Mesomerism, or Resonance

Mesomerism, or Resonance

The theory of mesomerism was developed on chemical grounds. It was found that no structural formula could satisfactorily explain all the properties of certain compounds, e.g., benzene. This led to the idea that such compounds exist in a state which is some combination of two or more electronic structures, all of which seem equally capable of describing most of the properties of the compound, but none of describing all the properties. Ingold (1933) called this phenomenon mesomerism (‘between the parts’, i.e., an intermediate structure). Heisenberg (1926), from quantum mechanics, supplied a theoretical background for mesomerism; he called it resonance, and this is the name which is widely used. 

The chief conditions for resonance are :
  •  The positions of the nuclei in each structure must be the same or nearly the same.
  • The number of unpaired electrons in each structure must be the same.
  • Each structure must have about the same internal energy, i.e., they various structures have approximately the same stability.

Let us consider carbon dioxide as an example. The electronic structure of carbon dioxide may be represented by at least three possible electronic arrangements which satisfy the above conditions :
Mesomerism, or Resonance
Structure (II) and (III) are identical as a whole, since both oxygen atoms are the same. Each structure, however, shows a given oxygen atom to be in a different state, e.g., the oxygen atom on the left in (II) is negative, whereas in (III) although two (or more) of the electronic structure may be the same when the molecule is considered as a whole, each one must be treated as a separate individual which makes its own contribution to the resonance state. Structure (I), (II) and (III) are called the resonating, unperturbed or canonical structures of carbon dioxide, and carbon dioxide is said to be a resonance hybrid of these structures, or in the mesomeric state.

It is hoped that the following crude analogy will help the reader to grasp the concept of resonance. Most readers will be familiar with the rotating disc experiment that shows the composite nature of white light. When stationary, the disc is seen to be coloured with the seven colours of the rainbow. When rotating quickly, the disc appears to be white.  The resonating structures of a resonance hybrid may be compared to the seven colours, and the actual state of the resonance hybrid to the ‘white’; i.e., the resonating structures may be regarded as superimposed on one another, the final result being one kind of molecule. In a resonance hybrid all the molecules are the same; a resonance hybrid cannot be expressed by any single structure.

In a resonance hybrid the molecules have, to some extent, the properties of each resonating structure. The greater the contribution of any one structure, the more closely does the actual state approach to that structure. At the same time, however, a number of properties differ from those of any one structure. The observed enthalpy of formation of carbon dioxide is greater than the calculated value by 132.2 kJ (31.6kcal). in other words, carbon dioxide requires 132.2kJ more energy that expected to break it up into its elements, i.e., carbon dioxide is more stable than anticipated on the structure O=C=O. How can this be explained? Argument based on quantum mechanics show that a resonance hybrid would be more stable than any single resonating structure, i.e., the internal energy of a resonance hybrid is less than that calculated for any one of the resonating structures. The difference between the enthalpy of formation of the actual compound, i.e., the observed value, and that of the resonating structure which has the lowest internal energy (obtained by calculation) is called the resonance energy. Thus the value of the resonance energy of any resonance hybrid is not an absolute value; it is a relative value the resonating structure containing the least internal energy being chosen as the arbitrary standard for the resonance hybrid. The greater the resonance energy, the greater is the stabilization. The resonance energy is a maximum when the resonating structures have equal energy content, and the more resonating structures there are, the greater is the resonance energy, provided that all contributing structures have similar stabilities.