Question

Explain the concept of resonance using Lewis structure model and molecular orbital model

Answer 1

In chemistry, resonance  is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis structure.

A molecule or ion with such delocalized electrons is represented by several contributing structures (also called resonance structures or canonical structures)

Example

The ozone molecule is represented by two resonance structures. In reality the two terminal oxygen atoms are equivalent and the hybrid structure is drawn on the right with a charge of − ¹⁄₂ on both oxygen atoms and partial double bonds with a full and dashed line

Consider the Lewis structure of the carbonate ion, CO₃^2-. The Lewis structure for this ion has a carbon-oxygen double bond, and two carbon-oxygen single bonds. Each of the singly bonded oxygen atoms bears a formal charge of 1^-. But which of the three oxygens forms the double bond? There are three possibilities:

Concept of resonance through molecular orbital model

Molecular orbitals provide what initially looks like a very different picture.

They are intrinsically delocalized descriptions, and much of the qualitative picture we get from resonance forms is tied up in the numerical results: MO energies, atomic charges.

However, the shapes of the HOMO (Highest Occupied Molecular Orbital) and the LU MO (Lowest Unoccupied Molecular Orbital) along with the ESP map provide a qualitiative presentation of molecular properties and reactivity. It helps to make some comparisons of the descriptions from both directions.

Note a couple things in the MO shapes: the HOMO is largest on the nitrogen--the most reactive electrons are there (or, the pi bond is polarized toward nitrogen). This corresponds to what the right-hand resonance form describes.

The LU MO is oriented toward the carbon. Anything with electrons will interact at this end of the pi bond.

The delocalization of the pi bond is consistent with the left-hand resonance form: this molecule will have a barrier to rotation about C-N (and this should be about 80 kcal/mol).

Because this molecule has a positive charge, the LU MO offers the more relevant picture of reactivity: it will react with things that have (at least partial) negative charge--and therefore electrons--which will interact with the LUMO to form a new set of MOs