[rogixmalix]

Acetophenone group C = O has a vibrational frequency of approximately 1685 cm -1. Substitution on the aromatic ring with an amino group in para (II) decreases this value to 1652 cm-1., While replacement with a nitro group (III) increases the value to 1693 cm-1. Explain the change in frequency vibrational in both cases.

Matching image for this exercise:

https://imgur.com/a/hp18P6M

[hollytara]

You can draw resonance structures for acetophenone where the carbonyl becomes a single bond with the O having a negative charge, there is a double bond from ring to carbonyl carbon, and the ring has two double bonds and a positive charge; there are 3 structures, with the charge in different places around the ring (ortho and para to the carbonyl). 

What is the influence of the amino and nitro groups on the contribution of these resonance structures?

[pgk]

Resonance occurs via hybridization of the involved orbitals and therefore, it changes the spring constant k of the involved bonds and consequently, it also changes the wavenumber of vibrational frequency v.

v = (1/2πc)/√(k/μ)

where, μ = m1m2/(m1 + m2)