Nuclear Magnetic Resonance Spectroscopy

Key Terminology
Term Definition
NMR The absorption of EM radiation by a nucleus having a magnetic moment when
in an external magnetic field, used as an analytical technique
Resonance The flipping of nuclei between parallel and antiparallel positions
13C NMR NMR Spectroscopy using the 13Carbon isotope
1H NMR NMR Spectroscopy of protons
TMS Chemical shift of TMS is used as a standard for δ values
Chemical Shift A quantity related to resonance frequency
Spin-Spin Coupling Spitting of 1H NMR peaks using the n+1 rule due to applied field felt by any H
atom is affected by the magnetic field of H atoms on neighbouring C atoms
Scientists have developed a range of analytical techniques which together enable the structures of new
compounds to be confirmed. NMR is a powerful technique used in organic chemistry to elucidate
molecular structure. A magnetic field is applied to a sample, which is surrounded by a source of radio
waves and a radio receiver. This generates an energy change in the nuclei of atoms in the sample that
can be detected. The emitted EM can be interpreted.
Nuclear magnetic resonance gives information about the position of 13C or 1H atoms in a molecule. 13C
NMR gives simpler spectra than 1H NMR.
13C atoms in different functional groups resonate differently in a magnetic field because their nuclei are
shielded from the field by the surrounding electrons. The greater the surrounding electron density, the
smaller the magnetic field is felt by the nucleus and the lower the frequency at which it resonates. The
NMR instrument produces a spectrum of energy absorbed against chemical shift, which is related to
the resonant frequency.
Chemical shift δ is measured in ppm from a defined zero related to TMS. Carbon atoms in different
molecular environments give different chemical shift values. On the spectrum, each peak indicates a
different carbon environment. δ values for different functional groups are given in the data booklet.
e.g. C6H1 0O2 is a cyclic compound and its 1 3C spectrum and IR spectrum are given below.
Work out its structure
Jagged O-H acid peak is present at 2500-3000cm-1 meaning there is a carboxylic acid group is present.
NMR trace shows 4 peaks so there are 4 different carbon environments. The peak at 160-185 shows
the C=O bond of the acid, and the peak at 40-50 shows the presence of a carbonyl group
Therefore the molecule is cyclopentanoic acid
Proton NMR
In proton NMR, the hydrogen nucleus is examined. The greater the electron density around the
hydrogen atom, the smaller the chemical shift δ. Each peak indicates a hydrogen environment. Again,
the chemical shift value refers to the functional group the H is part of.
For proton NMR, the area under the trace – the integration trace – is proportional to the number of
hydrogen atoms in the environment.
The δ values are measured by reference to tetramethylsilane, Si(CH3)4, TMS. The value for these H
atoms is zero by definition. A little TMS may be added to samples to calibrate the spectrum. TMS is
used as it is inert, non-toxic and easily removed.
NMR peaks split into parts – spin-spin coupling. The applied magnetic field felt by a H atom is affected
by the magnetic field of the H atoms on neighbouring carbons. n hydrogens on an adjacent carbon
atom will split into n+1 smaller peaks. The peaks produced are either doublets, triplets or quartets.
NMR spectra are run in solution; the solvent must not contain any H atoms and therefore CCl4 is often
used. Other solvents contain deuterium, another isotope of hydrogen which does not produce an NMR
signal in the same range as hydrogen.
e.g. An ester with molecular formula C5H1 0O2 is analysed by proton NMR. Identify the ester.
Splitting Patterns: singlet therefore there is no adjacent C with Hs attached; 7 peaks therefore adjacent
Cs have 6 Hs; doublet therefore adjacent C is bonded to 1H.
Chemical Shifts: Peak at 3.7ppm suggests bonded to an O, peak at 2.7ppm) indicates it is next to a C=O,
peak at 1.2ppm suggests bonded to other Cs.
In order to have a splitting part of 7 peaks, there must be 2 methyl groups attached to 1 carbon. There
must be a single CH group to cause the doublet. The singlet is caused by a methyl group attached to
the oxygen from the ester bond.
Therefore the molecule is (CH3)2CHCOOCH3