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.
- 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.
- 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.
