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Why Less Bands

The document discusses reasons why the number of observed infrared (IR) bands in molecules is often fewer than predicted by the 3N-5 or 3N-6 rules. Key factors include the lack of change in dipole moment during certain vibrations, the presence of degenerate vibrations, coalescence of closely spaced bands, weak absorption intensity, fundamental frequencies outside the IR range, instrumental limitations, and overlapping of overtone and combination bands with fundamental vibrations. These factors contribute to the discrepancy between expected and observed IR bands in molecular spectroscopy.

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24 views2 pages

Why Less Bands

The document discusses reasons why the number of observed infrared (IR) bands in molecules is often fewer than predicted by the 3N-5 or 3N-6 rules. Key factors include the lack of change in dipole moment during certain vibrations, the presence of degenerate vibrations, coalescence of closely spaced bands, weak absorption intensity, fundamental frequencies outside the IR range, instrumental limitations, and overlapping of overtone and combination bands with fundamental vibrations. These factors contribute to the discrepancy between expected and observed IR bands in molecular spectroscopy.

Uploaded by

hkm20545
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Why Observed IR Bands Are Fewer Than Expected (Based on 3N-5 or 3N-6 Rule)?

When predicting the number of fundamental vibrational modes of a molecule, we use the 3N-6

rule (for non-linear molecules) or 3N-5 rule (for linear molecules), where N is the number of

atoms. However, the number of IR absorption bands observed experimentally is often less than

the number predicted.

1- No Change in Dipole Moment During Vibration (IR Inactivity)

For a vibration to be IR active, it must cause a change in the dipole moment of the molecule. If a

vibration does not cause any change in dipole moment, it will not absorb infrared radiation, and

hence it will be IR inactive.

Example: Symmetric stretching of CO₂: Although the two C=O bonds stretch and contract

simultaneously, the center of positive and negative charges remains the same → No change in

dipole moment → IR inactive.

2- Degenerate Vibrations (Same Frequency)

Sometimes, different vibrational motions happen at the same energy and same frequency —

they are called degenerate vibrations. As a result, they appear as a single band, even though

multiple vibrations exist.

Example: In CO₂, the scissoring motions (bending vibrations) in two different planes are

degenerate → They absorb at the same frequency, producing only one IR band.

3- Bands Are Very Close Together (Coalescence)

Some vibrational frequencies are so close to each other that they merge into a single broad band.

The instrument may not be able to resolve them separately.

Example: In larger organic molecules, C-H bending vibrations often occur very close to each

other and appear as a single broad peak.

4- Weak Absorption Bands (Low Intensity)

Some vibrational modes produce very weak absorption because the change in dipole moment

during vibration is very small. Such bands may be too weak to detect with ordinary IR

spectrometers.
Example: Overtones (vibrations at multiples of fundamental frequencies) are typically very

weak and sometimes not observed.

5- Fundamental Frequency Outside the IR Range

Some vibrations may have frequencies too high or too low to fall within the normal IR range

(typically 4000 cm⁻¹ to 400 cm⁻¹). Therefore, they are missed during standard IR analysis.

Example:

 Very strong triple bonds (like C≡N) have stretching frequencies that can sometimes be

near the higher edge of IR range.

 Very low-frequency metal-ligand vibrations (in complexes) may fall in the far IR region

(below 400 cm⁻¹), which standard IR spectrometers cannot detect.

6- Instrumental Limitations

Sometimes, even if the band is present, limitations of the spectrometer such as resolution,

sensitivity, or detector range can prevent proper observation.

Example: In early IR spectrometers, weak overtones and combination bands were often

undetected.

7- Overlapping of Fundamental and Overtone/Combination Bands

Overtones (vibrations at twice or three times the fundamental frequency) and combination bands

(sum or difference of two vibrational modes) can overlap with fundamental vibrations. This can

cause confusion, missing out or misinterpreting expected bands.

Example: In aromatic compounds, C-H overtone bands sometimes overlap with fundamental

C=C stretching bands.

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