Peroxides bond order

Using MO theory, predict the electronic structure, bond order, and number of unpaired electrons in the peroxide ion, 022-. 

Although this reaction shows the formation of 02 +, it is also possible to add one electron to the 02 molecule to produce (),, the superoxide ion, or two electrons to form O/, the peroxide ion. In each case, the electrons are added to the antibonding 7r orbitals, which reduces the bond order from the value of 2 in the 02 molecule. For ()2 the bond order is 1.5, and it is only 1 for 022-, the peroxide ion. The 0-0 bond energy in the peroxide ion has a strength of only 142k) moT1 and, as expected, most peroxides are very reactive compounds. The superoxide ion is produced by the reaction... 

The ground state (0 kJ/mol) for the CL molecule is represented by the term symbol 3v . The first excited state (92 kJ/mol above the ground state) is a 1 singlet (electrons spin paired with both electrons in either the n x or the n y level). The 1 v state with paired spin electrons, one each in the 7i v and n y levels, is the next excited level 155 kJ/mol above the ground state. Reduction of 02 by one electron yields the superoxide ion (02), a radical anion. Reduction by two electrons yields the peroxide ion, (02 ). Bond lengths and bond orders for these are given in Table 4.2. As noted in equation 4.2, the reduction potential for 02 in the presence of protons is thermodynamically favorable. Therefore, reversible binding of O2 to a metal can only be achieved if competition with protons and further reduction to superoxide and peroxide are both controlled.8... 

Acetyl peroxynitrate (18) and perfluoroacetyl peroxynitrate (19), two important atmospheric oxidation products of hydrocarbons (formation of 18) or chlorofluorocarbon replacements, such as CF3CH3 (formation of 19), preferentially adopt a gauche conformation (C—O—O—N = 84.7° for 18 and 85.8° for 19 electron diffraction). The two peroxides are characterized by comparatively short 0—0 bonds on one side and long 0°—N connectivities (Table 5) on the other. The observed O —N distances may be explained on the basis of an no ct od-n orbital overlap. This type of interaction lowers the 0°—N bond order and could explain the low bond dissociation energies of this connectivity in peroxides 18 and 19 (118 4 klmol for both compounds). It should be noted that this interpretation does not reflect a possible r-type interaction between a lone pair at 0° and virtual orbitals of the nitro group and therefore requires future investigation. 

The standard state of fluorine is the difluorine molecule, F2, which has an electronic configuration identical with that of the peroxide ion. The two species are isoelectronic. The bond order is 1, and the bond dissociation energy of 155 kj mol-1 and bond length of 144 pm are very similar to the values for 022-. 

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 CP-02 complex the CP surface is an electron density donor. For example, in the case of PANI the bond orders in adsorbed O2 molecules decrease by about 30%, and the bond lengths L increase by about 24%. So, the adsorbed O2 molecules have a fairly high degree of activation and can readily interact with the protons in a solution. Further calculations show that in such case H2O2 compound forms even inside of adsorption complex. So, it is not necessary to spent high additional energy for formation of hydrogen peroxide. 

Figure 5.1 Bond length, vibration frequency and bond energy of the oxygen molecule (3E ) and its various ionic forms (oxygenyl, Oj, superoxide, O2, peroxide, and O3") as a function of bond order.

Although the peroxide ion, 02, and the acetylide ion, C2, have long been known, the diazenide ion (N2 ) has only recently been prepared. By comparison with the other diatomic species, predict the bond order, bond distance, and number of unpaired electrons for N2. (Reference G. Auffermann, Y. Prots, and R. Kniep, Angew. Chem., Int. Ed., 2001, 40, 547)... 

Prepare a molecular orbital energy level diagram of the peroxide ion, 022-, and predict the bond order of this ion. 

Solution The molecular orbital diagram is similar to that for 02, shown in Figure 2-4. The peroxide ion has two more antibonding electrons than neutral 02, giving a bond order of 1. 

If we add two electrons to O2 molecule we obtain the peroxide anion, 02 , with the electronic configuration log 2og Itig — note that this time the electrons are added to the Ittg antibonding orbital. The bond order for 02 is /2P - 2 +2 + 4 - 4] = 1. Of the three dioxygen species, 02 has the lowest bond order and the longest bond 149 pm. 

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