Chemguide: Support for CIE A level Chemistry

This statement is about NMR spectra and their interpretation.

Before you go on, you should find and read the statement in your copy of the syllabus.

Start by reading the Chemguide page about the background to NMR spectroscopy.

Judging by past CIE papers, the most important things you need to know from this page are:

• The hydrogen nucleus behaves as a tiny magnet. Take care that you talk about the hydrogen nucleus or a proton - not about a hydrogen atom.

• These "magnets" can align themselves with an applied magnetic field or against it.

• There is an energy gap between these two alignments which corresponds to the energy in radio frequencies.

• The size of energy gap between the two alignments depends on what else is present in the molecule close to the hydrogen you are concerned with.

But you should also be aware of everything else in this page.

Now read the page about low resolution NMR spectra.

You will be told values from an integration trace, and so you don't need to follow the link explaining how to work them out for yourself.

Don't leave this page until you are sure that you understand how to get information from a low resolution NMR spectrum. Unless you are comfortable with this, you won't get far with the next page!

Once you are confident, go on to the page about high resolution NMR spectra.

Don't forget to look carefully at the effect of adding D₂O to the substance, and the way this helps to identify an -OH group. This is mentioned specifically by the syllabus.

Most CIE questions about NMR will expect you to get information about the structure from the way the lines split, and so you should be sure that you understand and can use the "n+1 rule".

Up to the June 2013 exam, no questions had been asked about the reasons for this rule although the second edition of the Application Support Booklet mentions this in a "What the student needs to know" section.

I suggest that you only follow the link on the page to that explanation if you are totally confident that you can interpret high resolution spectra - that is the important thing to be able to do.

My guess is that if they ever ask a question about why splitting occurs, virtually nobody will be able to do it!

Interpreting high resolution NMR spectra needs a lot of practice. Try to get hold of as many past CIE papers as possible, and have a go at the NMR questions. There is no short cut to this!

A comment about NMR and benzene rings

This is a repeat of the same comment on the Chemguide page, but it is important that you don't worry unnecessarily about strange splitting patterns in the region of the spectrum containing hydrogens on benzene rings.

At this level, all you can safely say about hydrogens attached to a benzene ring is how many of them there are. If you have a molecular formula which has 6 or more carbon atoms in it, then it could well contain a benzene ring.

Look for NMR peaks in the 6.0 - 9.0 range. If you are given a number like 5 or 4 alongside that peak, this just tells you how many hydrogen atoms are attached to the ring.

If there are 5 hydrogens attached to the ring, then there is only one group substituted into the ring. If there are 4 hydrogens attached, then there are two separate groups substituted in, and so on. There should always be a total of 6 things attached to the ring. Every hydrogen atom that is missing has been replaced by something else.

Splitting patterns involving benzene rings are far too complicated for this level, and you shouldn't be expected to recognise or interpret them.

Go to the Section 11.2 Menu . . .

To return to the list of learning outcomes in Section 11.2

Go to the CIE Main Menu . . .

To return to the list of all the CIE sections

Go to Chemguide Main Menu . . .

This will take you to the main part of Chemguide.