1H NMR Splitting Patterns
Spin-Spin splitting
The NMR peaks we have encountered so far have been single peaks. I have some good news and some bad news for you. The bad news first. . . The bad news is that not all peaks in NMR spectra are single peaks. They get more complicated. The good news is that more complicated looking peaks give us much more information about a molecule! Let’s take a look at a 1H NMR spectrum that has some peaks that are not single looking peaks, they are “split” into more complicated peaks. This is called spin-spin splitting. The number of peaks it splits into is called the multiplicity of the peak.
N+1 rule
A 1H NMR resonance is split into peaks because of its hydrogen atom neighbors. We can tell how many peaks a resonance will split into, its multiplicity, by counting how many neighboring hydrogen atoms there are and adding one. This is the N+1 rule.
N+1 rule: The multiplicity of a 1H NMR resonance = the number of neighboring protons + 1.
Splitting patterns can be more complex than this. But, for now, this definition will suffice. Just realize that in a real-life scenario, your peaks may be a little more complex and more advanced training will be needed to deal with those situations.
Let’s walk through the 1H NMR spectrum for 2-butanone to become more familiar with this. The protons of 2-butanone are labeled, A, B, and C as are the corresponding peaks in the 1H NMR spectrum for these protons. Peak A is made from the three H atoms of the far-left methyl group, peak B is made from the two H atoms of the CH2 methylene, and peak C is made from the three H atoms of the far right methyl group.
1H NMR spectrum of 2-butanone
Peak A is split into three peaks. This is called a triplet. It is a triplet because the A protons are next to the CH2 group, B. Since there are two neighboring H atoms, the N+1 rule is 2+1 = 3 peaks, a triplet.
Peak B is split into four peaks. This is called a quartet. It is a quartet because the B protons have three H atoms, A, as neighbors to its left. It has no neighboring H atoms to its right, the C=O, carbonyl. Therefore, with three proton neighbors, the N+1 rule is 3+1 = 4 peaks, a quartet.
Peak C is not split into extra peaks. It is called a singlet. It is a singlet because its neighboring carbon atom does not contain any H atoms, it is a C=O, carbonyl. With 0 neighboring protons, the N+1 rule is 0+1=1 peak, a singlet.
The N+1 rule gives us valuable information about which proton groups are neighboring which proton groups in a molecule.
Why does spin-spin splitting happen?
Let’s analyze the 1H NMR spectrum of 1,1,2-trichloroethane.
1H NMR spectrum of 1,1,2-trichloroethane
The B protons have one neighboring proton, A. The N+1 rule makes peak B two peaks, a doublet. Proton A is a tiny magnet. It is a magnet that points up half with the H0 external magnetic field half the time and it points against H0 half the time. When it points up, it reinforces the magnet made by protons B making it bigger. When it points down, it makes the magnet made by protons B smaller. This makes the peak for protons B show up with two different chemical shifts, a doublet. The odds of each of these happening are 1:1, so the two peaks are equal size.
Why a doublet is formed from one neighboring proton
The A protons have two neighboring protons, the B protons. The N+1 rule makes peak A three peaks, a triplet. The B protons are both tiny magnets. The B protons can both point up, they can both point down, or one can point up while one points down (there are two ways to do that). This is how the three peaks of the triplet are formed. Because there are two ways to make the center peak of the triplet, up-down and down-up, the pattern for the triplet is a 1:2:1 pattern with the center peak twice as tall as the side peaks.
Why a triplet is formed from two neighboring protons
The pattern for multiplets
The shape or pattern for a normal multiplet is one where the peaks on the sides are smaller and the peaks get larger and larger towards the center peak of the multiplet. We saw this with the triplet above. The pattern can be predicted using Pascal’s triangle.
Pascal's triangle indicating the shape of multiplets
4. Predict the multiplicity for each set of hydrogen atoms in the following molecules. You may want to draw in hydrogen atoms if they are not drawn.
a)
b)
c)
d)
e)
5. Is the following peak a quartet or two doublets? How can you tell?
If they couple, they “point” towards each other
Usually, our multiplet shapes are not perfect. They give us a little hint. If a set of neighboring protons couple each other, their corresponding peaks point towards each other. If an arrow is drawn that points up, the peaks look like they point towards each other.
How peaks made from protons that couple each other "point" towards each other
6. In each box, put a letter, A, B, or C to indicate which protons of 3-chloro-2-butanone that 1H NMR peak represents. Then, draw in the “pointer” arrows to show that two of the peaks hint that protons A and B couple each other.
7. In each box, put a letter, A or B, to indicate which protons of bromoethane that 1H NMR peak represents. Notice the typical triplet-quartet pattern for an ethyl group. This is a common pattern, so it helps to learn it.
Coupling constants
When spin-spin splitting occurs, the distance between the peaks of the multiplet is called the coupling constant, J. The coupling constant is usually measured in Hz. The triplet and quartet of bromomethane are shown here. I’ve zoomed in and put them next to each other to make the coupling constants easier to see.
Equal coupling constants for two peaks from protons that couple each other
Notice three things.
1. The coupling constant remains the same, 8 Hz, between each of the peaks of the quartet and the triplet. The coupling constant does not change inside of a normal multiplet, it must remain constant throughout.
If two peaks are from coupling protons, J must be equal in both.
2. If I were to shake your hand, you shake my hand right back. It is impossible for me to shake your hand without you shaking my hand back. It is the same with neighboring protons. If two proton groups neighbor each other, they must split each other, or couple each other, in equal amounts. In the triplet-quartet pattern in our bromoethane example, notice that both the triplet and the quartet have a coupling constant of 8 Hz. This is because they are neighbors and couple each other.
In the spectrum below, there are four sets of doublets, labeled A, B, C, and D. One pair of the doublets is made by protons that couple each other and another pair is made by protons that couple each other.
8. Which of the following pairings makes sense?
a) A-B and C-D
b) A-C and B-D
c) A-D and B-C
3. The size of a coupling constant tells us something about what types of protons are neighboring or coupling each other. We won’t get too deep into this. But, a regular coupling of normal, aliphatic hydrogen atoms is 7-8 Hz. Protons on a double bond might have a coupling constant of 10-15 Hz.
Answers
4.
5. Two doublets. It does not have the 1:3:3:1 pattern that a quartet should have.
6.
7.
8. C