Analytical Techniques - High Resolution ¹H NMR (A-Level Chemistry)

High Resolution ¹H NMR

High Resolution ¹H NMR

Using a higher resolution, you will see that the peaks in a proton NMR spectra are actually split into particular patterns. This is called spin-spin coupling.

It occurs because the magnetic field of the hydrogen atoms creating the peak are affected by all the hydrogen atoms on neighbouring C atoms.

The splitting pattern gives information about the neighbouring hydrogen atoms which can be used to work out the overall structure.

The n+1 Rule

The number of split peaks is equal to the number of hydrogen atoms on the neighbouring C atoms, plus one. It is called the n+1 rule.

• One peak – if there is one peak, not split, it is called a singlet. There are no H atoms on the neighbouring C atoms.
• Two peaks – if the peak is split into two, it is called a doublet. There is 1 H atom on the neighbouring C atoms.
• Three peaks – if the peak is split into three, it is called a triplet. There are 2 H atoms on the neighbouring C atoms.

Example: CH₃ and Coupling

The figure shows the H atoms in the CH₃ part of the molecules produce one peak in low resolution ¹H NMR.

When we look at the peak under high resolution ¹H NMR, the hydrogen atom on the adjacent C atom to the CH₃, causes the peak to split.

As there is only one hydrogen atom, the peak will split into 1+1 = two peaks as shown.

Worked Example: Explain the splitting pattern obtained on a ¹H NMR spectrum of the peak for the hydrogen atoms on the C atom labelled “a”

Answer: The splitting pattern will be a triplet.

Explanation: There are two hydrogen atoms attached to the adjacent carbon atom. So there will be three peaks.

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Interpreting a High Resolution ¹H NMR Spectrum

The steps to interpreting a high resolution 1H NMR spectrum and deducing the structure of a molecule are as follows:

1. Count the number of peaks. The number of peaks represents the number of hydrogen atom environments
2. Use the integration ratio. If given, use the integration ratio to work out the simplest whole number ratio of H atom in each respective environment.
3. Look at the singlets first. Each singlet is either produced by an R-OH group (as long as the ???? shift = 0.5-5.0 ppm) or by a group with no hydrogen atoms attached on either adjacent C atom (such as RNH)
4. Consider possible structures of other peaks. Determine the possible structures of the other peaks using their ???? shift values and draw the structure with the correct number of H atoms, from the integration ratios.
5. Determine the number of H atoms. Determine the number of H atoms on the adjacent C atoms from the splitting pattern of each peak.
6. Draw the structure from each peak. Draw the structure from each peak and the possible adjacent C atoms.

Worked example: The diagram shows a high resolution ¹H NMR spectrum for an alcohol molecule with molecular formula C₆H₁₄O.

Deduce the structure of the molecule.

Answer:

Explanation:

1. There are four peaks which means there are four different hydrogen atom environments
2. The integration ratio is 1.6 : 0.4 : 1.2 : 2.4. The simplest whole number ratio = 4:1:3:6
3. The singlet with an integration ratio of 1 and a ???? shift value of 1.1 must be caused by the H in OH alcohol group as this is the only environment which has 1 hydrogen atom.
4. The singlet, with an integration ratio of 3 must be due to a CH₃ group. This is because there are 3 H atoms producing the peak. The singlet means there are no adjacent H atoms on the C atom. The ???? shift value of 1.2 which comes from R-CH₃. This adjacent C atom could be bonded to three other alkyl groups, but no H atom.
5. Triplet at ???? shift value = 0.9 suggests an R-CH₃ group, adjacent to a CH₂ group, as a triplet is produced.
6. The integration ratio of 4 and 6 indicate two equivalent CH₂CH₃ groups.