The Nuclear Magnetic Resonance (NMR) Spectroscopy is a powerful analytical tool used in various fields, including chemistry, biochemistry, and pharmacology. It provides detailed information about the molecular structure, dynamics, and environment of molecules. However, mastering NMR spectroscopy requires a thorough understanding of its principles, techniques, and interpretation of spectra. To help students and professionals improve their skills, we have compiled 12 challenging NMR practice problems, along with detailed explanations and solutions.
Key Points
- Understanding the basic principles of NMR spectroscopy, including the nuclear spin, magnetic field, and radiofrequency pulses.
- Interpreting 1D and 2D NMR spectra, including 1H, 13C, and HSQC spectra.
- Assigning NMR signals to specific atoms or groups in a molecule.
- Using NMR spectroscopy to determine molecular structure, including functional groups and stereochemistry.
- Applying NMR spectroscopy in various fields, including chemistry, biochemistry, and pharmacology.
Problem 1: Interpretation of 1H NMR Spectrum
The 1H NMR spectrum of a molecule shows three signals at 1.2 ppm, 3.4 ppm, and 7.1 ppm, with integrations of 3:2:1, respectively. The molecular formula of the compound is C6H10O. Assign the NMR signals to specific atoms or groups in the molecule and determine the molecular structure.
Solution to Problem 1
The signal at 1.2 ppm can be assigned to the methyl group (CH3) in the molecule, while the signal at 3.4 ppm can be assigned to the methylene group (CH2) adjacent to the methyl group. The signal at 7.1 ppm can be assigned to the aromatic proton (H) in the molecule. Based on the integration values and the molecular formula, the molecular structure can be determined as a substituted benzene ring with a methyl and a methylene group.
Problem 2: Assignment of 13C NMR Signals
The 13C NMR spectrum of a molecule shows five signals at 20.1 ppm, 40.5 ppm, 60.8 ppm, 120.3 ppm, and 150.2 ppm. The molecular formula of the compound is C8H12O2. Assign the 13C NMR signals to specific atoms or groups in the molecule and determine the molecular structure.
Solution to Problem 2
The signal at 20.1 ppm can be assigned to the methyl group (CH3) in the molecule, while the signal at 40.5 ppm can be assigned to the methylene group (CH2) adjacent to the methyl group. The signal at 60.8 ppm can be assigned to the methine group (CH) in the molecule, while the signals at 120.3 ppm and 150.2 ppm can be assigned to the aromatic carbons © in the molecule. Based on the 13C NMR signals and the molecular formula, the molecular structure can be determined as a substituted benzene ring with a methyl, a methylene, and a methine group.
Problem 3: Interpretation of HSQC Spectrum
The HSQC spectrum of a molecule shows three cross-peaks at (1.2 ppm, 20.1 ppm), (3.4 ppm, 40.5 ppm), and (7.1 ppm, 120.3 ppm). The molecular formula of the compound is C6H10O. Assign the cross-peaks to specific atoms or groups in the molecule and determine the molecular structure.
Solution to Problem 3
The cross-peak at (1.2 ppm, 20.1 ppm) can be assigned to the methyl group (CH3) in the molecule, while the cross-peak at (3.4 ppm, 40.5 ppm) can be assigned to the methylene group (CH2) adjacent to the methyl group. The cross-peak at (7.1 ppm, 120.3 ppm) can be assigned to the aromatic proton (H) and carbon © in the molecule. Based on the HSQC cross-peaks and the molecular formula, the molecular structure can be determined as a substituted benzene ring with a methyl and a methylene group.
| Molecule | 1H NMR Signals | 13C NMR Signals | HSQC Cross-Peaks |
|---|---|---|---|
| C6H10O | 1.2 ppm, 3.4 ppm, 7.1 ppm | 20.1 ppm, 40.5 ppm, 60.8 ppm, 120.3 ppm, 150.2 ppm | (1.2 ppm, 20.1 ppm), (3.4 ppm, 40.5 ppm), (7.1 ppm, 120.3 ppm) |
Problem 4: Determination of Molecular Structure
The molecular formula of a compound is C10H14O2. The 1H NMR spectrum shows four signals at 1.1 ppm, 2.3 ppm, 3.5 ppm, and 7.2 ppm, with integrations of 3:2:2:1, respectively. The 13C NMR spectrum shows six signals at 20.5 ppm, 30.8 ppm, 40.9 ppm, 60.1 ppm, 120.5 ppm, and 150.8 ppm. Determine the molecular structure of the compound.
Solution to Problem 4
Based on the 1H NMR and 13C NMR spectra, the molecular structure can be determined as a substituted benzene ring with a methyl, a methylene, and a methine group. The signals at 1.1 ppm and 20.5 ppm can be assigned to the methyl group (CH3), while the signals at 2.3 ppm and 30.8 ppm can be assigned to the methylene group (CH2) adjacent to the methyl group. The signals at 3.5 ppm and 40.9 ppm can be assigned to the methine group (CH), while the signals at 7.2 ppm and 120.5 ppm can be assigned to the aromatic proton (H) and carbon ©, respectively.
Problem 5: Assignment of NMR Signals
The 1H NMR spectrum of a molecule shows five signals at 1.2 ppm, 2.5 ppm, 3.7 ppm, 4.9 ppm, and 7.3 ppm, with integrations of 3:2:2:1:1, respectively. The 13C NMR spectrum shows seven signals at 20.2 ppm, 30.9 ppm, 40.6 ppm, 60.9 ppm, 80.1 ppm, 120.6 ppm, and 150.9 ppm. Assign the NMR signals to specific atoms or groups in the molecule and determine the molecular structure.
Solution to Problem 5
Based on the 1H NMR and 13C NMR spectra, the NMR signals can be assigned to specific atoms or groups in the molecule. The signals at 1.2 ppm and 20.2 ppm can be assigned to the methyl group (CH3), while the signals at 2.5 ppm and 30.9 ppm can be assigned to the methylene group (CH2) adjacent to the methyl group. The signals at 3.7 ppm and 40.6 ppm can be assigned to the methine group (CH), while the signals at 4.9 ppm and 80.1 ppm can be assigned to the oxygenated methine group (CH-O). The signals at 7.3 ppm and 120.6 ppm can be assigned to the aromatic proton (H) and carbon ©, respectively.
Problem 6: Interpretation of 2D NMR Spectra
The COSY spectrum of a molecule shows three cross-peaks at (1.2 ppm, 2.5 ppm), (2.5 ppm, 3.7 ppm), and (3.7 ppm, 4.9 ppm). The HSQC spectrum shows five cross-peaks at (1.2 ppm, 20.2 ppm), (2.5 ppm, 30.9 ppm), (3.7 ppm, 40.6 ppm), (4.9 ppm, 80.1 ppm), and (7.3 ppm, 120.6 ppm). Interpret the 2D NMR spectra and determine the molecular structure.
Solution to Problem 6
Based on the COSY and HSQC spectra, the molecular structure can be determined as a