NMR spectroscopy is used to determine the properties of atoms and the molecules which contain them. It uses something called magnetic resonance in order to provide data on specific molecules.
How it works
All atomic nuclei have an atomic spin, those used commonly in NMR spectroscopy are 1H and 13C, both of which have half integer spin. When a magnetic field is applied, these nuclei can have a spin of +1/2 or -1/2 – aligned with the magnetic field or not. The energy difference between the two spins is dependent on the strength of the magnetic field.
NMR spectrometers use powerful magnets, which increases the difference in energy between the 2 states but it is still very small. When radio waves (very low energy EM waves) are added, those nuclei in the lower energy state, +1/2, will be able to jump to the higher energy state, -1/2.
By studying the absorption and release of energy by the nucleus, we can then determine the type of atom present. This method only works for certain elements. We can also learn about what other elements a particular element is bonded to as it will resonate at a different frequency due to the magnetic effect of nearby nuclei.
What it tells us
1H NMR spectroscopy tells us firstly how many hydrogen atoms are in the molecule but also the number of different ‘environments’ which relates to which other atoms it is connected to. Each different environment will have a different peak on the spectra, allowing determination of functional groups and general structure of the organic molecule.
13C NMR works in the same way but with carbon atoms instead of hydrogen. NMR doesnt work for carbon-12 as it has an even atomic and mass number so there is no magnetic resonance. As carbon-13 makes up just 1% of naturally occuring carbon, the probability that the atoms will be resonant is much lower.
https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/spectrpy/nmr/nmr1.htm – spin of nuclei and links to NMR.
http://www.rsc.org/learn-chemistry/wiki/Introduction_to_NMR_spectroscopy – how NMR works, analysis