6.3.2 Spectrometry: Module 6: Organic Chemistry & Analysis Chemistry OCR A A Level

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

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 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 are other advantages

of using TMS?

Inert, non-toxic, easy to remove from the sample (as relatively volatile).

What is the n+1 rule?

If there are n inequivalent 1 H 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, C6D6CCl4 -tetrachloromethane

Why does the peak from O-H bonds disappear if D 2 O 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.