Edexcel A Level Chemistry:复习笔记7.7.3 Proton NMR

Proton NMR - Introduction

 

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

7.10.1-Structural-formula-of-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

     

8.1-Analytical-Techniques-Features-of-a-1H-NMR-Spectrum

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

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

     

7.10.2-Worked-example-environments-answer

 

Chemical Shift Values for 1H Molecular Environments Table

7.10.2-Chemical-shift-values-for-1H-molecular-environments-table.PNG

edexcel-proton-nmr-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

     

7.10.2-12-dichloroethane 

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

     

8.1-Analytical-Techniques-Low-Resolution-NMR-of-Ethanol

A low resolution NMR spectrum of ethanol showing 3 peaks for the 3 molecular environments

 

Using High Resolution Proton NMR Data

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)

 

7.10.2-High-Resolution-NMR-of-Ethanol

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 1:2:2:1 – the middle peaks are twice the intensity of the 2 on either side

     

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

     

7.10.2-Integrated-spectra

 

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

8.1-Analytical-Techniques-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

7.10.2-Common-pairs-of-splitting-patterns

Common pairs of splitting patterns

 

1H NMR spectrum of propane

7.10.2-Propane-spectrum

  • 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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