RESONANCE AND RESONANCE ENERGY

      It is generally observed that a single valence bond structure of a molecule cannot correctly account for the properties of the molecule. In such cases, the concept of resonance is introduced. According to this concept, if two or more alternate valence bond structures can be written for a molecule, the actual structure is said to be a resonance or mesomeric hybrid of all these alternate structures. For example, carbon dioxide molecule can be represented by the following three structures:

     The calculated values of bond distances between carbon and oxygen in C=O and C≡O are 1.22Å and 1.10Å, respectively but the observed bond distance between carbon and oxygen in carbon dioxide is 1.15Å. Thus, none of the above structures correctly explains the observed bond length.

     It is, thus, said that a hybrid form of these structures can provide the exact explanation. The various structure of which the molecule is a resonance hybrid are know  as canonical forms of mesomeric forms. Actually resonance hybrid does not oscillate between the canonical forms of mixture of these forms but it is a definite form and has a definite form and has a definite structure which cannot be written on paper.

Rules for selecting canonical or mesomeric forms

(i) The relative position of all the atoms of each of the canonical forms must  be the same. They should differ only in the position of electrons.

(ii)The number of unpaired and paired electrons in each of the canonical forms must be same.

(iii) The contributing structures should not differ much in energy.

(iv)The contributing structures should be such that negative charge resides on more electronegative and positive charge on the electropositive. Like charges should not reside on atoms close together in the canonical forms.

Resonance Energy

     It has been observed that the molecule which shows resonance has greater heat of formation than the calculated heat of formation of any one of canonical forms. The difference is represented by 𐤃E and is called resonance energy.

𐤃E= (Experimental value of heat of formation) - (calculated value of heat of formation of most stable canonical form)

     On account of 𐤃E, the hybrid form is more stable than any of the canonical forms.

Some Examples Showing resonance

(i) Nitric oxide molecule :  It is a resonance hybrid of following two structures:

(ii) Nitrogen dioxide molecule : It is a resonance hybrid of the following structures: