1 of 17© boardworks ltd 2010. 2 of 17© boardworks ltd 2010 how does nmr spectroscopy work?

1 of 17 © Boardworks Ltd 2010


How does NMR spectroscopy work?


What does an NMR spectrum tell us?

Different chemical environments (bonds and atoms surrounding a nucleus) affect the strength of magnetic field that must be applied to a nucleus in order for it to enter the resonance state.

By measuring the strength of magnetic field that must be applied, NMR spectroscopy gives us information about the local environment of specific atoms in a molecule. This can be used to deduce information about molecular structure.

The environments of 13C and 1H atoms are most commonly studied in NMR spectroscopy.


Carbon-13 NMR spectroscopy


Interpreting 13C NMR spectra

The 13C NMR spectrum of ethylamine contains two peaks. This is because ethylamine has two unique 13C environments, each requiring the application of a different magnetic field strength for that carbon nucleus to enter the resonance state.

C

H

H

H

C N

H

H

H

H

One peak is due to the carbon atom with three hydrogen atoms attached to it, and the second to the carbon

atom with two hydrogen atoms and

an amine group attached to it.


Interpreting 13C NMR spectra activity


Chemical shift and TMS

The horizontal scale on an NMR spectrum represents chemical shift (δ). Chemical shift is measured in parts per million (ppm) of the magnetic field strength needed for resonance in a reference chemical called TMS.

The signal from the carbon atoms in TMS is defined as having a chemical shift of 0.

Si

C

C

C

C

H

H

HH

H

HH

H

H

H H

HTMS (tetramethylsilane) is universally used as the reference compound for NMR as its methyl groups are particularly well shielded and so it produces a strong, single peak at the far right of an NMR spectrum.


13C NMR chemical shift assignment

The chemical shift values of peaks on the 13C NMR spectrum can help us identify the types of carbon atom in a compound. The likely source of spectrum peaks can be identified using a data table of typical chemical shift values.

5–40

20–50

190–220

Type of carbon

δ/ppm

chemical shift


13C NMR chemical shift activity


Proton NMR spectroscopy


Interpreting 1H NMR spectra activity


Integration and the number of hydrogens

The height of the peaks in an NMR spectrum does not give us any useful information.

The spectrum can be integrated to find this information.

However, the area under the peaks on a 1H NMR spectrum is proportional to the number of hydrogen atoms causing the signal. The ratio of the areas under the peaks tells you the ratio of 1H atoms in each environment.

21

3


Spin coupling


Splitting pattern activity


1H NMR chemical shift assignment

The chemical shift values of peaks on an 1H NMR spectrum give information about the likely types of proton environment in a compound.

0.7–1.2

2.1–2.6

9.0–10.0

Type of proton

δ/ppm


1H NMR chemical shift assignment activity


Uses of NMR spectroscopy

NMR spectroscopy uses the same technology as magnetic resonance imaging (MRI). This is an important non-invasive method of gaining information about internal structures in the body used in diagnostic medicine and scientific research.

NMR spectroscopy is also used in the pharmaceutical industry to check the purity of compounds.

Often, a combination of mass spectrometry, infrared spectroscopy and NMR spectroscopy is used in modern analysis to elucidate the structure of organic molecules.

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