How to use notation in NMR spectroscopy?

Every compound will have its name to easily identify it and differentiate it from others. We can have even IUPAC names to name each compound in an unique and unambiguous way. Similarly if a sample in NMR has notation, we can get a quick information about the compound and its spectra. So here we will see how we can give notation for a compound active in NMR spectroscopy.

Rules for notation in NMR

Even these are not the hard defined rules for specifying notation in NMR, we can start here with few points to simplify our discussion.

Rule 1: Each magnetically non-equivalent nuclei is designated by alphabetical letters like A,B,C etc.

Let’s take simple example, propane here. 

You can find that it has two types of protons one is of CH₃ and another one is CH₂. Another CH₃ is chemically and magnetically equivalent to first CH₃. Each type of nuclei can be designated with letters, so the notation will be AB.

Rule 2: Next the number of magnetically each equivalent nuclei is indicated as subscript.

If we continue with the above example, propane has totally 6 protons in two CH3 groups all being equivalent. Similarly, 2 protons exist in CH2. Therefore the notation of propane will be A₆B₂.

Let’s take another example.

In isopentane, you can find two sets of magnetically equivalent CH₃ protons indicated by letters ‘a’ and one CH group indicated by letter ‘b’ , one –CH₂ group indicated by letter ‘c’ and finally one –CH3 group indicated by letter ‘d’.

The corresponding number of protons are 3+3=6 of a, 1 of b, 2 of c and 3 of d. So its notation will be A₆BC₂D₃

Rule 3: Heteronuclear atom is designated by a different letter like X, Y and Z.

Suppose we are giving notation to the protons in the molecule and the compound contains a different nucleus which can also give NMR signal. Then we should label it with a different alphabet.

For example, consider fluoropropane.

It contains different protons as  -CH₃ , -CH₂ and -CH₂F. In the –CH₂F both proton and fluorine can give NMR signal but both are not of similar nuclei hence their chemical shifts differ by a lot. So the heteronuclei is indicated by letter X.

Combining all the rules, the notation of above compound will be A₃B₂C₂X.

Rule 3: Nuclei which are chemically equivalent but magnetically not equivalent are indicated by same letter differentiated by prime.

Consider nitrobenzene.

It has one proton on each carbon, but they differ in their position relative to the nitro group. Two protons are at ortho position indicated by A, two protons are at meta position indicated by A’ and one proton in at para position indicated by A’’.

All these protons differ slightly in their chemical shift hence indicated by same letter but differentiated by prime.

So the notation will be A₂A’₂ A’’

Advantages of notation in NMR

Notation in NMR spectroscopy has several advantages. First you can easily know how many types of chemically non-equivalent nuclei present in the sample. For example, let’s take ethanol.

It shows three signals each with protons in -CH₃, -CH₂ and –OH. We can easily designate it as ABC indicating that the sample produces totally three NMR signals by each type of nuclei.

Second, it can give the relative ratios of peak heights in NMR spectra. In the above example, the subscript of each letter indicates the number of corresponding protons. So if we take ratio, ethanol gives three signals with ratio of their heights as 3:2:1

Third, we can easily identify any hetero nuclei present in the sample. For example, fluoroethane can be designated as A₃B₂X which indicates that it gives totally  three NMR signals of which two are of similar type of nuclei and another one is different type of nuclei all being NMR active. Here A and B indicates signals by protons of CH₃ and CH₂ while X indicates the signal of Fluorine.

Finally, NMR notation can also give information about nuclei with a slight difference in magnetic environment. Let’s consider the following example.

Vinyl fluoride can have a notation of AA’BX which indicates totally four NMR signals. As seen above, of these four signals, three are due to protons and one is due to fluorine. But again here you can find that the protons which are cis and trans to fluorine are magnetically not equivalent hence produce different NMR signals.

Even they are magnetically non-equivalent, their chemical shifts fall very close to each other hence designated by same letter using prime. So Protons indicated by letter A and A’ appear as different signals but with less separation of peaks.