Proton Nuclear Magnetic Resonance Spectroscopy (¹H NMR)
How Does ¹H NMR Work?
- At the core of ¹H NMR is the interaction between hydrogen nuclei (protons) and a magnetic field.
- When placed in a strong magnetic field, these protons align either with or against the field, creating two distinct energy states: a lower-energy state (aligned with the field) and a higher-energy state (opposite to the field).
- When you apply radio waves of just the right energy, the protons absorb this energy and "flip" from the lower-energy state to the higher-energy state.
- This absorption occurs at specific frequencies depending on the chemical environment of each proton.
- The resulting NMR spectrum displays these frequencies as signals, providing a fingerprint of the molecule’s structure.
Key Features of ¹H NMR Spectra
To decode the structure of a molecule, focus on three main features of the ¹H NMR spectrum: the number of signals, their chemical shifts, and the integration of each signal.
Number of Signals
- The number of signals in a ¹H NMR spectrum corresponds to the number of unique hydrogen environments in the molecule.
- Hydrogens in identical environments produce a single signal, while hydrogens in different environments produce separate signals.
Example
- In methane (CH₄), all four hydrogens are in the same environment, resulting in one signal.
- In ethanol (CH₃CH₂OH), the hydrogens are in three distinct environments: the CH₃ group, the CH₂ group, and the OH group. This produces three signals.
Example
- Consider chloroethane (CH₃CH₂Cl).
- Its ¹H NMR spectrum shows two signals because the CH₃ group and CH₂ group represent two distinct hydrogen environments.
Chemical Shift (δ)
- The chemical shift tells you about the type of chemical environment surrounding a hydrogen atom.
- It’s measured in parts per million (ppm) relative to a reference compound, typically tetramethylsilane (TMS), which is assigned a chemical shift of 0 ppm.
- The chemical shift depends on the electron density around the hydrogen:
- Electron-withdrawing groups (such as oxygen or chlorine) reduce the electron density around the hydrogen, deshielding it.
- This shifts the signal to a higher δ value (downfield).
- Electron-donating groups (such as alkyl groups) increase the electron density around the hydrogen, shielding it.
- This shifts the signal to a lower δ value (upfield).
- Electron-withdrawing groups (such as oxygen or chlorine) reduce the electron density around the hydrogen, deshielding it.
Hint
Here are some common chemical shift ranges:
- Alkyl hydrogens (CH₃, CH₂, CH): 0.9–2.5 ppm
- Hydrogens on carbons attached to electronegative atoms (e.g., CH₂Cl): 2.5–4.5 ppm
- Aromatic hydrogens (benzene ring): 6.0–8.0 ppm
- Aldehyde hydrogens (CHO): 9.0–10.0 ppm
- Carboxylic acid hydrogens (COOH): 10.0–13.0 ppm
Tip
You can use the chemical shift values provided in the IB Chemistry Data Booklet to identify the hydrogen environments in your molecule.
Integration (Relative Area Under Signals)
- The area under each signal in a ¹H NMR spectrum is proportional to the number of hydrogens in that environment.
- This is often represented by an integration trace, which helps you determine the relative number of hydrogens contributing to each signal.
Example
- A signal with an integration of 3 corresponds to three hydrogens (e.g., a CH₃ group).
- A signal with an integration of 2 corresponds to two hydrogens (e.g., a CH₂ group).
Example
In the ¹H NMR spectrum of ethanol (CH₃CH₂OH), the integration ratio is 3:2:1, corresponding to the CH₃, CH₂, and OH groups, respectively.
Applications of ¹H NMR: Distinguishing Isomers
One of the most valuable uses of ¹H NMR is distinguishing between structural isomers, molecules with the same molecular formula but different arrangements of atoms.
Example
Propanol Isomers
- Propan-1-ol (CH₃CH₂CH₂OH):
- Three signals: one for the CH₃ group, one for the CH₂ group, and one for the OH group.
- Integration ratio: 3:2:1.
- Propan-2-ol (CH₃CHOHCH₃):
- Two signals: one for the CH₃ groups (which are in identical environments) and one for the OH group.
- Integration ratio: 6:1.
Self review
How would the ¹H NMR spectrum of propan-2-ol differ from that of propanal (CH₃CH₂CHO)?
Common Mistake
- Students often overlook symmetry in molecules, which can reduce the number of signals.
- For example, 2-bromopropane (CH₃CHBrCH₃) has only two signals, not three, because the two CH₃ groups are in identical environments.
- Another frequent error is misinterpreting the integration trace.
- Remember, the integration provides the relative number of hydrogens, not their absolute number.
