Spectroscopic Techniques (1) Nuclear Magnetic Resonance Spectroscopy (NMR)

NMR spectroscopy is one of the most versatile and valuable non-invasive investigative techniques chemists have these days to identify and put together the structures of molecules.

How does it work?

This is not an easy explanation for school and college students to get a hold of.

I’m just going to discuss ¹H proton nmr spectroscopy but if you delve deeper into this topic you will come across equally valuable ¹³C and ³¹P nmr as well.

You'll notice that these species all have an odd number of nucleons.

So if the proton is Deuterium with two nucleons this does not register in nmr.

Background NMR theory

Proton nmr tells us about the chemical environment of ¹H hydrogen nuclei in molecules.

¹H hydrogen nuclei spin about an axis.

They have spin quantum number ½

As they spin, the positive charge moves and creates its own very small magnetic field. 

Put this spinning ¹H hydrogen nucleus in an external magnetic field and it will either spin so as to line itself up with this external magnetic field or it will spin against the field. 

What happens is like what happens when you put a compass needle in the Earth’s magnetic field and the needle lines up with it pointing north. 

Now back to the proton, if the nucleus is lined up with the external field it has quantum number  + ½ . 

But if the spinning nucleus is aligned against the external field it has quantum number – ½ .

In fact, most nuclei line up with the field and far fewer line up against it. 

What this means is that there is an energy gap, ∆E, between the two states of the spinning nucleus. 

 

 

Radiation of the right frequency (low frequency radio waves) will match ∆E and lift some nuclei from spin state + ½   to spin state – ½: this is said to ‘flip’ the nuclei.

Energy is released (it's called relaxation) to the surroundings or other nuclei as the nuclei return to their original ground state. 

This energy registers as an nmr pulse in the nmr spectrum.  

What happens in an actual molecule to the ¹H protons if placed in an external magnetic field?

In an external magnetic field, the electrons around each ¹H nucleus in the molecule move and flow to produce a local, very small magnetic field that opposes the applied field. 

This means that all the ¹H nuclei in the molecule feel a magnetic field slightly smaller than the one applied to the molecule.

Each ¹H nucleus is then in a different magnetic environment according to the number and position of the electrons around it. 

These very slight differences in magnetic field show up in the nmr spectrum as the nuclei flip and relax.

These differences are very small for ¹H nuclei — in parts per million (ppm) of magnetic field density B.

However, it is possible to measure these so-called chemical shifts ∂.

The chemical shift ∂ value gives a sense of how much the electrons around the ¹H proton have “shielded” it from the full effect of the applied external magnetic field.

The more shielding, the larger the chemical shift ∂ value and the lower ∆E. 

The chemical shift ∂ values need a reference point. 

The magnetic signal from Tetra Methyl Silane TMS (CH₃)₄Si is used as a reference as its protons are hardly shielded from the effect of the external magnetic field. 

tetramethylsilane

All other magnetic signals of chemical shift ∂ are the result of the electrons in the molecule shielding that particular ¹H proton more or less.

You can see here the effect of the π system of electrons above and below the carbon ring.

Table of chemical shifts are available on the internet.

Equally, if there is a very electronegative atom in a molecule its electrons will also shield any ¹H protons and these protons will resonate at higher chemical shift ∂ values. 

C¹H—R                         C¹H—Cl

∂  1.5                               ∂  4.2

Spin–spin coupling

Another thing can happen if there are hydrogen atoms on different carbon atoms next to each other in the molecule. 

The signal from one ¹H proton will split the signal from the other proton and vice versa. 

Usually, the effect just happens over a distance of one C–C bond.

The effect is called spin–spin coupling.

To predict how the signal from the one proton will split the signal from the other you have to count the number of protons on the adjacent carbon atom. 

In simple cases a single signal will split (n+1) times where n is the number of ¹H hydrogen atoms on the adjacent carbon atom. 

For example:  Ethanol  CH₃CH₂OH

The signal from the hydrogen atoms on the –CH₂ – group will split the signal from the CH₃ – group because it will interact with the magnetic field produced by the CH₃ – group.

If we use the (n+1) rule then n for the CH₂ group is 2+1 =3 so that the signal from the CH₃ group will be split into a triplet.

Conversely, the –CH₃– group (n+1 or 3+1=4) will split the signal from the –CH₂ – group into a quartet.

Here then is the nmr spectrum of ethanol:

You can see several things here.

First, the –O–H group is a singlet because the oxygen decouples the hydrogen from the others in the molecule so there is no spin–spin coupling effect/no peak splitting. 

Second, the chemical shift ∂ for the –O–H hydrogen is shifted to 4.7 because of the electrons on the electronegative oxygen atom. 

Third, the CH₂ is a quartet as we predicted.

Fourth, the CH₃ is a triplet as we predicted.

Fifth, the peak areas (given in green) show us that they are in the ratio of the number of hydrogen atoms on each carbon. 

Sixth, the intensity of the peaks take up the pattern of Pascal’s triangle. 

Seventh, if the molecule were CH₃CH₂OD there would be no singlet because of the deuterium atom.  

At school and college level probably the most complex molecule you will have to deal with is something like butanone or ethyl ethanoate. 

Here are examples of both nmr spectra below:

Butanone

Ethyl acetate

Some important chemical shifts:

OH in alcohols ∂ between 2 and 4

OH in carboxylic acids ∂ between 10 and 12

Alkane CH between ∂ 0.25 and 1.75 but note: CH > CH₂ > CH₃

O–CH in alcohols ∂ between 3 and 4

Halogen –CH in halogenoalkanes  ∂ between 2.2 and 4.2

Beware symmetrical molecules such as  CH₂OH CH₂OH which has only two peaks and (CH₃)4C which has just one peak (like TMS)


You’ll also find many examples of spectra if you click here.

Some important chemical shifts