Nuclear magnetic resonance (NMR)

About this technique


Nuclear magnetic resonance (NMR) spectroscopy is most frequently used by chemists and biochemists to study organic molecules and has application in a wide range of disciplines including physics, engineering, plant biology, soil science, medicine, pharmacy, sports science and marine archaeology. It is based on the fact that certain nuclei (e.g. 1H, 13C, 31P) can absorb energy from the radio frequency range of the electromagnetic spectrum, when placed in a magnetic field. The utility of NMR arises because the frequency of the absorbed energy is characteristic of the chemical environment of each nucleus in the sample. Further interactions via the chemical bonds or through space can provide information showing the interconnection, proximity and angular relationship of nearby nuclei. This information can be used to determine the structure of a molecule or in the case of a mixture, the nature and quantities of the different components.
 
NMR in solution
One dimensional NMR spectra define the relative numbers of chemically different nuclei in a sample and provide information on their particular environments. 1-D spectra may be sufficient to define the structure of a compound, but often a combination of 1-D and 2-D techniques are required. In addition, 1-D spectra can be used to determine the nature and proportions of the components in a mixture, monitor the products of chemical reactions and study molecular dynamics. In the case of organic materials, NMR samples typically contain 1-20mg of material dissolved in 0.6ml of a suitable solvent.
Multidimensional NMR experiments spread the NMR information into several frequency dimensions. This provides clear access to information that is often difficult to obtain from 1-D experiments. A large number of different experiments are available, each providing specific information about the species in solution.
Typical 2-D experiments include:
COSY (COrrelation SpectroscopY) – shows the interconnection of nearby nuclei (usually 1H nuclei).
NOESY (Nuclear Overhauser Effect SpectroscopY) – shows the proximity of nuclei and thus provides valuable stereochemical information.
HSQC (Heteronuclear Single-Quantum Coherence) – shows which 1H and 13C (or 15N) nuclei are directly attached.
HMBC (Heteronuclear Multiple Bond Correlation) – identifies 1H and 13C (or 31P) nuclei within three bonds of each other.
DOSY (Diffusion-Ordered Spectroscopy) - used to determine diffusion coefficients or separate the signals of different compounds in solution.
Samples can be heated or cooled to study a variety of dynamic processes.
 
Multinuclear NMR spectroscopy
NMR spectra can be recorded for a wide range of nuclei in addition to 1H, 13C and 31P. However, the ease of measurement can vary dramatically depending on the characteristics of the particular nucleus. Other nuclei whose NMR spectra can provide valuable information include 2H, 6Li, 7Li, 11B, 15N, 19F, 23Na, 27Al, 29Si, 59Co, 63Cu, 77Se, 109Ag, 119Sn, 133Cs, 183W and 195Pt.
 
Solid-state NMR spectroscopy
Solid samples normally produce very broad NMR spectra. However, if a powdered sample is packed into a rotor and spun at high speed, with the axis of the rotor at a particular angle to the magnetic field (Magic Angle Spinning - MAS), a dramatic increase in the resolution of the spectrum can be obtained. A wide range of nuclei may be studied in the solid state using MAS.
This technique can be used in conjunction with cross polarisation and high power decoupling (CPMAS) to produce 13C spectra of solid samples.



Contact an expert

The University of Western Australia
Dr Gareth Nealon
T: 08 6488 1887
E: gareth.nealon@uwa.edu.au

Deakin University
Dr Luke O’Dell
T: 03 5227 3076
E: luke.odell@deakin.edu.au