Analytical Chemistry - 1-H NMR Chemical Shifts

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Today’s graphic is one for the chemists, with a guide to chemical shifts in proton nuclear magnetic resonance. At first glance, for those without a background in chemistry, this may well look largely nonsensical – however, if you’re interested in learning a little more about how chemists can work out the structures of organic compounds, read on below for an explanation that tries its best to be a simple one!

Looking at the graphics on the site, you may have wondered how chemists know that these are the structures of the particular organic (carbon-based) molecules being discussed. After all, molecules are incredibly small entities, so how can we possibly be so confident in knowing the precise arrangement of atoms within them? There are actually a number of techniques we can use to determine these arrangements – one of them is infrared spectroscopy, discussed previously – but by far the most important is nuclear magnetic resonance, or NMR spectroscopy.

All molecules are composed of atoms, and atoms contain at their centre a nucleus, which itself contains protons and neutrons. These spherical subatomic particles can be imagined as spinning on their axis; in many atoms, these spins cancel each other out, but in those with an odd number of protons the nucleus itself will have an overall spin. This generates a small magnetic field around the nucleus, much like that of a bar magnet (though much weaker!). Hydrogen’s nucleus is an example of one with spin.

If we place a bar magnet in an external magnetic field, it aligns parallel to it, much like a compass aligns with Earth’s magnetic field. If we place nuclei with spin in a magnetic field, they, too, will align with it. However, if we provide the required energy, we can ‘flip’ the nuclei, so that they are oriented against the external magnetic field. This energy required for this can be supplied in the form of radio waves. In an NMR spectrometer, we essentially place the sample we’re interested in finding the structure of in a magnetic field, then expose it to electromagnetic radiation in the form of radio waves. This ‘flips’ the hydrogen nuclei; it’s possible for us to detect this interaction, and it can be converted to a spectrum which we can then use to find information on the compound’s structure.

Not all hydrogen nuclei are seen at the same point on a spectrum as a result of the other atoms around them in the molecule. Electrons around the hydrogen nuclei in the molecule shield it from the effects of the magnetic field; the greater the number of electrons, the greater the shielding.  This is essentially what the graphic above shows; hydrogens next to varying groups of atoms show up at differing points, an effect referred to as the chemical shift. The exact point at which a hydrogen is seen on the spectrum gives us information on what atoms the hydrogen nucleus is near in the molecule.

However, this isn’t all an NMR spectrum can tell us about a molecule. An example spectrum is shown below, for ethanol. Hydrogens in identical environments in the molecule will show up at the same point on the spectrum, and the area under the peak at that point can be used to discern the relative number of hydrogens.

NMR spectrum of ethanol. Source: http://www.cryst.bbk.ac.uk/PPS2/projects/schirra/html/1dnmr.htm

NMR spectrum of ethanol. Source: http://www.cryst.bbk.ac.uk/PPS2/projects/schirra/html/1dnmr.htm

You might also notice the jagged appearance of two of the peaks shown in the above spectrum. This effect, referred to as ‘spin-spin coupling’, is caused by the proximity of other hydrogen nuclei. Essentially, the magnetic field felt by each hydrogen nucleus is affected by the magnetic field of hydrogen nuclei on neighbouring carbon atoms. This causes the splitting seen; the number of peaks the original peak is split into is equal to n + 1, where n is the number of hydrogens on adjacent carbons.

Hopefully this makes it clear why NMR is such a powerful tool. It allows us to work out the atoms near a hydrogen atom in a molecule, the number of hydrogens in specific environments in a molecule, and the number of hydrogens on adjacent carbons. Putting all this together, and perhaps combining it with infrared spectroscopy to more precisely determine the functional groups present, we can determine the structure of many organic compounds.

Note that this overview of NMR is very much a simplified one; there are further complexities which have not been discussed here. Understanding exactly how it works is a little bit complicated, to say the least, but hopefully this explanation has helped to an extent. If you’d still like to look into it more, check out the links below!

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