Ozone resonance structures

Seif-Test 2.1 IB Calculate the formal charges for the three oxygen atoms in one of the Lewis structures of the ozone resonance structure (Example 2.5). 

For singlet spin molecules at the equilibrium geometry, RHF and UHF wave functions are almost always identical. RHF wave functions are used for singlets because the calculation takes less CPU time. In a few rare cases, a singlet molecule has biradical resonance structures and UHF will give a better description of the molecule (i.e., ozone). 

By including electron correlation in the wave function the UHF method introduces more biradical character into the wave function than RHF. The spin contamination part is also purely biradical in nature, i.e. a UHF treatment in general will overestimate the biradical character. Most singlet states are well described by a closed-shell wave function near the equilibrium geometry, and in those cases it is not possible to generate a UHF solution which has a lower energy than the RHF. There are systems, however, for which this does not hold. An example is the ozone molecule, where two types of resonance structure can be drawn. Figure 4.8. 

The biradical resonance structure for ozone requires two singly occupied MOs, and it is clear that an RHF type wave function, which requires all orbitals to be doubly occupied, cannot describe this. A UHF type wave function, however, allows the a and /3 orbitals to be spatially different, and can to a certain extent incorporate both resonance structures. Systems with biradical character will often have a (singlet) UHF wave function different from an RHF. 

I Stratospheric ozone, O, protects life on Farth from harmful ultraviolet radiation from the Sun. Suggest two Lewis structures that contribute to the resonance structure for the 02 molecule. Experimental data show that the two bond lengths are the same. 

Ozone is a compound with the following resonance structures ... 

A clue to the nature of the third itt MO can be found in the placement of electrons in the two resonance structures for ozone, which are shown with color highlights in Figure 10-36a. Notice that in one resonance structure, the left outer atom has three lone pairs and a single bond, while the right outer atom has two lone pairs and a double bond. In the other resonance structure, the third lone pair is on the right outer atom, with the double bond to the left outer atom. The double bond appears in different positions in the two stmctures, and one of the lone pairs also appears in different positions. These variations signal delocalized orbitals. 

A structure midway between the two resonance structures represents the ozone structure best. The bonds in this structure are stronger than a single bond but weaker than a double one. 

Fig. 13.1 Three important resonance structures for the ground state of the ozone molecule, Oj. The third structure represents a problematic diradical.

Sometimes a given set of atoms can covalently bond with each other in multiple ways to form a compound. This situation leads to something called resonance. Each of the possible bonded structures is called a resonance structure. The actual structure of the compound is a resonance hybrid, a sort of weighted average of all the resonance structures. For example, if two atoms are connected by a single bond in one resonance structure and the same two atoms are connected by a double bond in a second resonance structure, then in the resonance hybrid, those atoms are connected by a bond that is worth 1.5 bonds. A common example of resonance is found in ozone, 0, shown in Figure 5-7. 

FIGURE 7.21 The ozone molecule is a hybrid of two resonance forms that differ only in the location of p electrons. The nuclei and the cr-bond electrons are in the same position in both resonance structures. 

As discussed in Section 7.7, two resonance structures are required to explain the structure of ozone because the two 0-0 bonds have equal lengths ... 

The extreme forms of resonance structures in ozone molecules can be represented as shown in Figure 4.7. This structure illustrates that the ozone molecule has two types of dipoles, which can serve as electrophilic and nucleophilic agents, respectively. In organic solvents, the following three reactions can be observed ... 

Extreme forms of resonance structures in an ozone molecule. (From Langlais, B. et al., Ozone in Water Treatment Application and Engineering, Lewis Publishers, Boca Raton, FL, 1991. With permission.)... 

Sometimes more than one satisfactory Lewis structure can be written and there is no reason to select one over another. In such cases a single structural formula is inadequate for a correct representation, and we say that the true structure is a resonance hybrid of the several structures. Common examples of species requiring resonance structures are ozone, 03, carbonate ion, CO " and benzene, C6H6. These... 

EXAMPLE 4 Figure 9-3 shows a single resonance structure for ozone. Each oxygen is assigned a 6 due to the number of valence electrons present (a Group VIA element—6 electrons). 

Ozone is known as a very reactive agent in both water and air. The high reactivity of the ozone molecule is due to its electronic configuration. Ozone can be represented as a hybrid of four molecular resonance structures (see Fig. 2). As can be seen, these structures present negative and positively charged oxygen atoms, which in theory imparts to the ozone molecule the characteristics of an electrophilic, dipolar and, even, nucleophilic agent. 

The GMS wave function for the ozone molecule supports the GVB(pp) picture for the ozone molecule in a straightforward way [54]. The two classical resonance structures were considered together with the biradical one in a three structure GMS-CI calculation. The ground state of the O3 molecule... 

I. Some of the pollutants in the atmosphere are ozone, sulfur dioxide, and sulfur trioxide. Draw Lewis structures for these three molecules. Show all resonance structures. 

The other form of elemental oxygen is ozone (O3), a molecule that can be represented by the resonance structures... 

The biradical resonance structure for ozone requires two singly occupied MOs, and it is clear that an RHF type wave function, which requires all orbitals to be doubly... 

The calculated frequencies at different levels of theory with the cc-pVTZ basis are given in Table 11.23, corresponding results with more sophisticated methods are shown with ozone as a resonance structure between... 

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