Dioxygen ground 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 above reaction scheme was established by a combination of uv-visible absorption and fluorescence, ir isotopic substitution, esr and kinetic measurements (37). The important point to note here is that in 02 rich Xe matrices, ground state Cu(2Sj/2) cannot avoid reactive encounters with 02 to form Cu(02)2 and Cu(02) dioxygen complexes,whereas it is proposed that the formation of CuO, Cu(03) and 03 in dilute 02/Xe matrices arises from the reaction of a long lived mobile excited state Cu(2D) with 02. On the other hand the reactions of photoexcited Ag(2P) with 02 are different (37), electron transfer being favoured to form Ag 02. ... 

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. 

As mentioned above, NO can also be reduced to the nitroxyl anion (NO-) (Eq. (5)), which is isoelectronic with 02, and like dioxygen has a triplet ground state (15). 

In some cases, the ground state is not a singlet state, e.g. dioxygen, anion and cation radicals of aromatic molecules. 

The singlet (S = 0) state lies about 1000 cm above the ground state triplet (S = 1) in the EPR spectrum of free dioxygen. Transitions associated with triplet oxygen in solution are detectable by EPR at low temperatures, but dioxygen complexes with even electron metal centers (e.g., ferroheme) are not generally observable by this method. Usually, only odd electron systems (Kramers systems) are detectable by magnetic resonance. 

Fig. 5. The spin septet ground state of the iron-dioxygen complex as geometry optimized using DFT (B3LYP) (120). Distances are given in angstroms.

Binding of dioxygen to the water-free iron complex 4a leads to 5a (Fig. 10), which has a septet ground state originating from the triplet dioxygen and the quintet iron complex 4a and which is... 

Dioxygen could overcome the kinetic barrier of its unpaired electrons and triplet ground state by excitation to its first excited state (xAg), in which all electrons are paired. Unfortunately, this species, referred to as singlet oxygen, is generally too reactive and too short-lived for most situations (lb, lc). However, dioxygen complexation to a transition metal can also result in activation and create stable complexes that can be studied, modified, and used in further reactions in a controlled manner (2). This latter type of activation is the subject of this chapter. 

Thus, since ground-state Co+ does not react with dioxygen or N20, in order to produce appreciable amounts of CoO+, N20 can be allowed to react with translationally (produced through radio-frequency excitation) or electronically (produced via laser desorption) excited Co+, leading to a conversion of Co+ to CoO+ of about 20% (222). The... 

As noted earlier, there is a kinetic restriction on the reactivity of dioxygen, as the one-electron reduction to superoxide is unfavoured. This is demonstrated in the redox potentials shown in Figure 51. The triplet ground state of the molecule may also impose spin restrictions on its reactions. 

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. 

Owing to its nonpolar nature, O2 is more soluble in organic solvents than in H2O. Since the 3 ground state of dioxygen is a spin triplet, concerted oxygenation reaction is subject to spin restriction. As a result, many types of dioxygen reactions proceed slowly, even in cases where such reactions are... 

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