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Showing posts with label What does an NMR spectrum tell you?. Show all posts
Showing posts with label What does an NMR spectrum tell you?. Show all posts

Friday, 8 July 2016

What does an NMR spectrum tell you?

Modern NMR spectrometers can do many weird and wonderful things.
I shall limit this discussion to One-Dimensional NMR Spectroscopy, in which frequency is plotted along the horizontal axis and signal intensity is plotted along the vertical axis.
1H NMR
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  • Chemical shift gives type of H atom.
  • Integration gives numbers of each type of H atom
  • Splitting tells which H atoms (and how many) are on adjacent C atoms.
13C NMR
(a) Broad Band Decoupled Spectra
These are the "normal" spectra.
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They give the numbers of each type of C atom
(b) Off-Resonance Decoupled Spectra
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They give the numbers of each type of C atom and number of H atoms attached to each C.
DEPT — Distortionless Enhancement by Polarization Transfer
A technique called DEPT can identify the multiplicity of peaks in an NMR spectrum.
Often you run several different spectra.
The DEPT spectra for an alkyne are shown below.
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They show:
  • DEPT-135: CH and CH3 peaks up, CH2 peaks inverted
  • DEPT-90:CH peaks only
  • "Normal": all C atoms
Note that they do not show the quaternary alkyne carbons, as the normal spectra dp.
The DEPT spectra can be combined to yield "edited DEPT" spectra, in which each spectrum contains peaks of only one multiplicity (CH, CH2 or CH3), as shown below in the spectrum of isobutyl butyrate.
chemwiki.ucdavis.educhemwiki.ucdavis.edu
Put this information all together, and you have much information to help you identify a molecule.