxygen is manufactured by fractional distillation of liquefied air and is used in making steel. The O2 molecule is paramagnetic and has an... 

Molecular orbital theory predicts that O2 is paramagnetic, in agreement with experiment. Note that the Lewis structure of O2 does not indicate that it has two unpaired electrons, even through it does imply the presence of a double bond. In fact, the prediction/confirmation of paramagnetism in O2 was one of the early successes of molecular orbital theory. Also, the ions 0+ (dioxygen cation), Oj (superoxide anion), and 0 (peroxide anion) have bond orders 2V2, U/2, and 1, respectively. The experimental energy levels of the molecular orbital for the O2 molecule are shown in Fig. 3.3.3(b). 

In the ground state of O2, the outermost two electrons occupy a doubly degenerate set of antibonding tt orbitals with parallel spins. Dioxygen is thus a paramagnetic molecule with a triplet ground state (3 ), and its formal double bond has a length of 120.752 pm. 

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