¹H NMR (Oxford AQA International A Level Chemistry)

¹H NMR

Features of an NMR spectrum

• NMR spectra show the intensity of each peak against its chemical shift

• The area under each peak gives information about the number of protons in a particular environment

• The height of each peak shows the intensity/absorption from protons

• A single sharp peak is seen to the far right of the spectrum

  • This is the reference peak from TMS

  • Usually at chemical shift 0 ppm

Low resolution ¹H NMR for ethanol

Molecular environments

• Hydrogen atoms of an organic compound are said to reside in different molecular environments

  • E.g. Methanol has the molecular formula CH₃OH

  • There are 2 molecular environments: -CH₃ and -OH

• The hydrogen atoms in these environments will appear at 2 different chemical shifts

• Different types of protons are given their own range of chemical shifts

¹H NMR chemical shift data table

• Protons in the same chemical environment are chemically equivalent

  • 1,2-dichloroethane, Cl-CH₂-CH₂-Cl has one chemical environment as these four hydrogens are all exactly equivalent

• Each individual peak on a ¹H NMR spectrum relates to protons in the same environment

  • Therefore, 1,2-dichloroethane would produce one single peak on the NMR spectrum as the protons are in the same environment

Identifying molecular environments in 1,2-dichloroethane

Low resolution ¹H NMR

• Peaks on a low resolution NMR spectrum refer to molecular environments of an organic compound

  • E.g. Ethanol has the molecular formula CH₃CH₂OH

  • This molecule has 3 separate ¹H environments:

    • -CH₃

    • -CH₂

    • OH

  • So 3 peaks would be seen on its spectrum at:

    • 1.2 ppm (-CH₃)

    • 3.7 ppm (-CH₂)

    • 5.4 ppm (-OH)

Low resolution NMR spectrum of ethanol

Deuterated solvents

• When samples are analysed through NMR spectroscopy, they must be dissolved in a solvent

• Tetramethylsilane (TMS) is a commonly used solvent in NMR

• Despite TMS showing one sharp reference peak on NMR spectra, the proton atoms can still interfere with peaks of a sample compound

• To avoid this interference, solvents containing deuterium can be used instead

  • For example CDCl₃

  • Deuterium (²H) is an isotope of hydrogen (¹H)

• Deuterium nuclei absorb radio waves in a different region to the protons analysed in organic compounds

• Therefore, the reference solvent peak will not interfere with those of the sample

Integrated Spectra

• In ¹H NMR, the spectrometer measures the area under each peak and produces an integrated spectrum

  • This can provide useful information for identifying an unknown compound

• The relative area under a peak indicates the number of protons responsible for that peak

• The ¹H NMR of methyl chloroethanoate, ClCH₂COOCH_3, will show an integration spectra in the peak area ratio of 2:3

  • 2 for the protons in the CH₂

  • 3 for the protons in CH₃

¹H NMR of methyl chloroethanoate

Spin-Spin Splitting

• High resolution ¹H NMR spectroscopy can show you the structure of the molecule but also the peaks can be split into sub-peaks or splitting patterns

• These are caused by a proton's spin interacting with the spin states of nearby protons that are in different environments

  • This can provide information about the number of protons bonded to adjacent carbon atoms

  • The splitting of a main peak into sub-peaks is called spin-spin splitting or spin-spin coupling

High resolution ¹H NMR spectrum of ethanol

The n+1 rule

• The number of sub-peaks is one greater than the number of adjacent protons causing the splitting

  • For a proton with n protons attached to an adjacent carbon atom, the number of sub-peaks in a splitting pattern = n+1

• When analysing spin-spin splitting, it shows you the number of hydrogen atoms on the adjacent carbon atom

• These are the splitting patterns that you need to be able to recognise from a ¹H spectra:

¹H NMR peak splitting patterns table

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
1	2, doublet	1 : 1
2	3, triplet	1 : 2 : 1
3	4, quartet	1 : 3 : 3 : 1

• Splitting patterns must occur in pairs because each proton splits the signal of the other

• There are some common splitting pairs you will see in a spectrum however you don't need to learn these but can be worked out using the n+1 rule

  • You will quickly come to recognise the triplet / quartet combination for a CH₃CH₂ because it is so common

Common pair of splitting patterns

• A quartet and a triplet in the same spectrum usually indicate an ethyl group, CH₃CH₂-

• The signal from the CH₃ protons is split as a triplet by having two neighbours

• The signal from the CH₂ protons is split as a quartet by having three neighbours

• Here are some more common pairs of splitting patterns

Common pairs of splitting patterns

¹H NMR spectrum of propane

• The CH₂ signal in propane (blue) is observed as a heptet because it has six neighbouring equivalent H atoms (n+1 rule), three on either side in two equivalent CH₃ groups

• The CH₃ groups (red) produce identical triplets by coupling with the CH₂ group

• In summary, a ¹H NMR spectrum provides several types of information:

  • Number of peaks = the number of different proton environments

  • Chemical shift = the general environment of the protons

  • Peak area = the relative number of protons in each environment

  • Splitting patterns = the number of protons on adjacent atoms