Interpreting NMR Spectra (OCR A-Level Chemistry A): Revision Notes

Worked Example: Complete Interpretation of a Proton NMR Spectrum

Let's work through a complete interpretation of a proton NMR spectrum for a compound with molecular formula $\text{C}_4\text{H}_8\text{O}_2$.

Step 1: Analyse the types of proton present

The spectrum shows three distinct peaks, indicating three different proton environments. Looking at the relative peak areas (integration values), the ratio is 3:2:3.

This ratio suggests:

• A peak at $\delta = 3.7$ ppm representing 3 protons (likely $\text{CH}_3$)
• A peak at $\delta = 2.3$ ppm representing 2 protons (likely $\text{CH}_2$)
• A peak at $\delta = 1.1$ ppm representing 3 protons (likely $\text{CH}_3$)

Step 2: Analyse the splitting patterns

Examining each peak more carefully:

• The peak at $\delta = 3.7$ ppm is a singlet, which tells us there are no adjacent protons (by the $n + 1$ rule: $0 + 1 = 1$)
• The peak at $\delta = 1.1$ ppm is a triplet, indicating an adjacent $\text{CH}_2$ group (by the $n + 1$ rule: $2 + 1 = 3$)
• The peak at $\delta = 2.3$ ppm is a quartet, indicating an adjacent $\text{CH}_3$ group (by the $n + 1$ rule: $3 + 1 = 4$)

The combination of a triplet at $\delta = 1.1$ ppm and a quartet at $\delta = 2.3$ ppm is characteristic of an ethyl group ($\text{CH}_3\text{CH}_2$).

Step 3: Analyse the chemical shifts

Using chemical shift data:

• The $\text{CH}_3$ peak at $\delta = 3.7$ ppm indicates protons in the environment $\text{H-C-O}$, suggesting a sequence like $\text{CH}_3\text{-O}$
• The $\text{CH}_2$ peak at $\delta = 2.3$ ppm indicates protons in the environment $\text{H-C=O}$, suggesting a sequence like $\text{CH}_2\text{-C=O}$
• The $\text{CH}_3$ peak at $\delta = 1.1$ ppm indicates protons in the environment $\text{H-C-R}$, suggesting a sequence like $\text{CH}_3\text{-C}$

Step 4: Combine the information

Putting together all the evidence:

• We have a $\text{CH}_3\text{CH}_2$ group (from the triplet-quartet combination)
• We have a $\text{CH}_2\text{-C=O}$ group (from the chemical shift at 2.3 ppm)
• We have a $\text{CH}_3\text{-O}$ group (from the singlet at 3.7 ppm)
• The molecular formula is $\text{C}_4\text{H}_8\text{O}_2$, suggesting an ester

The structure must be methyl propanoate: $\text{CH}_3\text{CH}_2\text{COOCH}_3$

This annotated spectrum shows how each group of protons corresponds to its peak, confirming our interpretation.

Worked Example: Identifying Structure from Heptet-Doublet Pattern

Now let's interpret a proton NMR spectrum for a compound with molecular formula $\text{C}_3\text{H}_7\text{Cl}$.

Step 1: Analyse the types of proton present

This spectrum shows two peaks, so there are two different proton environments. The integration ratio is 1:6.

This suggests:

• A peak at $\delta = 3.8$ ppm representing 1 proton ($\text{CH}$)
• A peak at $\delta = 1.6$ ppm representing 6 protons ($2 \times \text{CH}_3$)

Step 2: Analyse the splitting patterns

Looking at the splitting:

• The peak at $\delta = 3.8$ ppm is a heptet (7 peaks), which indicates 6 adjacent protons (by the $n + 1$ rule: $6 + 1 = 7$). This can only arise from two adjacent $\text{CH}_3$ groups: $(\text{CH}_3)_2$
• The peak at $\delta = 1.6$ ppm is a doublet, indicating 1 adjacent proton (by the $n + 1$ rule: $1 + 1 = 2$)

The combination of a doublet at $\delta = 1.6$ ppm and a heptet at $\delta = 3.8$ ppm tells us the structure contains $(\text{CH}_3)_2\text{CH}$.

Step 3: Analyse the chemical shifts

Using chemical shift data:

• The $\text{CH}$ peak at $\delta = 3.8$ ppm indicates the environment $\text{H-C-Cl}$, supporting a $\text{CH-Cl}$ sequence
• The $(\text{CH}_3)_2$ peak at $\delta = 1.6$ ppm indicates the environment $\text{H-C-R}$, supporting the sequence $\text{C}(\text{CH}_3)_2$

Step 4: Combine the information

Combining all the evidence from steps 1-3:

• We have two equivalent $\text{CH}_3$ groups
• We have one $\text{CH}$ group bonded to chlorine
• The $\text{CH}$ is adjacent to both $\text{CH}_3$ groups

The structure must be 2-chloropropane: $(\text{CH}_3)_2\text{CHCl}$

This annotated spectrum confirms our deduced structure, showing how the two methyl groups produce the large peak at 1.6 ppm and the single CH group produces the smaller, split peak at 3.8 ppm.

