July 02, 2022, 02:09:36 PM
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Topic: I need some help interpreting these NMR and IR spectra. (Identify Unknowns lab)  (Read 5028 times)

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Offline thescepticalchymist

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In this lab we had to identify one solid unknown and one liquid unknown using melting point/boiling point data, spectra, and a list of compounds.  The melting point range of the solid was 51.9-54.6 C, and the boiling point range of the liquid was 94.5-96.2 C.

Here (https://i.imgur.com/W6RD3qF.jpg) and here (https://i.imgur.com/JKUBuBk.jpg) are the NMR spectra for the liquid. Here (https://i.imgur.com/XvdcYuo.jpg) is the IR for the liquid.

Here (https://i.imgur.com/5DjVaqb.jpg) is the NMR of the solid, and here (https://i.imgur.com/IwvE2bQ.jpg) is the IR.

I think I understand the basics of interpreting an IR, but I don't really understand how to determine the structure from an NMR. What exactly do the peaks tell you, and what is a chemical shift? How can I tell how many carbons are in each molecule, and what other functional groups they may be bonded to?

Offline wildfyr

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Its going to be really really difficult for us to explain how to interpret NMR by typing over the internet. I think you should find a textbook/professor/TA/kid who is really good in class to help you with this. There may be some youtube videos that help if you are good at learning in that way. If you have a more specific question (Such as "what is the characteristic NMR peak of the methylene next to the carbonyl on a linear long chain carboxylic acid") then we could help you.

I'll provide on practical hint to help you though: the NMR peaks on your 1H NMRs at 0 and 7.26 are due to the the tetramethylsilane (TMS) standard and CHCl3 in herent in the CDCl3 solvent.

Offline phth

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here is a good resource for some interactive practice problems with posted solutions http://www-usr.rider.edu/~grushow/nmr/NMR_tutor/selftests/problems_fs_start.html

Offline crawlingmcedge

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You really do need to look up a textbook to do this properly. There are countless guides on youtube to learn how to do this.

As a (somewhat quasi accurate) intro, you can think of hydrogen atoms with less electron density (less shielded) having a higher chemical shift c.w. those on the lower end with more electron density around said protons.

« Last Edit: April 14, 2018, 08:26:09 AM by crawlingmcedge »

Offline Babcock_Hall

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Sometimes students try to propose a structure based on one type of information in the spectrum, such as chemical shift (the location of the peaks), but ignore other kinds of information, such as the integrals or coupling constants.  The structure should be consistent with all of the information in the spectrum.

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