3.3.15 Nuclear magnetic resonance spectroscopy - Chemistry Alevel

¹³C NMR - How to determine the number of peaks (example CH₃CH₂CH(CH₃)₂)

A unique carbon atom creates a peak. Look at what it is directly bonded to and then the carbon chain. (There is one signal for each set of equivalent C atoms).

For example;

CH₃CH₂CH(CH₃)₂ has 4 peaks.

NB: Sometimes there is a peak at 0 chemical shift due to TMS.

¹³C NMR - use

Identifying compounds containing carbon. (Enables the structures of new compounds to be confirmed).

¹³C NMR - what determines the x-axis position of the peak

The closer the atom to electronegative atoms and the more of those electronegative atoms the higher the chemical shift (δ) and so the further to the left on the x-axis.

¹³C NMR - use of the chemical shift value

Gives a lot of information about the chemical environment of the C atoms.

¹³C NMR vs ¹H NMR

¹³C NMR is a lot simpler.

NMR - What is used for calibration?

Tetraethyl silane (TMS/Si(CH₃)₄) is used for calibration as it provides a single peak as an internal standard to which other peaks are compared.

NMR - Why is TMS used for calibration

It gives a signal that is further to the right than most of the signals from organic compounds (as Si has a low electronegativity).

It only gives one signal (as all H atoms are chemically equivalent).

It is non-toxic.

It is inert.

It has a low boiling point (26°C) and so can be easily removed from the sample afterwards.

¹H NMR - solvent examples and why they are used

CCl₄ and deuterated solvents such as CDCl₃ and C₆D₆ are used. These solvents are used as solvents containing ¹H atoms will appear in the spectrum and so solvents must have no ¹H atoms (could be ²H atoms as they have no spin).

¹H NMR - conditions

Always recorded with solution of the sample.

¹H NMR - How to determine the number of peaks (example CH₃CH₂CH(CH₃)₂)

A unique hydrogen atom creates a peak. Look at what it is directly bonded to and then the carbon chain. Depending in the number of equivalent H atoms you get a different ratio (and therefore different peak area - look at splitting).

For example;

CH₃CH₂CH(CH₃)₂ has 4 peaks with a ratio:

6:1:2:3

NB: Sometimes there is a peak at 0 chemical shift due to TMS.

¹H NMR - how to determine area of peaks

The area of each signal is proportional to the number of equivalent H atoms (ratio). Look at the height of the integration trace or the number given NOT the actual height of the peak.

¹H NMR - chemical shift value

The closer the H is to electronegative atoms (for example O, Cl) the greater the shift. Look at the best match in the data booklet!

¹H NMR - how to determine the number of lines (splits) in each peak

Number of lines = 1 + number of inequivalent H atoms on adjacent C atoms

Equivalent H don't split each other.

¹H NMR - splitting (coupling/multiplicity) - 1 line - name & why

Singlet as there are no inequivalent H neighbouring atoms (could be no H atoms on the neighbouring C)

¹H NMR - splitting (coupling/multiplicity) - 2 lines - name & why

Doublet as there is one inequivalent H atom on the neighbouring atoms. (e.g CH₃ of CH₃CH₂...)

¹H NMR - splitting (coupling/multiplicity) - 3 lines - name & why

Triplet as there are two inequivalent H atoms on the neighbouring atoms. (e.g CH₂ of CH₃CH₂CHO aldehyde ketone = ...CO...).

¹H NMR - splitting (coupling/multiplicity) - 4 lines - name & why

Quartet as there are three inequivalent H atoms on the neighbouring atoms. (e.g CH of (CH₃)₂CHCH₂...)

¹H NMR - splitting (coupling/multiplicity) - 5+ lines - name

Multiplet.