Worked Example: Predicting a Carbon-13 NMR Spectrum

Let's predict the carbon-13 NMR spectrum for 2-hydroxypropanoic acid (lactic acid), $\text{CH}_3\text{CHOHCOOH}$.

Step 1: Draw out the structure

The structure shows the skeletal formula of lactic acid with three carbon atoms.

Step 2: Identify the number of chemical environments

Looking at the carbon atoms in the structure:

• Carbon 1: $\text{CH}_3$ group bonded to C-OH
• Carbon 2: $\text{CH}$ group bonded to OH and to the rest of the chain
• Carbon 3: $\text{COOH}$ carboxylic acid carbon with C=O

These three carbon atoms are in different chemical environments, so we expect three peaks in the carbon-13 NMR spectrum.

Step 3: Predict the chemical shifts

Using carbon-13 chemical shift data:

• Carbon-1 is of type $\text{C-C}$, so we predict a peak at $\delta = 0\text{-}50$ ppm
• Carbon-2 is of type $\text{C-O}$, so we predict a peak at $\delta = 50\text{-}90$ ppm
• Carbon-3 is of type $\text{C=O}$ (carboxylic acid), so we predict a peak at $\delta = 160\text{-}220$ ppm

Checking the prediction

The actual carbon-13 NMR spectrum shows three peaks at approximately 20 ppm, 80 ppm, and 170 ppm, which matches our predictions very well. The peaks appear in the expected regions for the three different carbon environments.

Worked Example: Predicting a Proton NMR Spectrum with Splitting Patterns

Now let's predict the proton NMR spectrum for the same compound, 2-hydroxypropanoic acid, $\text{CH}_3\text{CHOHCOOH}$.

Step 1: Draw the structure and identify the number of chemical environments

This numbered structure shows four different types of protons, so we expect four peaks in the proton NMR spectrum:

• Protons 1: $\text{CH}_3$ group
• Proton 2: $\text{CH}$ group
• Proton 3: $\text{OH}$ group on carbon 2
• Proton 4: $\text{COOH}$ group

Step 2: Predict the chemical shifts

Using proton chemical shift data:

• Protons 1 ($\text{CH}_3$) are of type $\text{H-C-R}$, giving a peak at $\delta = 0.5\text{-}2.0$ ppm
• Proton 2 ($\text{CH}$) is of type $\text{H-C-O}$, giving a peak at $\delta = 3.0\text{-}4.2$ ppm
• Proton 3 ($\text{OH}$) can appear anywhere in the range $\delta = 0\text{-}12$ ppm (broad, variable)
• Proton 4 ($\text{COOH}$) is of type carboxylic acid, giving a peak at $\delta = 10\text{-}12$ ppm

Step 3: Predict the relative peak heights

The relative peak heights depend on the number of each type of proton. For protons 1-4, the ratio would be 3:1:1:1 ($\text{CH}_3 : \text{CH} : \text{OH} : \text{COOH}$).

Step 4: Predict the splitting patterns

Using the $n + 1$ rule:

• Protons 1 ($\text{CH}_3$) have one adjacent proton (the $\text{CH}$), so they appear as a doublet ($1 + 1 = 2$)
• Proton 2 ($\text{CH}$) has three adjacent protons (the $\text{CH}_3$ group), so it appears as a quartet ($3 + 1 = 4$)
• Protons 3 and 4 ($\text{OH}$ and $\text{COOH}$) typically give broad peaks without clear splitting due to rapid proton exchange

Checking the prediction

The actual proton NMR spectrum shows the predicted features:

• A peak with integration 3 around $\delta = 1$ ppm for the $\text{CH}_3$ group
• Peaks around $\delta = 4$ ppm for the $\text{CH}$ group
• A peak around $\delta = 11$ ppm for the $\text{COOH}$ proton

Notice that the quartet for the $\text{CH}$ group at $\delta = 4.2$ ppm appears at the upper end of the predicted range (3.0-4.2 ppm). This is because this $\text{CH}$ group is adjacent to both an oxygen atom (from the $\text{OH}$) and a carbonyl group (from $\text{C=O}$), both of which cause additional deshielding and shift the signal further downfield.