Reasons for unusual infrared spectrum?

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Discussion Overview

The discussion revolves around the unusual infrared (IR) spectrum obtained from usnic acid, a natural product isolated from dried lichen. Participants explore potential reasons for the absence of expected spectral features, particularly the C=O bond, and consider various factors that may contribute to the unexpected results.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes the absence of a C=O bond in the IR spectrum of usnic acid and questions the reasons for this anomaly, suggesting contamination as a possible factor.
  • Another participant questions the method used, asking if a Nujol mull was employed, indicating that usnic acid may not dissolve well in mineral oil.
  • A later reply suggests that while Nujol is a heavy mineral oil, a suspension of the sample should suffice for measurement, but too much oil could obscure the sample peaks.
  • Another participant emphasizes that a small crystal may not provide enough material for a proper IR measurement, particularly if not prepared as a mull, which could lead to insufficient signal contribution.

Areas of Agreement / Disagreement

Participants express differing views on the adequacy of the sample preparation method and its impact on the IR spectrum. There is no consensus on the specific reasons for the unusual spectrum, as various factors are proposed without agreement on their significance.

Contextual Notes

Limitations include the potential for contamination, the dependence on the sample preparation method, and the unresolved nature of the spectral analysis due to the use of mineral oil versus Nujol.

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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