High Resolution Proton NMR (Edexcel International A Level Chemistry)

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High Resolution Proton 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, ClCH2COOCH3, 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

Integrated spectra, downloadable AS & A Level Chemistry revision notes

Spin-Spin Splitting

  • 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 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

Analytical Techniques - 1H NMR peak splitting patterns table, downloadable AS & A Level Chemistry revision notes

  • 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 CH3CH 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, downloadable AS & A Level Chemistry revision notes

Common pairs of splitting patterns

1H NMR spectrum of propane

Propane spectrum, downloadable AS & A Level Chemistry revision notes

  • 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

Worked example

For the compound (CH3)2CHOH predict the following:

i) the number of peaks

ii) the type of proton and chemical shift (using the Data sheet)

iii) the relative peak areas

iv) the split pattern

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)

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Richard

Author: Richard

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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.