Summary notes for AQA A-Level Chemistry Unit 3.3.15 - Nuclear magnetic resonance spectroscopy (A-level only) by an Imperial College London MSci Chemistry graduate. Notes divided into the following sections: Chemical Shift, H NMR spectroscopy, C NMR Spectroscopy
Nuclear Magnetic Resonance Spectroscopy
Nuclear magnetic resonance (NMR) is an analytical technique which gives info. about the position of
13C or 1H atoms in a molecule. It is used in combination w/ a range of other analytical techniques to
enable the structures of new compounds to be con rmed.
13C or 1H atoms are used because they are atoms w/ nuclear spin + ∴ behave like magnets when placed
in a strong magnetic eld. Hence when the nuclei are placed in a magnetic eld, the magnetic moments
(spin) will all line up parallel to the applied eld, either…
• Spin aligned (lined up w/ the external eld) = lower energy.
• Spin opposed (lined up against the external eld) = higher energy.
In an NMR spectrometer, EM radiation of a speci c frequency is absorbed by the nuclei + used to ‘ ip’
the alignment of nuclear spin from the lower energy spin aligned state to the higher energy spin
opposed state.
• The energy re-emitted from the nuclei as the alignment of their nuclear spins drop from a higher to a
lower energy state is absorbed by the spectrometer. This can be measured + processed to produce an
NMR spectrum.
Chemical Shift
The x-axis of a spectrum is the chemical shift (δ) measured in ppm (parts per million) + the scale ↑
from right to left. The chemical shift is the difference in the energy absorbed by nuclei in different
env.s relative to a standard substance w/ δ = 0.
• The standard substance is TMS (tetramethylsilane) in a 1H + a 13C NMR spectrum.
- TMS is a suitable substance to use as a standard because it…
• Gives a signal that is further right than most of the signals from organic compounds
(owing to the electronegativity of Si).
• Only gives one signal as all 12 H atoms are chemically equivalent + all 4 C
atoms are chemically equivalent.
• Is non-toxic + inert.
• Has a low b.p. (i.e. is volatile) + so can be easily removed from the sample afterwards.
• An NMR spectrum allows the structure of molecules to be determined because nuclei positioned in
different env.s in a molecule will give rise to separate signals, which will lead to separate peaks w/
different δ.
• The strength of the external magnetic eld is not the only variable that affects δ - it also depends on
molecular env. …
- Proximity of nuclei to areas of high electron density or other electronegative nuclei ➜ deshielding
of nuclei ➜ ↑ δ.
- ↑ in electron density or no. of electronegative nuclei ➜ deshielding of nuclei ➜ ↑ δ.
1H NMR Spectroscopy
Hydrogen Environments
Each peak on a 1H NMR spectrum is due to one or more protons in a particular env. + the relative area
under each peak tells you the relative no. of H atoms in each env.
Number of Signals - Number of Different Hydrogen Environments
H atoms that appear at the same δ on the spectra + are ∴ in the same env. w/in the molecule are said to
be chemically equivalent H atoms.
• Chemically equivalent H atoms are not distinguishable on an NMR spectrum.
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Relative Intensity of Signals - Number of Equivalent Hydrogen Atoms
In 1H NMR (but not 13C NMR) the area of the signal is proportional to the no. of H atoms it represents.
There are a no. of ways in which the relative size (area) of the signals can be shown…
• The most common one is to indicate the relative intensity of the signals from which the simplest
whole no. ratio can be calculated.
- E.g. relative intensity = 0.3 : 0.15 : 0.3 : 0.6 = 2 : 1 : 2 : 6
• Sometimes an integration trace can be drawn on the spectrum. the
relative height of these traces give the relative no. of H atoms
represented by each signal. The heights shown on the integration
trace are proportional to the areas of the peaks.
- E.g. relative intensity = 24 : 12 = 2 : 1
Table of Chemical Shift Data for 1H NMR Spectra
The following table will be found on the data sheet during the exam…
Splitting Pattern
Equivalent 1H nuclei will appear at the same chemical shift value on the spectrum, but the peak for
these 1H nuclei will be split into a no. of peaks depending on the no. of non-equivalent 1H nuclei bonded
to the adjacent C atoms. These split peaks are called multiplets.
N.B. the H atom of the OH group in alcohols rarely causes splitting, or is split itself. Sometimes the H
atom of an OH group appears as a broad hump.
Splitting of the peaks into a set of peaks is known as the splitting pattern. The splitting pattern
follows the n+1 rule (i.e. if 5 peaks → 4 1H nuclei on adjacent C atoms).
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