The Triplet Ground State of Dioxygen

A Chemist s Guide to Valence Bond Theory, by Sason Shaik and Philippe C. Hiberty Copyright 2008 John Wiley Sons, Inc. 

Goddard et al. (1) and subsequently the present authors (4) also provided a simple VB explanation for the choice of the ground state. Let us reiterate this explanation based on our qualitative VB theory, outlined in Chapter 3. 

Knowing the VB wave function for a 3e bond (Eq. 3.47), we can construct the wave function for structure 1 by considering this electronic distribution as made of two independent three-electron/two-orbital systems orthogonal to 

FIGURE 5.1 The four possible distributions of six electrons in four atomic orbitals of dioxygen, leading to a total spin component Sz = 0. 

On the other hand, the energy of structure 2 is given by Equation 5.2, in the VB framework, where the two terms represent, respectively, the 2e bonding energy and the 4e repulsion  

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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. 

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... 

Because the ground state of dioxygen is a triplet with two unpaired electrons with parallel spins, its reactivity with organic molecules in the singlet state is severely... 

Because the ground state of dioxygen is a triplet with two unpaired electrons... 

The ground state of dioxygen is a triplet with two unpaired electrons with parallel spins. The first two electronically excited states are both sii lets, formed by relocation and/or pairing of the unpaired electrons in the 2pn antibonding orbitals. The half-filled antibonding molecular orbitals of 2 can accommodate two additional electrons. The addition of one electron affords the superoxide anion (O2 ) and two-electron reduction gives the peroxide ion (02 ). 

When this deoxyheme interacts with the triplet ground state dioxygen 02(X Zg ), there are six unpaired electrons (two from O2 plus 4 from heme). Their interaction can provide a variety of possible total spin states. The maximum spin corresponds to the septet A", S = 5) state, when both subsystems have parallel spins. If they are anti-parallel, the triplet state A" occurs. At long Fe-O distances (R > 2.5 A) these states are degenerate because of the absence... 

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. 

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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