Hydrogen NMR
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).
Calibration
A standard, Si(CH3)4 or tetramethylsilane (TMS), is used to calibrate chemical shifts for 1H and 13C NMR spectroscopy in organic solvents:
- It gives a single peak that is further right than most of the peaks
- It gives one signal/peak as all 12 H atoms are chemicaclly equivalent
- It is inert
- It is non-toxic
- It has a low boiling point so it can be easily removed from the sample
Solvents
- CCl4, CDCl3 or any solvents which have 2H (Deuterium)
- This is to dissolve the sample and to prevent peaks being mafe by the solvent
Chemical shift
- 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.
Splitting
- Peaks that are created by different environments of hydrogen may split into two, three or four. Known as doublet, triplet and quartet.
- The number of lines a signal is split into gives information about neighbouring H atoms
- Example: A peak that has split into two has one hydrogen on the next carbon as aneighbour. Splitting of three means that the next carbon has two hydrogens. Splitting into four means that the next carbon has three hydrogens.
- Use Pascal's triangle to know what the peak splits into. The middle number on each row is the amount of hydrogens that are the neighbours
- The number of lines = number of equivalent H(s) on adjacent C plus 1 [the "n+1" rule]
Common exam style question in AS/A-level chemistry:👨🏫
How many separate peaks would you expect to see in the 1H NMR spectra of ethanol?
There are three different hydrogen enviornments indicated by the colours.
- 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 bluewill be split into a triplet using the row 1,2,1 in Pascal's triangle (2 beingthe amount of hydrogens in red).
- Green: 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 row1,3,3,1 in Pascal's triangle (3 being the number of neighbours).
- Red: The hydrogens indicated in this colour have noneighbours 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.
- On the graph, the quartet will be shifted more to the left as its carbon is attached to a highly electronegative oxygen.
- The triplet is shifted to the right as it is further away from the oxygen, the hydrogens have higher shileding.
This is the end of the topic!
Drafted by Cherry (Chemistry)