NMR

Question 1

Why is NMR used ?

Answer 1

NMR is such a powerful technique because:
* It is easy to obtain a spectrum.
* Only a small amount of sample is necessary.
* It is non-destructive.

Question 2

13C NMR

Answer 2

(1) No of peaks tells you the No of carbon environments.
(2) Location (i.e. chemical shift) of peaks tells you the type of carbon environment.

Question 3

TMS – Tetra Methyl Silane

Answer 3

Reasons why TMS is used as a standard are: Si(CH3)4
- In generates one sharp peak at 0ppm
- It is unreactive, so won’t affect the sample being analysed.
- It is highly volatile. This means it can be removed from the sample being tested after the analysis fairly easily.
- Peak always found on the far right of the spectrum.

Question 4

1H NMR (Proton NMR)

Answer 4

(1) No of peaks tells you the No of proton environments
(2) Location of peaks (i.e. chemical shift) show the type of proton environment.
(3) Area under each peak (i.e. the integration ratio) tells you the number of protons in each environment.
(4) The SPLITTING (i.e. coupling) shows the number of protons in the adjacent environments

Question 5

Spin – Spin Coupling (i.e. splitting)

Answer 5

n + 1 rule
(where n is the number of protons on the adjacent carbons)

Question 6

H NMR example

Answer 6

Question 7

Solvents Used in NMR spectra

Answer 7

Deuterium is an isotope of hydrogen with a mass number of 2 (it has one proton and one neutron in the nucleus). For this reason it doesn’t create a peak in an NMR spectrum.

This means any solvent molecule where H has been replaced by D could be used but common examples are:

• Deuterated Trichloromethane: CDCl3 – for polar samples
• Deuterated Benzene: C6D6- for non-polar samples

Alternatively solvent molecules with no hydrogen atoms to start with could be used like tetrachloromethane: CCl4

Question 8

State why Si(CH3)4, CCl4 and CDCl3 are used in 1H NMR spectroscopy.
Explain how their properties make them suitable for use in 1H NMR spectroscopy.

Answer 8

• CDCl3 or CCl4 solvent
• TMS as reference
• all are Inert

CCl4 or CDCl3 as solvent:
* Both have no H, no peaks
* CCl4 non polar
* CDCl3 polar covalent molecule

TMS as reference:
* one single environment
* Signal furthest right on spectrum
* Easy to remove

Question 9

Integration ratio question

Answer 9