3.3.15 - Nuclear Magnetic Resonance Spectroscopy Flashcards
Name 2 types of NMR
- 1H (or proton) NMR
- 13C NMR
What type of atomic nuclei spin?
Atomic nuclei with an odd number of nucleons (protons and neutrons)
What does nuclear spin cause an atom to have?
a weak magnetic field
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Why do hydrogen nuclei have spin?
Hydrogen nuclei are single protons
Why doesn’t carbon have nuclear spin?
C usually has 6 protons and 6 neutrons
1% of carbon atoms are isotope 13C (___ protons and ____ neutrons) so does have ____
But about 1% of carbon atoms are isotope 13C (6 protons and 7 neutrons) so does have spin
What partly shields nuclei from the effects of external magnetic fields?
- Its surrounding electrons
- Any atoms and groups of atoms around the nucleus
- Affect its amount of electron shielding
State what will happen to the amount of electron shielding around its nucleus, if carbon atoms bond to a more electronegative atom (e.g. oxygen)?
Will decrease
Why do nuclei in different environments absorb different amounts of energy at different frequencies?
∵ in different environments, nuclei will feel different magnetic fields
What does NMR spectroscopy essentially look for?
Differences in energy absorption between environments
For an atom to be in same environment, what must happen?
2 atoms must be joined to exactly the same things
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What substance is used as a standard in NMR spectroscopy?
Tetramethylsilane (TMS), Si(CH3)4
What is chemical shift (δ)?
The differences (in energy absorbed) NMR spectroscopy measures relative to the standard substance (TMS)
Tetramethylsilane has 12 H atoms in ____ ________
identical environments
Name 5 reasons why TMS is used as a standard
- So produces single absorption peak
- Well away from most other absorption peaks
- Inert (doesn’t react with sample)
- Non-toxic
- Volatile (easy to remove from sample)
What is chemical shift measured in?
parts per million (ppm) relative to TMS
State the chemical shift of the single peak produced by TMS
0
Why do you often see a peak at δ = 0 on spectra?
∵ TMS is added to test compound for calibration purposes
Draw tetramethylsilane (TMS)
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What does the number of peaks in 13C NMR tell you?
no. of different carbon environments in molecule
State the number of carbon environments in cyclohexane-1,3-diol
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What does 1H NMR tell you?
Tells you how many environments and how many hydrogens are in each
(1H NMR is about how hydrogen nuclei react to magnetic field)
What is each peak in 1H NMR spectrum due to?
one or more hydrogen nuclei (protons) in a particular environment
1H NMR
What does the numbers above the peaks on a spectrum tell you?
- The ratio of areas under peaks
- This relative area under each peak = relative number of H atoms in each environment
1H NMR: Splitting Patterns
What is meant by spin-spin coupling?
When peaks are split into smaller peaks
1H NMR
What is the n+1 rule?
Peaks always split into number of hydrogens on the neighbouring carbon, plus one
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1H NMR
What are used instead of peak ratios to show the area more clearly?
Integration Traces
Explain how integration traces work
The increases in height are proportional to areas under each peak
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Describe how you can calculate a integration ratio from integration traces
Can use ruler to measure height of each vertical bit of trace and then use heights to work out ratio of peak areas
State the integration ratio for this spectrum and what it means
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- 1:2
- Means there’s 1 H atoms in 1st environment for every 2 H atoms in 2nd environment
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1H NMR
What solvents are used to dissolve samples?
Deuterated solvents
What are the hydrogen atoms in deuterated solvents replaced by?
D or 2H
What is deuterium?
Isotope of H that has 2 nucleons (proton and neutron)
Why does deuterium not have a spin (and doesn’t create a magnetic field)?
∵ deuterium has even no. of nucleons
1H NMR
Name a solvent that is used to dissolve samples
CCl4
Explain why CCl4 can be used as a solvent to dissolve a sample
∵ doesn’t contain any 1H atoms
Two isomers of C6H12O2 are both esters. Each isomer has only two peaks in their 1H NMR spectrum. The integration ratio for both esters is 3:1. Draw the two esters. (2)
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Another isomer of C6H12O2 is a carboxylic acid with a chiral centre. This isomer has five peaks in its 13C NMR spectrum. Draw the structure of this isomer.
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What solvent should you use when obtaininng the 1H NMR spectrum of an amino acid?
D2O