3.15 NMR Spectroscopy
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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 resonance of nuclei with spin
How would you carry out NMR spectroscopy?
Dissolve the liquid sample in 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 one 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 e,g. 1H, 13C
What percentage of carbon atoms are 13C?
1% but modern instruments are sensitive enough to detect this
What defines the resonant frequency of a 13C 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 is its symbol? What are its units?
The resonant frequency of the nuclei, compared to that of a 1H atom in TMS. Symbol δ Parts per million (ppm)
What is the range of chemical shift for 13C NMR?
0-200 ppm
What means 13C atoms show a different chemical shift value?
Having different chemical environments (but equivalent atoms show the same peak)
What kind of environment leads to a greater chemical shift?
A C atom next to more electronegative atom has a greater chemical shift
Summarise what these mean for 13C NMR: number of signals Chemical shifts Area under peak Splitting
number of signals: one signal for each carbon environment (each set of inequivalent 13C atoms) Chemical shifts: Greater δ from atoms closer to electronegative atoms or C=C Area under peak: no meaning Splitting: there is no splitting for 13C NMR
Why is it easier to get a spectrum of 1H NMR than 13C NMR?
Most H atoms are 1H- it is much more abundant than 13C. This means almost all H atoms have spin so show up
What is the range of chemical shift for 1H NMR?
0-10 ppm
What leads to a lower chemical shift value for H NMR?
1H with more electrons around them I.e. 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 in equivalent 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 1H 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 (name and structure)?
Tetramethylsilane Slide 42
What state is TMS at room temperature?
Liquid
Why is TMS used
Can be added to sample to calibrate the NMR equipment. It provides a peak at exactly () = 0ppm. It is the reference point against which all () are measured
What are the other advantages of using TMS?
inert, non toxic, easy to remove from the sample
When does splitting/spin-spin coupling occur?
Neighbouring hydrogen atoms (3 or fewer bonds away, or on the adjacent carbon) affect the magnetic field of 1H atoms and causes their peaks to split