In high-resolution proton (\(^1\text{H}\)) NMR spectroscopy, many peaks do not appear as simple single lines. Instead, they are split into sub-peaks. This phenomenon, known as spin-spin coupling, provides critical information about the neighbouring atoms in the molecule.
🔑 Key Principle
Spin-spin coupling occurs because the magnetic field experienced by a proton is slightly altered by the magnetic states of non-equivalent protons on adjacent carbon atoms. This magnetic interaction splits the NMR absorption signal of a proton environment into a multiplet.
The n+1 Rule
The number of peaks in a split signal (a multiplet) is governed by the n+1 rule:
If a proton environment has n non-equivalent hydrogen atoms on adjacent carbon atoms, its NMR peak will be split into a multiplet containing n + 1 sub-peaks.
The relative intensities of the sub-peaks in a multiplet follow the coefficients of Pascal's Triangle:
- n = 0: Singlet (1 line), relative intensity 1
- n = 1: Doublet (2 lines), relative intensity 1:1
- n = 2: Triplet (3 lines), relative intensity 1:2:1
- n = 3: Quartet (4 lines), relative intensity 1:3:3:1
- n = 5: Sextet (6 lines), relative intensity 1:5:10:10:5:1
The interaction between the nuclear spins of adjacent, non-equivalent protons that leads to the splitting of NMR signals.
A group of closely spaced lines representing a single, split NMR signal, such as a doublet, triplet, or quartet.
Equivalent protons do not couple with each other! For example, in 1,2-dichloroethane (\(\text{Cl-CH}_2\text{-CH}_2\text{-Cl}\)), all 4 protons are in identical chemical environments due to symmetry. Even though they are on adjacent carbons, they do not split each other, producing a single, clean singlet.
Simulated High-Resolution Proton NMR Spectrum
Below is the simulated high-resolution proton NMR spectrum of 1,1,2-trichloroethane, \(\text{CHCl}_2\text{CH}_2\text{Cl}\). The molecule has two proton environments:
- A \(\text{-CH}_2\text{-}\) group adjacent to a \(\text{-CH-}\) group (n = 1 adjacent proton), split into a doublet (1:1 ratio) at \(\delta \approx 3.9\text{ ppm}\).
- A \(\text{-CH-}\) group adjacent to a \(\text{-CH}_2\text{-}\) group (n = 2 adjacent protons), split into a triplet (1:2:1 ratio) at \(\delta \approx 5.8\text{ ppm}\).
Carboxyl (\(\text{-COOH}\)) and hydroxyl (\(\text{-OH}\)) protons appear as singlets in high-resolution NMR and do not split neighbouring protons. This is due to rapid proton exchange with trace water or acidic impurities, which averages the coupling constant to zero. Always look for an unsplit singlet whenever an \(\text{-OH}\) or \(\text{-NH-}\) group is present.
Common Structural Fragments Identified by Splitting
Recognizing characteristic splitting patterns is the fastest way to identify common structural fragments in exam questions:
| Fragment | Expected Splitting | Integration Ratio | Explanation |
|---|---|---|---|
| Ethyl group (\(\text{-CH}_2\text{CH}_3\)) | Triplet & Quartet | 3 : 2 | The \(\text{CH}_3\) is split by 2 adjacent protons into a triplet. The \(\text{CH}_2\) is split by 3 adjacent protons into a quartet. |
| Isopropyl group (\(\text{-CH(CH}_3)_2\)) | Doublet & Septet | 6 : 1 | The 6 equivalent methyl protons are split by 1 adjacent proton into a doublet. The central \(\text{CH}\) is split by 6 adjacent protons into a septet. |
| Isolated Methyl (\(\text{-C-CH}_3\)) | Singlet | 3 | The methyl group has no adjacent protons, so it remains a singlet. |
Worked Examples of Splitting Analysis
Solution:
Identify the three proton environments and their adjacent neighbours:
- \(\text{-CH}_3\) of the ethyl group: It is adjacent to a \(\text{-CH}_2\text{-}\) group (2 protons). According to the n+1 rule, \(2 + 1 = 3\). It will be a triplet.
- \(\text{-CH}_2\text{-}\) of the ethyl group: It is adjacent to a \(\text{-CH}_3\) group (3 protons) on one side and a carbonyl carbon (no protons) on the other. According to the n+1 rule, \(3 + 1 = 4\). It will be a quartet.
- \(\text{-OCH}_3\) of the ester group: It is adjacent to an oxygen atom (no protons). According to the n+1 rule, \(0 + 1 = 1\). It will be a singlet.
Solution:
- Symmetry and Proton Count Analysis: The molecular formula \(\text{C}_6\text{H}_{10}\text{O}_4\) has 10 protons. The integration trace shows a ratio of 3 : 2. Since the total number of protons is 10, this ratio represents 6 protons and 4 protons (a 6:4 ratio). The presence of only two signals indicates high molecular symmetry.
- Ethyl groups (\(\text{-CH}_2\text{CH}_3\)): The triplet (integrating to 6H) and quartet (integrating to 4H) indicate two equivalent ethyl groups.
- The triplet is split by the adjacent \(\text{-CH}_2\text{-}\) protons (\(2 + 1 = 3\)).
- The quartet is split by the adjacent \(\text{-CH}_3\) protons (\(3 + 1 = 4\)).
- Chemical Shift: The quartet at 4.3 ppm is significantly downfield, indicating that the methylene group is attached directly to an electronegative ester oxygen: \(\text{-O-CH}_2\text{CH}_3\).
- Structure: Subtracting two equivalent \(\text{-OCH}_2\text{CH}_3\) groups from the molecular formula:
\(\text{C}_6\text{H}_{10}\text{O}_4 - 2 \times (\text{C}_2\text{H}_5\text{O}) = \text{C}_2\text{O}_2\). This leaves two carbonyl groups (\(\text{-C=O}\)). Connecting the pieces symmetrically gives diethyl oxalate (diethyl ethanedioate), with the structure \(\text{CH}_3\text{CH}_2\text{OOCCOOCH}_2\text{CH}_3\). - Peak Assignment:
- Triplet at \(\delta = 1.3\text{ ppm}\): 6 equivalent methyl protons (\(\text{-CH}_3\)) split by adjacent \(\text{-CH}_2\text{-}\) protons.
- Quartet at \(\delta = 4.3\text{ ppm}\): 4 equivalent methylene protons (\(\text{-CH}_2\text{-}\)) split by adjacent \(\text{-CH}_3\) protons, deshielded by the adjacent oxygen atom.
Get flashcards and quizzes in ChemEasy, or plan your revision with ChemPlan IB.