Unit 3: Section 9 - Further Synthesis and Analysis Flashcards
What does NMR stand for?
Nuclear Magnetic Resonance
What are the basic principles of NMR?
- You can find the structures of complex molecules by placing them in a magnetic field and applying EM waves of radio frequency to them
- If radio waves of the right frequency are absorbed, the nuclei flips from parallel to applied magnetic to field to anti-parallel
- This energy change can be monitored and recorded
- Uses the the resonance of nuclei with spin
How would you carry out NMR spectroscopy?
- Dissolve the liquid sample in a suitable solvent
- Put in a tube along with a small amount of TMS and put the tube into an NMR machine
- The sample is spun to even out any imperfections in the magnetic field and the spectrometer is zeroed against the TMS
- Radiation with different radio frequencies but a constant magnetic field is applied to the sample and any absorptions (due to resonance) are detected
Give a use of NMR?
MRI scans
What kind of nuclei does NMR work with (and examples)?
Those with an uneven number of nucleons meaning they will spin
What percentage of carbon atoms are C13?
1% - modern instruments are sensitive enough to detect this
What defines the resonant frequency of a C13 atom?
The chemical environment that it is in; the amount of electron shielding it has
What graph is produced by NMR spectroscopy?
Energy absorbed against chemical shift
What is chemical shift? What are its units?
The resonant frequency of a nuclei, compared to that of a H1 atom in TMS
ppm (parts per million)
What is the range of chemical shift for C13 NMR?
0-200ppm
What means C13 atoms show a different chemical shift value?
Having different chemical environments (but equivalent atoms show the same peak)
What kind of envrionment leads to a greater chemical shift?
A C atom next to a more electronegative atom has a greater chemical shift
What does the number of signals mean for C13 NMR?
One signal for each carbon environment
What does the chemical shift mean for C13 NMR?
Greater δ from atoms closer to electronegative atoms or C=C
What does the area under the peak mean for C13 NMR?
No meaning
Is there splitting in C13 NMR?
No
Why is it easier to get a spectrum of H1 NMR than C13 NMR?
Most H atoms are 1H- - it is much more abundant than C13
This means almost all H atoms have spin so show up
What is the range of chemical shift for H1 NMR?
0-10ppm
What leads to a lower chemical shift value for H NMR?
H1 with more electrons around them - further from electronegative groups/atoms
On a low resolution spectrum, what peaks would you expect to see for H NMR?
One peak for each set of inequivalent H atoms (each chemical environment shows 1 peak)
What does the area under the peak represent for H NMR?
The area under the peak is proportional to the number of H1 atoms represented by the peak
What is the integration trace?
A stepped line that makes it easier to measure the area under the curve (height of line = area under that peak)
What is TMS?
Tetramethylsilane
Silicon bonded to 4 methyl groups
What state is TMS at room temperature?
Liquid
Why is TMS used?
- Can be added to sample to calibrate the NMR equipment
- It has a very strong signal
- The signal is far away from any other signal
- It is inert so won’t react with the sample, affecting the result
- Volatile so can be used to remove from the sample
When does splitting/spin-spin coupling occur?
Neighbouring hydrogen atoms affect the magnetic field of 1H atoms and cause their peaks to split
What is the n+1 rule?
If there are n inequivalent 1H atoms on the neighbouring carbon then the peak will split into n+1 peaks
Why must solvents used for 1H NMR not contain any hydrogen atoms?
Signals from the solvent would get confused with the sample, as there is much more solvent than the sample
Which solvents are used?
CDCl3 or CCl4
