Peak Splitting in Proton NMR (HL) (DP IB Chemistry): Revision Note

Philippa Platt

Written by: Philippa Platt

Reviewed by: Richard Boole

Updated on

Peak Splitting in Proton NMR

  • In the first part of NMR spectroscopy, we have seen that the nuclei of H atoms:

    • Behave as tiny magnets

    • Interact with an applied magnetic field

  • Only atoms with odd mass numbers show signals on NMR spectra and have the property of nuclear spin

Table showing nuclei which posses spin

Nuclei

Protons

Neutrons

Spin

1H

1

0

2H

1

1

x

12C

6

6

x

13C

6

7

19F

9

10

31P

15

16

  • Atoms with odd mass numbers either align themselves:

    • With the external magnetic field (lower energy state)

    • Against the external field (higher energy state)

  • Samples are irradiated with radio frequency energy while subjected to a strong magnetic field

  • Energy from the radio frequency end of the electromagnetic spectrum excites the nuclei

    • This causes them to resonate or ‘flip’ between the lower and higher energy states

  • Protons in different molecular environments absorb and emit (resonate) different radio frequencies

  • The magnetic field strength of protons in organic compounds is measured and recorded on a spectrum

  • The resonance energy is unique to specific H atoms in molecules that are located in the same chemical environment

  • Information from the spectrum tells us the number of different H environments

  • A reminder about low resolution 1H NMR:

Low resolution 1H NMR of ethanol

Low resolution 1H NMR for ethanol
A low resolution 1H NMR for ethanol showing the key features of a spectrum

Chemical shift

  • The horizontal scale on an NMR spectrum represents chemical shift (δ)

  • Chemical shift:

    • Is measured in parts per million (ppm) of the magnetic field strength needed for resonance

    • Measured against a standard or reference chemical called tetramethylsilane, abbreviated to TMS

Displayed formula of TMS
The displayed formula of tetramethylsilane
  • TMS is used universally as the reference compound for NMR as its methyl groups are particularly well shielded

    • This means that it produces a strong, single peak at the far right of an NMR spectrum

    • The signal from the carbon atoms in TMS is defined as having a chemical shift of 0 ppm

    • So, TMS gives a strong single reference peak on an NMR spectrum at 0 ppm:

The NMR reference peak for TMS
The NMR reference peak for TMS
  • The chemical shift values of peaks on an 1H NMR spectrum give information about the likely types of proton environment in a compound

    • Relevant data for interpreting ¹H NMR spectra is provided in the IB Chemistry data booklet (Section 21)

Worked Example

Use the 1H NMR data table located in the IB Chemistry data booklet (section 21) to assign the 3 proton peaks for propanal.

NMR spectrum with three peaks at 9.7, 3.8, and 1.1 ppm; chemical structure of propanal shown above with coloured hydrogen and carbon atoms.

Answer:

  • From the molecule, there are 3 different proton environments:

    • CH3-, highlighted in blue

    • -CH2-, highlighted in green

    • -CHO, highlighted in red

  • There are 3 peaks on the 1H NMR spectrum:

    • 9.7 ppm

    • 2.6 ppm

    • 1.0 ppm

  • Using the 1H NMR data table, each peak can be assigned to a proton environment:

    • 9.7 ppm = R-CHO

    • 2.6 ppm = R-CO-CH2-

    • 1.0 ppm = -CH3

NMR spectrum of ethanol showing the different chemical shifts of the protons

Peak splitting

  • High resolution NMR provides more structural detail than low resolution NMR

  • Each signal in a high resolution spectrum can appear as a cluster of smaller peaks

    • This is know as multiplicity

  • Splitting occurs because neighbouring protons influence the magnetic field experienced by a proton

  • It depends on how the spins align with or oppose the external field

  • A neighbouring proton can either align with or oppose the magnetic field

    • When aligned:

      • The neighbouring spin slightly increases the effective magnetic field

      • This causes a slightly higher chemical shift

    • When opposed

      • The neighbouring spin slightly reduces the effective magnetic field

      • This causes a slightly lower chemical shift

One neighbouring proton

  • If a proton has one neighbouring proton, there are two possible alignments:

    • Protons aligned

    • Protons opposed

Aligned and opposite spins on neighbouring protons
Aligned and opposite spins on neighbouring protons
  • This causes the single low resolution 1H NMR peak to split into two equal peaks

    • The peaks are equal because of their 1:1 intensity ratio

    • This is called a doublet

  • This pattern is only obtained when there is one neighbouring proton

Two neighbouring protons

  • If a proton has two neighbouring protons, there are four possible combinations of alignment:

First neighbour

Second neighbour

Field strength

Frequency

+

+

stronger

1

+

unchanged

2

+

unchanged

weaker

1

  • Two of the possible combinations have the same outcome on field strength

    • They are equivalent peaks

    • So, three separate peaks are obtained

      • But one peak is twice as strong as the others

    • The peaks have an 1:2:1 intensity ratio

    • This is called a triplet

  • This pattern is obtained when a proton is next to a -CH2- group

Three neighbouring protons

  • If a proton has three neighbouring protons, there are eight possible alignments:

First neighbour

Second neighbour

Third neighbour

Field strength

Frequency

+

+

+

strong increas

1

+

+

slight increase

3

+

+

slight increase

+

+

slight increase

+

slight decrease

3

+

slight decrease

+

slight decrease

strong decrease

1

  • Three of the eight combinations cause the same increase in field strength (+, +, -)

  • Another three cause the same shift decrease in field strength (+, -, -)

    • These groups form two medium-intensity peaks

  • Overall, four separate peaks are obtained

    • But two peak are three times as strong

    • The peaks have an 1:3:3:1 intensity ratio

    • This is called a quartet

  • This pattern is obtained when a proton is next to a -CH3- group

The n+1 rule and splitting patterns

  • The number of neighbouring protons determines how a signal is split

  • This follows the n+1 rule

    • A proton with n equivalent neighbouring protons produces n + 1 peaks

  • Each pattern has a characteristic name, intensity ratio and shape:

Number of adjacent protons (n)

Splitting pattern using the n+1 rule the peak will split into ....

Relative intensities in splitting pattern

Shape

0

1, singlet

1

NMR singlet peak

1

2, doublet

1 : 1

NMR doublet peak

2

3, triplet

1 : 2 : 1

NMR triplet peak

3

4, quartet

1 : 3 : 3 : 1

NMR quartet peak

Final summary

  • An NMR spectrum provides several types of information:

    • The number of signal groups shows the number of different proton environments

    • The chemical shift indicates the general environment of the protons

    • The area under each peak (integration) shows the relative number of protons in each environment

    • The multiplicity (splitting pattern) reveals how many protons are on adjacent atoms

  • This information is often enough to deduce the structure of an organic molecule

  • Other techniques are often used alongside NMR to confirm the structure, such as:

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Philippa Platt

Author: Philippa Platt

Expertise: Chemistry Content Creator

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener

Richard Boole

Reviewer: Richard Boole

Expertise: Chemistry Content Creator

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.

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