IR Absorption of Compounds: Why the 3300 cm-1 Peak?

In summary, when analyzing the absorption pattern of compounds radiated by IR radiation, the wavelengths of the absorbed IR light correspond to the frequency of bond vibrations in the molecule. The concept of threshold frequency is not applicable as bonds can be excited by more than one quantum at a time, but this results in weaker spectral lines. In a real molecule, the selection rule for harmonic oscillators holds, meaning only transitions to the first excited vibrational state are observed in the IR spectrum.
  • #1
MathewsMD
433
7
When analyzing the absorption pattern for different compounds when radiated by IR radiation, we see the wavelengths of the IR lights that is absorbed. These wavelengths correspond to the frequency at which the bonds in the molecule vibrate. My question is why a compound, for example an alcohol, has a peak in absorption for wavenumbers ~3300 cm-1 and no where higher. Is the concept of threshold frequency no longer in play? Why not? I understand that this would cause a greater stretching in the bonds than normal, but why can't radiation of higher energy be absorbed?

Any explanations would be great! Thanks! :)
 
Chemistry news on Phys.org
  • #2
If you are asking why you can't excited a bond by more than one quantum at a time, you actually can, but the corresponding line will be much weaker. For a harmonic oscillator, electric dipole selection rules show that you can only excite by one quantum at a time. In a real molecule, bonds are of course not harmonic oscillators, but close enough that the selection rule holds to a good approximation. From the ground vibrational state, the transition dipole moment will be close to zero except for a transition to the first excited vibrational state, which is essentially the only spectral line you will see in an IR spectrum.
 

1. What is the significance of the 3300 cm-1 peak in IR absorption spectra?

The 3300 cm-1 peak in IR absorption spectra is associated with the stretching vibration of the O-H bond. This peak is particularly important in identifying compounds that contain a hydroxyl group, such as alcohols, carboxylic acids, and phenols.

2. Why is the 3300 cm-1 peak considered a diagnostic peak?

The 3300 cm-1 peak is considered a diagnostic peak because it is unique to compounds with O-H bonds. This peak is not present in compounds without O-H bonds, making it a useful tool in identifying the presence of hydroxyl groups in a sample.

3. Can the intensity of the 3300 cm-1 peak provide information about the strength of the O-H bond?

Yes, the intensity of the 3300 cm-1 peak can provide information about the strength of the O-H bond. A stronger O-H bond will result in a more intense peak, while a weaker O-H bond will result in a less intense peak. This can be useful in determining the relative strength of different O-H bonds in a compound.

4. How does the environment of the O-H bond affect the position of the 3300 cm-1 peak?

The position of the 3300 cm-1 peak can be affected by the environment of the O-H bond. For example, the peak may shift to a lower wavenumber if the O-H bond is involved in hydrogen bonding. This can provide valuable information about the molecular structure and interactions within a compound.

5. Are there any other factors that can affect the intensity or position of the 3300 cm-1 peak?

Yes, there are several factors that can affect the intensity or position of the 3300 cm-1 peak, including the presence of other functional groups, the molecular geometry, and the type of molecule (e.g. gas, liquid, solid). It is important to consider these factors when interpreting IR spectra to accurately identify compounds.

Similar threads

  • Chemistry
Replies
5
Views
4K
Replies
8
Views
1K
  • Chemistry
Replies
1
Views
2K
Replies
1
Views
2K
  • Chemistry
Replies
1
Views
6K
Replies
3
Views
1K
Replies
4
Views
972
Replies
1
Views
10K
  • Chemistry
Replies
4
Views
10K
Replies
2
Views
8K
Back
Top