Hydrogen NMR
In A2/A-Level Chemistry, NMR of carbon and hydrogen is a form of analytical techniques that can enable scientists to identify the structures of new compounds. This is achieved because the NMR gives the position of 13C or 1H atoms in a molecule. 13C NMR gives a simpler spectra than 1H NMR.
A symbol δ is used for chemical shift which is a scale measured in parts per million (ppm). A standard, Si(CH3)4 or tetramethylsilane, is used because:
- It gives a single peak which is away from all peaks
- It is inert
- It has a low boiling so it can be removed from the sample easily
In A2/A-Level Chemistry, the chemical shift depends on the molecular environment. The y-axis is signal intensity and the x-axis is δ/ppm. A peak near to 0.0 (Si(CH3)4) means that the hydrogen(s) on the carbon have high shielding because it is not near a highly electronegative element like oxygen. The further away the peak is from the standard peak at 0.0 the lower the shielding where the hydrogen(s) attached to the carbon is closer to highly electronegative element. This is what we mean by the term molecular environment. A simple graph with only the axis on illustrates:
Example:
How many separate peaks would you expect to see would you expect to see in the 1H NMR spectra of ethanol.
Firstly, know how ethanol looks like: CH3CH2OH
Notice the three different hydrogen environments indicated by the colours. Let's take a look at each environment separately:
Blue: The hydrogens indicated with this colour have two neighbours being the hydrogens circled in red. This means that this peak for the hydrogens circled in blue will be split into a triplet using the row 1,2,1 in Pascal's triangle (2 being the amount of hydrogens in red).
Red: The hydrogens indicated with this colour have three neighbours being the hydrogens in blue. This mean that for this peak it will be split into a quartet using the row 1,3,3,1 in Pascal's triangle (3 being the number of neighbours).
Green: The hydrogens indicated in this colour have no neighbours as it is not attached to a carbon which has to be attached to another carbon. As it is isolated, we use the first row 1 in Pascal's triangle to show it has no neighbours and therefore makes the peak a singlet.
In A2/A-Level Chemistry, solvents that are used include CCl4 or any solvents which have 2H (called deuterium). This is to dissolve the sample and to prevent peaks being made by the solvent.
On the graph the green peak will be the one plotted more towards the left with the blue peak more towards the right. Integration values will be to do with each peak having a whole number where it represents how many hydrogens there were in the environment. E.g. the blue peak will have an integration value of 3 as it has 3 hydrogens for the blue environment.
That's the end~
