What does an NMR spectrum tell you?

1 Answer
Ernest Z.
Jun 25, 2015

NMR spectra tell us how many #"C"# and #"H"# atoms are in a molecule and which atoms are attached to which.

Explanation:

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.

#""^1"H"# 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.

#""^13"C"# 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 — #color(red)"D"#istortionless #color(red)"E"#nhancement by #color(red)"P"#olarization #color(red)"T"#ransfer

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 #"CH"_3# peaks up, #"CH"_2# 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"#, #"CH"_2# or #"CH"_3#), as shown below in the spectrum of isobutyl butyrate.

chemwiki.ucdavis.edu

Put this information all together, and you have much information to help you identify a molecule.

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