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What Is Nuclear Magnetic Resonance?
Good question!
The Nuclear Magnetic Resonance phenomenon was first observed at Harvard by Purcell, Torrey and Pound. Like all good scientists (?), they were working on a Saturday evening, Decmeber 15, 1945.
When placed in a magnetic field, the nuclei of certain atoms can absorb electromagnetic radiation in the form of radio waves. This is not that unlike the way coloured objects abosorb light or the way water molecules absorb microwaves when heating a burger in 7-11!
Each type of atom with a nucleus which is NMR active has its own characteristic absorption frequency - just like radio stations broadcast on their own specific frequency.
The bigger the magnetic field, the higher the absorption frequency.
Commonly studied examples of appropriate nuclei, which are "NMR active" are those of hydrogen atoms (the proton) and carbon-13.
Atoms of the same type which are in different chemical environments have slightly different absorption frequencies -this is known as the chemical shift.
Here's a quick example. This is a model of an everyday sugar molecule, sucrose:
Each white sphere represents a hydrogen atom (black = carbon, red = oxygen). Because there are different locations for hydrogens within the molecule, they have different absorption frequencies. So when we do an NMR experiment, we get a spectrum of various frequencies due to each type. Here's a spectrum:
Intensity 
<--- Frequency
The biggest peak (off scale) is from the water. The other peaks are due to the sugar molecule. (In fact only about half of the hydrogens in the sucrose give rise to a visible signal in this spectrum, but that's another story!) There are clearly more absorption peaks in the spectrum than there are hydrogen atoms. This is because of a phenomenon known as coupling. Signals from protons which are 3 or fewer chemical bonds (the black lines in the model) away from other protons effectively "see" these other protons and their absorption line is split into multiple lines. This coupling phenomenon is very useful as it allows us to deduce the structure of the molecule by analysing the splitting patterns.
Another useful feature of NMR for determining structures of molecules is that experiments can be performed which show which hydrogens are close to each other in space, rather than connected by a series of chemical bonds. This can't be seen in this example though.
The frequency scale at the bottom is in parts per million. This means that the absorption frequencies are very similar - less than 0.001 % different from each other in this case. The actual frequency of the radio waves at the centre of this spectrum is about 500.1 MHz. As a comparison, if you live here in Sydney, you can tune into Triple J radio on 105.7 MHz. This is pretty close to the frequency of a platinum-195 nucleus in the instrument used to collect this spectrum (107.5 MHz).
(If you have a Java enabled browser and want a closer look at the sucrose molecule, visit the movable IUMSC sucrose model)
What is it used for?
NMR spectroscopy is one of the most powerful and widely applicable analytical tools currently available for various types of chemists, biochemists and physisicists.
Applications include:
- elucidating the composition and structure of synthetic or isolated molecules
- defining the structure of large biomolecules like proteins and nucleic acids (DNA, RNA)
- studies of kinetics and mechanisms of reactions and molecular dynamics
- solid state studies of properties of materials
The NMR effect is the basis for magnetic resonance imaging (MRI). Many hospitals are equipped with MRI instruments which allow the condition of internal organs and tissues to be examined without having to go under the surgeons knife!!
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