This document discusses nuclear magnetic resonance (NMR) spectroscopy. It explains that NMR occurs when nuclei absorb radiofrequency energy when in a strong magnetic field, causing them to resonate. It provides instructions for proper sample preparation for NMR. It describes the differences between proton (1H) NMR and carbon-13 (13C) NMR, including their applications and sensitivity. It also discusses techniques for obtaining 13C NMR spectra such as broadband decoupling, off-resonance decoupling, and DEPT spectra. Finally, it notes that combining NMR data with other techniques like mass spectrometry and infrared spectroscopy can allow determination of molecular structures.

1. Dima Depp; Master’s Student

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NMR signal is recorded
Nucleus is In resonance
Absorption of energy occurs
Applied radiofrequency (energy)
Applied magnetic field
Atoms as little a magnet

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1) Use the Correct Quantity of Material
(about 5-25mg)
2) Remove All Solid Particles
3) Make Samples to the Correct Depth
4) Use Deuterated Solvents
5) Use Clean Tubes and Caps
6) Label Your Samples
7) Use an internal reference (TMS)
8) Degassing Samples

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98.9% 1.08%

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This atom acts as
a little magnet 

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 C13 has only about 1.1% natural abundance (of carbon
atoms)
C12 does not exhibit NMR behavior
As a result, C is about 400 times less sensitive than H
nucleus to the NMR phenomena
Due to low abundance, we do not usually see C13-C13

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 Chemical shift range is normally 0 to 220 ppm
 "Normal" 13C spectra are "broadband, proton
decoupled" so the peaks show as single lines
 Number of peaks indicates the number of types of
Carbon atoms

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 H1-NMR is used to determine the number of
Hydrogen atoms in a molecule
 C13-NMR is used to determine the type of Carbon
atoms in a molecule

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 Old technique
 Time consuming
 The sample is continuously irradiated with a
frequencies (one frequency at a time)
 The spectrum
(RF;X-Axiz – intencity of absorbtion; Y-Axis)

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 pulsed NMR allows for many FID’s (Free
Induction Decay) in a NMR spectra to be
accumulated over time
 These FID’s are added together and averaged
 Signals (resonances) build up while the “noise”
is random and cancels out during the averaging.

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Broad Band hydrogen Decoupling
 Usually used mode
 C-H coupling removed
 all 13C resonances appear as singlets.

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Off – Resonance Decoupling (C13-H1
spin-spin coupling)
 13C nuclei are split only by the protons directly
bonded to them and 13C resonances appear as
singlets.
(13 C – C – 1H )Off-Resonance Decoupling are not observed.
 tells how many protons are attached to the 13C
nuclei.

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DEPT spectra (Distortionless Enhancement
by Polarization Transfer)
 a modern 13C NMR spectra that allows you to
determine the number of attached hydrogen
 Run: broad-band decoupled spectra
 DEPT-90º: only CH’s show up
 DEPT-135º: CH’s and CH3’s give positive
resonances CH2’s give negative resonances

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 To solve a combined spectra problem (find the molecule formula)
• Mass Spectra:
 Molecular Formula
• Infrared Spectra:
 Functional Groups
• 13C NMR:
 symmetry of carbon framework
• 1H NMR:
 finial check on the structure assignment
 Each piece of evidence gives a fragment (puzzle piece) of the
structure. Piece the puzzle together to give a proposed structure. The
proposed structure should be consistent with all the evidence

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Thank you
