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.
Note
Think of each proton as a tiny spinning magnet.
- 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.
Analogy
Think of the protons as musical notes in a symphony. Each note (or signal) is unique, depending on the environment of the hydrogen, and together, they create a recognizable "melody" that represents the molecule.
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.
1. 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.
2.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).
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.
3.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 Mistakes in ¹H NMR Analysis
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.
Common Mistake
Another frequent error is misinterpreting the integration trace. Remember, the integration provides therelativenumber of hydrogens, not their absolute number.
Reflection
Theory of Knowledge
How does the use of ¹H NMR in drug development illustrate the interconnectedness of science and society? What ethical considerations arise when using such techniques to create new medicines?
