What is NMR Spectroscopy?

NMR spectroscopy means Nuclear magnetic resonance spectroscopy. NMR is the study of molecules by recording the interactions of radiofrequency (Rf) electromagnetic radiation with the nuclei of molecules placed in a strong magnetic field.

Zeeman first observed the strange behavior of some nuclei when subjected to a strong magnetic field in the late nineteenth century, but the so-called “Zeeman effect” was only put to practical use in the 1950s when NMR spectrometers became commercially available.

Basis of NMR Spectroscopy

Nuclear magnetic resonance (NMR) was first detected experimentally in late 1945, with approximately concurrent work by groups Felix Bloch, Stanford University, and Edward Purcell, Harvard University. The first NMR spectrum was first published in the January 1946 issue of Physical Review. Bloch and Purcell were jointly awarded the 1952 Nobel Prize in Physics for their research on nuclear magnetic resonance spectroscopy.

Nuclear magnetic resonance (NMR) spectroscopy is an important analytical tool for organic chemists. Research in the biological laboratory has been greatly improved with the help of NMR. Not only can it provide information about the structure of a molecule, but it can also determine the content and purity of a sample. Proton (1H) NMR is one of the most widely used NMR methods by organic chemists. The protons in the molecule will behave differently depending on the surrounding chemical environment, making it possible to elucidate their structure.

NMR spectroscopy principle

According to NMR theory, many nuclei have spins, and all nuclei are electrically charged. The transfer of energy from the ground state to the higher energy level is achieved when an external magnetic field is supplied.

  • All nuclei are electrically charged and many have spins.
  • Transfer of energy from ground state to higher energy levels is possible when an external magnetic field is applied.
  • The transfer of energy occurs at a wavelength which coincides with the radio frequency.
  • Furthermore, the energy is emitted at the same frequency when the spin returns to its ground state.
  • Therefore, processing the NMR spectrum for the corresponding nucleus yields a measurement of the signal matching this transfer.

NMR Spectroscopy Working

  • Place the sample in the magnetic field.
  • Excite the nuclear sample in 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 field surrounding it.
  • It describes the individual functional groups of a molecule and its electronic structure.
  • Nuclear magnetic resonance spectroscopy is a conclusive method of identifying monomolecular organic compounds.
  • This method provides a description of the reaction conditions, structure, chemical environment and dynamics of a molecule.

Chemical Shift in NMR Spectroscopy

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

The chemical shift is characterized as the difference between the resonant frequency of the spinning proton and the signal of the reference molecule. Nuclear magnetic resonance chemical shift is one of the most important properties usable for molecular structure determination. There are also various nuclei that can be detected by NMR spectroscopy, 1H (proton), 13C (carbon 13), 15N (nitrogen 15), 19F (fluorine 19), and many others. 1H and 13C are the most widely used. The definition of 1H as it is very descriptive of the spectroscopy of NMR. Both the nuts have a good charge and they constantly move like a cloud. Through mechanics, we learn that a charge in motion produces a magnetic field. In NMR, when we get radio frequency (Rf) radiation reaching the nucleus, it causes the nucleus and its magnetic field to spin (or it pulsates the nuclear magnet, thus the term NMR).

NMR Spectroscopy Instrumentation

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

Sample holder – It is a glass tube which is 8.5 cm long and 0.3 cm in diameter.

Magnetic coil – A magnetic coil produces a magnetic field whenever current flows through it.

Permanent Magnet – This helps provide a uniform magnetic field at 60 – 100 MHz

Sweep generator – modifies the strength of an already applied magnetic field.

Radiofrequency Transmitter – This produces a powerful but short pulse of radio waves.

Radiofrequency – This receiver helps to detect radio frequency.

RF Detector – It helps to determine the unabsorbed radio frequencies.

Recorder – It records the NMR signals that are received by the RF detector.

Readout System – A computer that records data.

NMR Spectroscopy Techniques

  1. Resonant Frequency

It relates to the signal’s energy of absorption and intensity, which are directly proportional to the strength of the magnetic field. When placed in a magnetic field, NMR active nuclei absorb electromagnetic radiation at a frequency specific to the isotope.

  1. Acquisition of Spectra

Upon exciting the sample with a radiofrequency pulse, a nuclear magnetic resonance response is obtained. This is a very weak signal and requires a sensitive radio receiver to pick it up.

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 type of sample that has a spin-containing nucleus.

For example, NMR can quantitatively analyze a mixture containing known compounds. NMR can be used either to match against spectral libraries or to directly predict native structures for unknown compounds.

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

Limitations of the NMR Spectroscopy and Ways to Overcome

One of the key techniques used in organic chemistry is NMR spectroscopy. However, there are some limitations of this technique which may come in your way while using it. Below you’ll find the limitations of NMR spectroscopy and what you can do to overcome them.

  1. Sensitivity

NMR spectroscopy is a flexible method, however because of its poor sensitivity, it frequently fails. This is a major disadvantage when probing metabolomics or other complex reactions.The sensitivity issue in NMR spectroscopy can be solved in a number of ways. For example, you can use advanced hardware to improve sensitivity during NMR experiments.

  1. Magnetic Field Drift

Drift in the magnetic field has a huge effect on NMR spectroscopy, leading to distorted lines and spectral leakage. This can make interpretation of results very difficult. In such cases, you can make some corrections to the magnetic field when recording the measurements. For this, deuterium is detected in NMR solvents, which enables the drift of the magnetic field to be tracked. Next, adjustments are made to the core field using electromagnets at room temperature.

Read Also