Spectrometry 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. (1)H, (13)C
Summarise what these mean for 13C NMR:
No. signals
Chemical shift
Area under peak
Splitting
NO. 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: there is no splitting for (13)C NMR
Why is it easier to get a spectrum of (1)H NMR that (13)C NMR?
Most H atoms are (1)H — it is much more abundant than (13)C. 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 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 (1)H atoms represented by the peak.
What is the integration trace?
A stepped line that makes it easier to measure the area underneath the curve (height of line = area under that peak)
What is TMS?
Tetramethylsilane
Why is TMS used?
Can be added to sample to calibrate the NMR equipment. It provides a peak at exactly δ=0ppm
It is the reference point against which all shifts are measured.
What are 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 (1)H atoms and causes their peaks to split.
What is the n+1 rule?
If there are inequivalent (1)H atoms on the neighbouring carbon then the peak will split into (n+1) smaller peaks.
Why much solvents used for (1)H 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 (1)H NMR:
NO. signals
Chemical shift
Splitting
Area under peak
NO. signals: one main signal for each set of inequivalent (1)H atoms (for each H environment)
Chemical shift: Larger for (1)H atoms closer to electronegative atoms or C=C
Splitting: NO. smaller peaks = 1 + no. inequivalent hydrogen atoms 3 bonds away
Area under peak: proportional to the no. 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.
