Infrared spectroscopy:Coupled Interactions:

I- Coupling interactions due to stretching vibrations:

This results from a mechanical coupling interaction between the oscillators.

- It appears when:

i) Two bond oscillators share a common atom and

ii) Their individual oscillation frequencies are not widely different.

For example, carbon dioxide molecule, which consists of two C=O bonds with a

common carbon atom, has two fundamental stretching vibrations an asymmetrical and

a symmetrical stretching mode. (N.B. It shows also bending vibrations at 667 cm-1 as has been

mentioned):

a) The symmetrical stretching mode:

- It consists of an in-phase stretching or contracting of the C=O bonds.

← O=C=O →

→ O=C=O ←

- Absorption occurs at a frequency lower than that observed for the carbonyl

group in an aliphatic ketone.

- The symmetrical stretching mode produces no change in the dipole moment of

the molecule and it is therefore "IR inactive" and can't be observed in IR

spectrum but it is easily observed in the Raman spectrum near 1340 cm -1. (N.B.

Band intensity in Raman spectra depends on bond polarizability rather than molecular dipole

changes(

b) In the asymmetrical stretching mode,

The two C=O bonds stretch out of phase; one C=O bond stretches as the other

contracts. → O=C=O →

- The asymmetrical stretching mode is IR active, since it produces a change in the

dipole moment.

- The absorption of asymmetrical stretching occurs at 2350 cm-1, i.e., at a higher

frequency than observed for a carbonyl group in aliphatic ketones.

- This difference in carbonyl absorption frequencies displayed by the carbon

dioxide molecule results from strong mechanical coupling or interaction.

- Other examples for coupling interactions due to stretching vibrations are the

following:

2) The two N—H stretching bands in the 3497-3077 cm-1 region in primary amine,

and primary amide spectra:

3) Also for the two C=O stretching bands in the 1818-1720 cm-1 region for carboxylic

anhydride and imide spectra.

4) The two C—H stretching bands in the 3000-2760 cm-1 region for both methylene

and methyl groups.

II-Coupling interactions due to bending vibrations:

-Vibrations resulting from bond angle changes frequently couple in a manner similar

to stretching vibrations.

-For example: The ring C—H out-of-plane bending frequencies of aromatic

molecules depend on the number of adjacent hydrogen atoms on the ring .

-Coupling between the hydrogen atoms is affected by the bending of the C—C bond

in the ring to which the hydrogen atoms are attached.

III-Coupling due to interactions between stretching and bending vibrations:

- Coupling between stretching and bending vibrations can occur if the stretching

bond forms one side of the changing angle.

- This type of interaction can be illustrated by the absorption of secondary acyclic

amides, which exist predominantly in the trans conformation.

N

O

H

- It shows strong absorption in the 1563-1515 cm-1 region.

- This absorption involves coupling of the N—H bending and C—N stretching

vibrations.

* We can summarize the requirements for effective coupling interaction as

follows:

1. The vibrations must be of the same symmetry species if interaction is to occur.

2. Strong coupling between stretching vibrations requires a common atom between

the groups.

3. Interaction is greatest when the coupled groups absorbs, individually, near the same

frequency.

4. Coupling between bending and stretching vibrations can occur if the stretching

bond forms one side of the changing angle.

5. A common bond is required for coupling of bending vibrations.

6. Coupling is negligible when groups are separated by one or more carbon atoms.

As we have seen in our discussion of coupling interactions of two fundamental

vibrational modes will produce two new modes of vibration, with frequencies higher

and lower than that observed when interaction is absent.

IV-Coupling due to interactions between fundamental vibrations and

overtones (Fermi resonance):

When coupling occur between fundamental vibrations and overtones or combination

tone vibrations. Such interaction is known as Fermi resonance.

One example of Fermi resonance is afforded by the absorption pattern of carbon

dioxide.

As we have previously discussed that the symmetrical stretching band of C=O;

appears in the Raman spectrum near 1340 cm-1. Actually there are two bands are

observed: one at 1286 cm-1 and one at 1388 cm-1. The splitting results from coupling

between the fundamental C=O stretching vibration, near 1340 cm -1, and the first

overtone of the bending vibration. (N.B. The fundamental bending vibration occurs near 667 cm -1 and the first

overtone near 1334 cm-1).

- Fermi resonance is a common phenomenon in both IR and Raman spectra. It

requires that;

a) The vibrational levels be of the same symmetry species and,

b) The interacting groups must be located in the molecule so that mechanical

coupling is appreciable.