NMR signals may have different number of peaks (the number of lines). This is called the splitting of the signal or the multiplicity.

Signal splitting is arguably the most unique and important feature that makes NMR spectroscopy a comprehensive tool in structure determination. We will see this importance in a little bit, but first, let’s go over the concept of signal splitting.

The simplest signal consists of one line and is called a singlet, followed by the doublet, triplet, etc. A signal with more than seven lines is referred to as a multiplet.

 

 

Do not confuse this with the integration, these two are not related, e.g. a singlet may have an integration of nine, and a quartet may have an integration of one:

 

 

The Origin of Splitting

The splitting is caused by the hydrogens on the same (geminal hydrogens) or on the neighboring carbons (vicinal hydrogens). Only nonequivalent protons split the signal of the given proton(s).

One adjacent proton splits an NMR signal into a doublet and two adjacent protons split the signal into a triplet.

For example: Ha and Hb are nonequivalent protons so they split each other’s NMR signals.

 

 

Let’s first see how the doublet originates. If there was no adjacent hydrogen a singlet would’ve been observed:

 

If there is a neighboring protons(s), its spin can be aligned with or against the magnetic field (B₀). As a result, proton Ha feels two magnetic fields; one slightly stronger than B₀, the other one slightly weaker than B₀.

 

 

With the same mechanism, two adjacent protons split the signal into three peaks and three protons split it into a quartet.

There is a formula for predicating the number of peaks base on the neighboring hydrogens and that is known as the n + 1 rule, where n is the number of neighboring protons.

The more general formula for this is 2nI + 1, where I is the magnetic spin number of the given nucleus. And since it is equal to 1/2 for hydrogen, the formula that we use in ¹H NMR is n + 1.

Below is a summary table for the splitting patterns in NMR spectroscopy.

 

 

When two protons split each other’s NMR signals, they are said to be coupled. However, not all neighboring protons are coupled.

Signal splitting occurs only between nonequivalent protons.

It is not observed for homotopic and enantiotopic protons since they are chemically equivalent.

For example, starting with simple methane and ethane, both have equivalent protons which do not split each other, and the signal appears a singlet.

1,2-dichloroethane is also a molecule where the hydrogens are equivalent and therefore there is only one singlet.

 

 

However, if we replace one of the Cl atoms with a bromine, the hydrogens on the two carbons are not equivalent anymore and they split each other’s signal into triplets:

 

 

Having one or two chlorine atoms also disrupts the symmetry making the protons on adjacent carbons nonequivalent:

 

 

It is also possible to have a signal splitting by a proton on the same carbon if these protons are diastereotopic:

 

 

Let’s look at an example where we can distinguish two sets of protons only based on their splitting pattern.

For example, how do we distinguish between the two methyl groups in the following molecule?

They are both next to electron-withdrawing groups with comparable power and they will both have the same integration.

This is where the spin splitting gets into play as, based on it, one of the methyl groups is expected to be a triplet and the other – a doublet (n+1 rule).

 

 

Does this all work so far or not quite so? Here are some more practice problems on the multiplicity in the NMR spectroscopy:

NMR Signal Splitting N+1 Rule Multiplicity Practice Problems

 

Check Also

• NMR spectroscopy – An Easy Introduction
• NMR Chemical Shift
• NMR Chemical Shift Range and Value Table
• NMR Number of Signals and Equivalent Protons
• Homotopic Enantiotopic Diastereotopic and Heterotopic
• Homotopic Enantiotopic Diastereotopic Practice Problems
• Integration in NMR Spectroscopy
• Splitting and Multiplicity (N+1 rule) in NMR Spectroscopy
• NMR Signal Splitting N+1 Rule Multiplicity Practice Problems
• ¹³C NMR NMR
• DEPT NMR: Signals and Problem Solving
• NMR Spectroscopy-Carbon-Dept-IR Practice Problems