Question

What functional groups might molecules contain if they show IR absorptions at the following frequencies? (a) 1715 cm-1 (b) 1540 cm-1 (c) 2210 cm-1 (d) 1720 and 2500–3100 cm#-1 (e) 3500 and 1735 cm-1

Answer 1

Hi,
There are different normal modes of vibration in the molecules, since these are associated with a movement that makes atoms characteristic, the main ones are: bond deformations, valence angles, dihedral angles, out-of-plane deformations, etc.
Each absorption in the spectrum corresponds to a certain vibration of some bond within the molecule.
Therefore, when identifying the functional groups with IR spectroscopy, the different areas that are considered below are considered:

From 4000 to 2900 cm-1: Voltage of C-H, O-H and N-H

From 2500 to 2000 cm-1: Tension of triple bonds and double accumulated bonds.

From 2000 to 1500 cm-1: Voltage of C = O, C = N and C = C.

From 1500 to 600 cm-1: Fingerprint zone (Flexion of CH, CO, CN, CC links, etc.)

Knowing the above, various functional groups can be identified:

(a) 1715 cm-1
Aldehydes (-CHO) and ketones (carbonyl bonded to two carbon atoms) α, β-unsaturated
(b) 1540 cm-1
Nitro (NO2)
(c) 2210 cm-1
Alkynes (-C ≡ C-)