Proton (1H) NMR Spectroscopy (Cambridge International A-Level Chemistry)

Interpreting & Explaining Proton (1H) NMR Spectra

• Nuclear Magnetic Resonance (NMR) spectroscopy is used for analysing organic compounds

• Atoms with odd mass numbers usually show signals on NMR

• In ¹H NMR, the magnetic field strengths of protons in organic compounds are measured and recorded on a spectrum

• Protons on different parts of a molecule (in different molecular environments) emit different frequencies when an external magnetic field is applied

• All samples are measured against a reference compound – Tetramethylsilane (TMS)

  • TMS shows a single sharp peak on NMR spectra, at a value of zero
  • Sample peaks are then plotted as a ‘shift’ away from this reference peak
  • This gives rise to ‘chemical shift’ values for protons on the sample compound
  • Chemical shifts are measured in parts per million (ppm)

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

The key features of this spectrum are the number and position of the peaks

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

Worked Example How many different ¹H chemical environments occur in 2-methylpropane?

Answer:

• Two different ¹H chemical environments occur in 2-methylpropane

• The three methyl groups are in the same ¹H environment

  • The lone hydrogen is in its own ¹H environment

Chemical shift values for ¹H molecular environments table

• δ values for O–H protons and N–H protons vary depending on the solvent and concentration

• 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

All four protons in the 1,2-dichloroethane molecule are equivalent

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 as 3 separate environments: -CH₃, -CH₂, -OH
  • So 3 peaks would be seen on its spectrum at 1.2 ppm (-CH₃), 3.7 ppm (-CH₂) and 5.4 ppm (-OH)

Low resolution NMR spectrum of ethanol

The low resolution NMR spectrum of ethanol shows 3 peaks for the 3 molecular environments

High resolution ¹H NMR

• More structural details can be deduced using high resolution NMR
• The peaks observed on a high resolution NMR may sometimes have smaller peaks clustered together
• The splitting pattern of each peak is determined by the number of protons on neighbouring environments

The number of peaks a signal splits into = n + 1

• (Where n = the number of protons on the adjacent carbon atom)

Predicting Shifts & Splitting Patterns

Spin-Spin Splitting

• A high resolution ¹H NMR spectrum 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 high resolution¹H NMR spectrum of ethanol showing the splitting patterns of each of the 3 peaks. Using the n+1, it is possible to interpret the splitting pattern

Exam Hint

• It is very rare that the spin-spin splitting of equivalent protons is covered in teaching because it is so rarely asked in exams

• Equivalent protons do not cause spin-spin splitting

  • The simplest example of this is benzene

    • In benzene, all of the protons are equivalent
    • This means that they are seen as one singlet in the high resolution ¹H NMR spectrum of benzene

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

• 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

Worked Example For the compound (CH₃)₂CHOH, predict the following:

1. The number of peaks
2. The type of proton and chemical shift
3. The relative peak areas
4. The splitting pattern

Answers:

1. The number of peaks

• 3 peaks

1. The type of proton and chemical shift

• (CH₃)₂CHOH at 0.9 - 1.7 ppm
• (CH₃)₂CHOH at 3.2 - 4.0 ppm
• (CH₃)₂CHO**H **at 0.5 - 6.0 ppm

1. The relative peak areas

• Ratio 6 : 1 : 1

1. The splitting pattern

• (CH₃)₂CHOH split into a doublet (1+1=2)
• (CH₃)₂CHOH split into a heptet (6+1=7)