Proton NMR (OCR A Level Chemistry)

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Low & High Resolution Proton NMR

Features of a 1H NMR spectrum

  • An NMR spectrum shows 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 area under 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
    • By definition the chemical shift is at 0 ppm

Analytical Techniques - Features of a 1H NMR Spectrum, downloadable AS & A Level Chemistry revision notes

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

Molecular environments

  • 1H nuclei that have different neighbouring atoms (said to have different chemical environments) absorb at slightly different field strengths
  • The difference environments are said to cause a chemical shift away from TMS
    • 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

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

Worked example environments answer, downloadable AS & A Level Chemistry revision notes

Chemical shift values for 1H molecular environments table

Environment of proton Chemical shift range, δ / ppm
R–CH 0.5 - 2.0
toluyl-1h-nmrketone-1h-nmrN–CH
2.0 - 3.0
O–CH
Cl–CH
Br–CH
3.0 - 4.2
alkenyl-1h-nmr 4.5 - 6.0
benzyl-1h-nmr 6.2 - 8.0
aldehyde-1h-nmr 9.0 - 10.0
carboxyl-1h-nmr 10.0 - 12.0
R–OH
R–NH
0.5 - 12.0

  • 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

1,2-dichloroethane, downloadable AS & A Level Chemistry revision notes

Low resolution 1H NMR

  • Peaks on a low resolution NMR spectrum refer to molecular environments of an organic compound
    • Ethanol has the molecular formula CH3CH2OH
    • This molecule as 3 separate proton 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 and are proportional to the area underneath each absorption peak
    • Hence, the areas of absorptions of -CH3, -CH2, -OH are in the ratio of 3:2:1 respectively

Analytical Techniques - Low Resolution NMR of Ethanol, downloadable AS & A Level Chemistry revision notes

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 in neighbouring environments

Spin-Spin 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 showing the splitting patterns of each of the 3 peaks. Using the n+1, it is possible to interpret the splitting pattern

Examiner Tip

  • 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 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

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 splitting 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)

Tetramethylsilane as a Standard

Tetramethylsilane

  • The horizontal scale on an NMR spectrum represents chemical shift (δ)
  • Chemical shift is measured in parts per million (ppm) of the radio frequency needed for resonance compared to a reference chemical called tetramethylsilane, abbreviated to TMS

Structural formula of TMS, downloadable AS & A Level Chemistry revision notes

The displayed formula of tetramethylsilane

  • TMS is used universally as the reference compound for NMR as its methyl groups are particularly well shielded and so it produces a strong, single peak at the far right of an NMR spectrum
  • The signal from the hydrogen atoms in TMS is defined as having a chemical shift of 0 ppm

Analytical Techniques - TMS 1H NMR, downloadable AS & A Level Chemistry revision notes

The NMR reference peak for TMS

Hydroxyl and amino protons

  • Organic compounds often contain protons from part of a functional group rather than being bonded to a carbon, examples include:
    • Alcohols, ROH
    • Carboxylic acids, RCOOH
    • Phenols, ArOH
    • Amines, RNH2 
    • Amides, RCONH2 
    • Amino acids, H2NCHRCOOH
  • These hydroxyl, OH, protons and amino, NH, protons can be involved in hydrogen bonding which causes their peaks to be broader than normal and they can appear at almost any chemical shift
    • The broadening of the peak means that they are not usually involved in spin-spin coupling and therefore appear as singlets

Deuterated solvents

  • Deuterated solvents such as D2O (heavy water) and CDCl3 can be used to identify OH and NH protons
  • This is achieved by:
    • Performing a standard proton NMR on the sample compound
    • A small amount of the deuterated solvent is added to the sample compound and mixed
    • A second proton NMR is then performed
  • Deuterium atoms from the solvent replace the OH and NH protons in the sample, e.g for ethanol

C2H5OH + CDCl3 ⇌ C2H5OD + CHCl3 

  • This means that the second proton NMR will still have the peaks for the CH3 and CH2 protons of ethanol but will not have the OH peak as that proton has been exchanged

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Richard

Author: Richard

Expertise: Chemistry

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.