Reasons for unusual infrared spectrum?

In summary, the conversation was about a strange IR spectrum obtained from usnic acid, a natural product isolated and crystallized from dried lichen. The individual was confused by the lack of a C=O bond band, and asked for possible reasons for this unusual result. The conversation also mentioned the use of mineral oil as a solvent and the possibility of contamination affecting the spectrum. The conversation concluded by discussing the use of Nujol as a heavy mineral oil and the importance of having a finely divided sample for accurate IR measurements.
  • #1
chops369
56
0
In my organic lab we recently isolated and crystallized the natural product usnic acid from dried lichen. An IR spectrum of the crystals dissolved in mineral oil was obtained, but it came out looking a bit strange. The thing that's really confusing me is that there is no band that would be suggestive of a C=O bond, which one would expect to see granted the structure of the molecule (http://en.wikipedia.org/wiki/Usnic_acid)

My question is: what are some of the reasons for an unusual/unexpected IR spectrum? I would assume that contamination would be one factor, but are there others?

The actual spectrum can be found here by searching for usnic acid: http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi

See my spectrum attached.
 

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  • #2
chops369 said:
In my organic lab we recently isolated and crystallized the natural product usnic acid from dried lichen. An IR spectrum of the crystals dissolved in mineral oil was obtained, but it came out looking a bit strange. The thing that's really confusing me is that there is no band that would be suggestive of a C=O bond, which one would expect to see granted the structure of the molecule (http://en.wikipedia.org/wiki/Usnic_acid)

My question is: what are some of the reasons for an unusual/unexpected IR spectrum? I would assume that contamination would be one factor, but are there others?

The actual spectrum can be found here by searching for usnic acid: http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi

See my spectrum attached.


You didn't use a Nujol mull? I don't think this stuff will 'dissolve' in mineral oil.
 
  • #3
chemisttree said:
You didn't use a Nujol mull? I don't think this stuff will 'dissolve' in mineral oil.
Nope, we just put a small crystal into a drop of the oil. Come to think of it, it didn't really look like it dissolved very well. I remember trying to mix it around for a while.
 
Last edited:
  • #4
So now you know.
 
  • #5
Isn't Nujol simply a heavy mineral oil? Your sample shouldn't have to dissolve to be measured; a suspension works fine. It's possible that you had too much oil and your sample peaks got lost in the noise.
 
  • #6
"A small crystal into a drop of oil" isn't enough sample expecially if it wasn't in the form of a mull. Suspensions are OK if it is a suspension of finely divided ground up sample (mull). A larger crystal likely won't pass any IR and won't contribute anything to the signal.
 

1. What is an infrared spectrum?

An infrared spectrum is a graph that shows the absorption of infrared light by a substance. It is used to identify the types of molecules present in a substance and to study their chemical bonds.

2. What are some reasons for an unusual infrared spectrum?

There are a few possible reasons for an unusual infrared spectrum. It could be due to the presence of impurities in the sample, a change in the molecular structure of the substance, or a technical error in the measurement process.

3. How can impurities affect an infrared spectrum?

Impurities can absorb infrared light at different wavelengths than the main substance, causing peaks to appear in the spectrum that do not correspond to the substance's molecular structure. This can make the spectrum appear unusual or inconsistent.

4. Can changes in molecular structure affect an infrared spectrum?

Yes, changes in the molecular structure of a substance can result in a different infrared spectrum. This is because the positions and strengths of the chemical bonds within the molecule can affect how it absorbs infrared light.

5. How can technical errors affect an infrared spectrum?

Technical errors, such as incorrect sample preparation or calibration, can result in an unusual infrared spectrum. It is important to carefully follow proper procedures and ensure accurate measurements to avoid these errors.

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