November 29, 2021, 05:01:50 AM
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Topic: NMR spectroscopy - How to describe the spectrum??  (Read 110 times)

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Offline fergijane88

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NMR spectroscopy - How to describe the spectrum??
« on: November 09, 2021, 09:51:39 AM »
I have the NMR spectrums for commercial aspirin, aspirin synthesised in the lab and salicylic acid. I need to describe them for figure legends, and analyse and compare the differences for my discussion.
I am a final year BSc biology student, so I'm not that well versed on NMR. All I have done is describe where the peaks are, and note the differences between the spectrums.
What else could I say? Feeling extremely lost on this lab project and getting no help from my tutor.

Offline Corribus

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Re: NMR spectroscopy - How to describe the spectrum??
« Reply #1 on: November 09, 2021, 10:07:04 AM »
A scientific paper basically comes down to this:
(1) Tell people what the problem is, how you plan to solve it, and what you expect to find. Introduction.
(2) Tell people specifically what you did. Methods.
(3) Describe what you observed. Results
(4) Provide an explanation for the observation. Discussion.
(5) Sum it all up and note some limitations of the study and some future directions for further exploration. Conclusions.

So you're going to want to describe what you observed, particularly focusing on what the differences between the spectra are. That's your Results. And then provide some explanation for why the differences exist. That's your Discussion.

For example, if you see additional peaks in the commercial aspirin versus salicylic acid, why do you suspect that might be the case? Is this consistent with what your predictions were? Can you take some guesses (based on knowledge of NMR peaks or literature search) about what those peaks could be representing? This is the kind of stuff that should be in your discussion.

By the way, if you think about what you should find before you do the experiment, this can make writing a discussion much easier.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline fergijane88

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Re: NMR spectroscopy - How to describe the spectrum??
« Reply #2 on: November 09, 2021, 10:33:00 AM »
Thanks for your reply ! I seem to be going along the right lines then.

One thing I have noticed is that each of the NMR spectrums have a single peak at 7.2814ppm. This is very close to the 7.24ppm of CDCI3. Could this single peak be caused by the CDCI3 solvent?

The relative intensities on the lab samples are higher than those of commercial aspirin too, is that worth mentioning?


Offline Babcock_Hall

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Re: NMR spectroscopy - How to describe the spectrum??
« Reply #3 on: November 09, 2021, 10:38:41 AM »
A H-1 NMR spectrum has up to four kinds of information:
The chemical shift relates to the environment of the equivalent nuclei.
The area of the peak provides a relative measure of how many nuclei contributed.
The number of peaks provides the number of groups of equivalent nuclei.
The coupling constant provides information about vicinal hydrogen nuclei.

The peak near 7.28 ppm is very likely to arise from residual CHCl3.  However, molecules such as aspirin will have signals not very distant from this.

One way you might approach this is to ask how one could tell salicylic acid apart from aspirin and other such questions.
« Last Edit: November 09, 2021, 10:52:39 AM by Babcock_Hall »

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