6.3.2- Spectroscopy Flashcards
What are basic principles of NMR?
You can find 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.
What does NMR stand for?
Nuclear Magnetic Resonance
Give one use of NMR?
MRI scans
What kind of nuclei does NMR work with?
Those with an uneven number of nucleons, meaning they will spin e.g. 1H and 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
Chemical shift: Greater shift from atoms closer to electronegative atoms or C=C
Area under peak:: no meaning
Splitting: 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.
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?
Area under the peak is proportional to the number of 1H atoms represented by the peak
What is TMS?
Tetramethylsilane
What is the integration trace?
A stepped line that makes it easier to measure area under the curve (height of line= area under that peak)
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 chemical shifts are measured
What are other advantages of using TMS?
Inert, non-toxic, easy to remove from the sample (as relatively volatile)
When does splitting occur?
Neighbouring hydrogen atoms (on adjacent carbon) affect the magntic 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.
Which solvents are used?
- Deuterated solvents: CDCl3, D2O. C6D6
-CCl4- tetrachloromethane
Summarise what these indicate for 1H NMR:
-Number of sigals
-Chemical shift
-Splitting
-Area under peak
Number of signals: One main signal for each set of inequivalent 1H atoms.
Chemical shift: Larger for 1H atoms closer to electronegative atoms or C=C
Splitting: Number smaller peaks= 1 + number of inequivalent hydrogen bonds on adjacent carbon
Area under peak: Proportional to number of atoms represented by that peak.
Why does the peak from O-H bonds disappear if D2O is used as a solvent?
O-D bond is formed in preference to O-H due to labile protons that move/swap from one molecule to another
