7.7.3 Proton NMR

• Nuclear Magnetic Resonance (NMR) spectroscopy is used for analysing organic compounds
• All samples are measured against a reference compound – Tetramethylsilane (TMS)

 Tetramethylsilane is the common reference compound for NMR spectroscopy

• TMS shows a single sharp peak on NMR spectra, at a value of zero

• TMS is also used because it is:

  • Non toxic.
  • Does not react with the sample.
  • Easily separated from the sample molecule due to its low boiling point.
  • Produces one strong, sharp absorption peak on the spectrum.
• 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 a ¹H NMR spectrum

• NMR spectra shows the intensity of each peak against their 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

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

Molecular environments

• ¹H nuclei that have different neighboring atoms (said to have different chemical environments) absorb at slightly different field strengths
• The difference environments are said to cause a chemical shift of the absorption
  • Ethanol has the structural formula CH₃CH₂OH
  • There are 3 chemical environments: -CH₃, -CH₂ and -OH

• The hydrogen atoms in these environments will appear at 3 different chemical shifts
• Different types of protons are given their own range of chemical shifts

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

• 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

Low resolution ¹H NMR

• Peaks on a low resolution NMR spectrum refers to molecular environments of an organic compound
  • 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)
  • The strengths of the absorptions are proportional to the number of equivalent ¹H atoms causing the absorption and are measured by the area underneath each absorption peak
  • Hence, the areas of absorptions of -CH₃, -CH₂, -OH are in the ratio of 3:2:1 respectively

A low resolution NMR spectrum of ethanol showing 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)

High resolution ¹H 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

• Each splitting pattern also gives information on relative intensities
  • A doublet has an intensity ratio of 1:1 – each peak is the same intensity as the other
  • In a triplet, the intensity ratio is 1:2:1 – the middle of the peak is twice the intensity of the 2 on either side
  • In a quartet, the intensity ratio is 1:3:3:1 – the middle peaks are three times the intensity of the 2 outer peaks

Integrated spectra

• In ¹H NMR, the relative areas under each peak give the ratio of the number of protons responsible for each peak
• The NMR spectrometer measures the area under each peak, as an integration spectra
  • This provides invaluable information for identifying an unknown compound

• 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

Spin-Spin Splitting

• A ¹H NMR peak 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

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 immediately 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 protons 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 either side in two equivalent CH3 groups
• The CH3 groups (red) produce identical triplets by coupling with the CH₂ group

Answers:

i) 3 peaks

ii) (CH₃)₂CHOH at 0.7 - 1.2 ppm, (CH₃)₂CHOH at 3.1 - 3.9 ppm, (CH₃)₂CHOH at 0.5 - 5.5 ppm

iii) Ratio 6 : 1 : 1

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