6.3.2: Spectroscopy 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 resonance of nuclei with spin.
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
Summarise what these mean for 13C NMR: Number of signals Chemical shift Area under peak Splitting
Number of signals: One signal for each carbon environment (each set of inequivalent 13C atoms)
Chemical shift: Greater from atoms closer to electronegative atoms or C=C
Area under peak: no meaning
Splitting: there is no splitting for 13C NM
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
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 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?
Tetramethylsilane
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 other advantages of using TMS?
Inert, non-toxic, easy to remove from the sample (as relatively volatile)
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
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) smaller peaks
Why must solvents used for 1H NMR not contain any
hydrogen atoms?
Signals from the solvent would swamp signals
from the sample, as there is much more solvent
than sample.