Nuclear Magnetic Resonance Spectroscopy
Hydrogen atoms have the property of spin, which means they have a magnetic field. NMR spectroscopy is used to give information about the relative numbers and positions of hydrogen atoms because the magnetic field of the hydrogen can either be aligned or not with an external field.
An NMR spectrum is obtained by dissolving a sample in a solvent that is deuterated meaning it has no protons that would interfere with the result, for example CCl4. This sample is then placed in a giant magnet and the test is run.
Different peaks are produced on the spectrum because resonances are produced depending on the particular chemical environments that the protons are in. To understand this, have a look at the ethanol molecule below.
Ethanol has three different chemical environments. The first is highlighted in red, as all three hydrogen atoms are bonded to a carbon that is bonded to CH2OH. And so the blue and green hydrogens are also seperate chemical environments. Each of these three will resonate at different frequencies and produce seperate peaks.
An NMR spectrum uses the molecule tetramethylsilane (TMS, below) as a standard, which means that all resonances are referenced from this point. This gives the chemical shift: δ which is the movement caused by shielding. It is measured in parts per million (ppm).
There are several reasons why this molecule is used. Firstly, it gives a resonance higher than almost all organic compounds and this peak is intense because it has 12 hydrogens with the same chemical environment. TMS is non-toxic and inert so is safe to work with, and it has a low boiling point (26.5°C) so can be easily removed by distillation.
A useful feature of NMR spectra is that they give an indication of the numbers of hydrogen atoms in each environment. This can be done by two ways. Firstly, the integration values. The area under each peak will indicate how many atoms there are, relative to each other. So taking our example of ethanol, the following will be observed. Where the area under the CH3 peak is three times that of the OH peak.
Similar information can be obtained using high resolution NMR spectra. Here, we see individual peaks being split in to several peaks, this is as a result of spin-spin couplings where the magnetic fields of neighbouring protons interact. Again, let's look at a high resolution NMR of ethanol.
The splitting of the peaks occurs according to the n+1 rule. Where a peak splits in to, one more than the number of protons in the group next to it, peaks. For example, take a look at the CH2 group, it is next to a CH3 which has 3 protons; therefore it splits to 4 (a quartet).
Interpreting NMR Spectra
You now know of three features of the NMR spectrum that will help you determine the structure of an organic compound. Firstly, you can check the chemical shift of the peak, which will indicate the group.
The integration ratios will give an indication of how many protons there are in that particular group.
The splitting patterns of the peaks will give an indication of where each group is, especially if you already know the integration ratios.