NMR Spectroscopy - Nuclear Magnetic Resonance Spectroscopy

What is NMR Spectroscopy?

NMR Spectroscopy is abbreviated as Nuclear Magnetic Resonance spectroscopy. It is a research technique that exploits the magnetic properties of certain atomic nuclei. The NMR spectroscopy determines the physical and chemical properties of atoms or molecules.

NMR Spectroscopy instrumentation

NMR Spectroscopy instrumentation

It relies on the phenomenon of nuclear magnetic resonance and provides detailed information about the structure, dynamics, reaction state, and chemical environment of molecules.

NMR Spectroscopy Principle:

All nuclei are electrically charged and and many have spin. Transfer of energy is possible from base energy to higher energy level when an external magnetic field is applied. The transfer of energy occurs at a wavelength which coincides the radio frequency. Also, energy is emitted at the same frequency when the spin comes back to its base level. Therefore, by measuring the signal which matches this transfer the processing of NMR spectrum for the concerned nucleus is yield.

NMR Spectroscopy working:

  • Place the sample in a magnetic field.
  • Excite the nuclei sample into nuclear magnetic resonance with the help of radio waves to produce NMR signals.
  • These NMR signals are detected with sensitive radio receivers.
  • The resonance frequency of an atom in a molecule is changed by the intramolecular magnetic filed surrounding it.
  • This gives details of a molecule’s individual functional groups and its electronic structure.
  • Nuclear magnetic resonance spectroscopy is a conclusive method of identifying monomolecular organic compounds.
  • This method provides details of the reaction state, structure, chemical environment and dynamics of a molecule.

NMR Spectroscopy Instrumentation:

This instrument consists of nine major parts. They are discussed below:

1. Sample holder: It is a glass tube which is 8.5 cm long and 0.3 cm diameter.

2. Magnetic coils: Magnetic coil generates magnetic field whenever current flows through it

3. Permanent magnet: It helps in providing homogenous magnetic field at 60 – 100 MHZ

4. Sweep generator: Modifies the strength of magnetic field which is already applied.

5. Radio frequency transmitter: It produces a powerful but short pulse of radio wave.

6. Radio frequency: It helps in detecting receiver radio frequencies.

7. RF detector: It helps in determining unabsorbed radio frequencies.

8. Recorder: It records the NMR signals which are received by the RF detector.

9. Readout system: A computer that records the data.

NMR Spectroscopy Applications

NMR spectroscopy is a Spectroscopy technique used by chemists and biochemists to investigate the properties of organic molecules, although it is applicable to any kind of sample that contains nuclei possessing spin. For example, the NMR can quantitatively analyze mixtures containing known compounds. NMR can either be used to match against spectral libraries or to infer the basic structure directly for unknown compounds.

Once the basic structure is known, NMR can be used to determine molecular conformation in solutions as well as in studying physical properties at the molecular level such as conformational exchange, phase changes, solubility, and diffusion.

NMR Spectroscopy Techniques

  • Resonant frequency:

    It refers to the energy of the absorption, and the intensity of the signal that is proportional to the strength of the magnetic field. NMR active nuclei absorb electromagnetic radiation at a frequency characteristic of the isotope when placed in a magnetic field.

  • Acquisition of spectra:

    Upon excitation of the sample with radio frequency pulse, a nuclear magnetic resonance response is obtained. It is a very weak signal and requires sensitive radio receivers to pick up.

  • Chemical shift:

    A spinning charge generates a magnetic field that results in a magnetic moment proportional to the spin. In the presence of an external magnetic field, two spin states exist; one spin up and one spin down, where one aligns with the magnetic field and the other opposes it.

    Read more:

  • Nomenclature of functional groups
  • Classification of organic compounds

Practise This Question

Chemical equilibrium is dynamic in nature because