Reasons for unusual infrared spectrum?

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SUMMARY

The forum discussion centers on the unexpected infrared (IR) spectrum of usnic acid, a natural product isolated from dried lichen. The primary issue raised is the absence of a C=O bond signal, which is typically expected based on the molecular structure. Participants identified potential reasons for this anomaly, including improper sample preparation, specifically the use of mineral oil instead of a Nujol mull, which is essential for accurate IR measurements. The consensus is that insufficient sample quantity and improper suspension techniques can lead to misleading spectral results.

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  • Experience with Nujol mull and its application in spectroscopy
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Organic chemists, laboratory technicians, and students involved in spectroscopy or natural product analysis will benefit from this discussion, particularly those working with infrared spectroscopy and sample preparation techniques.

chops369
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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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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.
 
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:
So now you know.
 
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.
 
"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.
 

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