Ozone canonical structures

Ozonium Ion (H03+). Ozone is a resonance hybrid of canonical structures 50a-50d.135 Ozone does in fact act as a 1,3-dipole—that is, either as an electrophile or a nucleophile. The electrophilic nature of ozone has been recognized for a long time in its reactions with alkenes, alkynes, arenes, amines, phosphines,... 

When the canonical forms all contribute equally to the bonding, they are called equivalent canonical forms when they contribute unequally, they are nonequivalent canonical forms. Therefore, the structures shown in Figure 6.5 for ozone represent equivalent canonical forms. They are related to each other by a symmetry element—in this case, a twofold rotational axis that passes through the central oxygen. As a result, each canonical structure contributes exactly 50% to the resonance hybrid. The double-headed arrow is used to indicate the concept of resonance. The carbonate ion, shown in Figure 6.6, is another example of equivalent canonical forms only in this case, each canonical structure contributes 33.3% to the resonance hybrid. 

For example, rotation of the ozone molecule by 180 around an imaginary axis that bisects the 0-0-0 bond angle leads to an equivalent configuration for the molecule. In each case, there is one 0-0 single bond and one 0=0 double bond in the contributing canonical structures the only difference between them is the placement of the single and double bonds. Likewise, in the carbonate ion, rotation of each subsequent canonical structure by 120° or 240° leads to a structure that is identical to the original. As we see in a later chapter, symmetry plays an important role in the structure and physical properties of molecules. 

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