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Perhaps the most useful and powerful tool for structural elucidation is nuclear magnetic resonance (NMR). Not only does it allow you to determine the number of a particular atom, it can demonstrate what is bonded to what. Essentially, providing puzzle pieces which you can use to determine the identity of an unknown sample in the lab.
The fundamental idea in NMR is that a magnetic field is used to align the spins of nucleus of atoms. The spins align (parallel) since this allows them to achieve the lowest possible energy configuration. Similar to placing a boat in a flowing stream, its going to line up with the flow of water. A radio wave is then used to increase the energy of the nuclei, resulting in them flipping in the magnetic field to a higher energy state (anti-parallel). Eventually, they will return to the lowest energy (parallel), releasing the energy from the radio wave they absorbed. This allows us to determine what their structure is.
The same process is used in hospitals to determine the profile of soft tissue – this is known as an MRI.
NMR requires a nucleus which is spin active, meaning it will align in magnetic fields. Examples include 1H, 19C, 31P, 19F. The nucleus act as miniature magnets.
Usually the magnet used is super conducting, hence it must be kept at a cool temperature in order to function. A current approach to this is to create an outer jacket and an inner jacket respectively containing liquid N2 and He.
13C NMR Spectroscopy
The Carbon-13 isotope is naturally abundant at 1.1%, this means in an unknown organic sample, these will be present. This can tell us how many carbon environments are present and what it looks lie around the carbon atom. This is due to electron shielding which will show whether there are single, double or triple bonds present as well as the types of atoms attached.
The reference compound (TMS) tetramethyl silane and the frequency axis sets this as the reference peak at 0 parts per million (ppm). The high field is where the scale is smallest and low field is where the scale is largest. Keep in mind this scale decreases from right to left.
Carbon environments correlate to the number of visible spikes in the molecule. Two different carbons can, however, show up in the same carbon environment. The type of environment will change where it shows up. The closer one is to an electro-negative atom the lower field it will be. The movement of the peaks is known as a chemical shift and the list below provides a rough guide.
- Approximate peak position for 13C – NMR
- C=O is 200-175 ppm
- Aromatic is 150-110 ppm
- C=C is 120-100 ppm
- C-O, C-N, C-Cl is 75-40 ppm
- Alkane 30-0
1H NMR Spectroscopy
The1H NMR is similar to 13C NMR in that the frequency axis is also in ppm, however, the information will can get from this is a bit more detailed.
Just as before, there are chemical environments, but this time they are Hydrogen environments. The reference compound is still TMS
The Integral, just meaning the area under each peak tells you the relative number of hydrogen’s per peak. Likewise, a hydrogen environment will be further downstream when it is closer to an electronegative element such as chlorine.
Multiplicity as seen by splitting tells you how many neighboring molecules have hydrogen. This follows the n + 1 rule where the umber of peaks seen is equal to n + 1 where n is the number of neighboring hydrogen. As you can see this gives us a snap shot of what the molecule looks like. We can piece it together from here.
This list summarizes 0-4 neighboring hydrogen peak the number of peaks (n+1) to the naming of the peaks (multiplicity) in 1H NMR.
- 1 peak is singlet
- 2 peaks is doublet
- 3 peaks is triplet
- 4 peaks is quartet
- 5 peaks is quintet
Remember: multiplicity is telling you how many neighboring hydrogen atoms there are – if there are zero then it is a singlet (n + 1 rule)
The following is a great video explaining this:
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