Nuclear Magnetic Resonance Spectroscopy (NMR)

Nuclear Magnetic Resonance Spectroscopy (NMR)

Nuclear Magnetic Resonance Spectroscopy (NMR) is used for the compositional analysis of organic materials. NMR utilizes the fact that certain atomic nuclei have magnetic moments and can interact with externally applied magnetic fields. These typically have odd mass numbers, such as  ¹H, ¹³C, ¹⁹F, ²⁹Si, ¹⁵N. When subjected to a strong constant external magnetic field, these magnetic moments get aligned with (parallel) or opposed to (anti-parallel) to the applied field. The parallel orientation represents a lower energy state relative to the anti-parallel alignment. If the nuclei are now subjected to electromagnetic radiation in the Radiofrequency (RF) range, the lower energy nuclei absorb energy and at a resonant frequency, their alignment flips-over (spin-flip) to the higher energy state. The resonant frequency depends on the type of nuclei and bonding environment as well as the applied magnetic field. Hence NMR provides information on composition and structure of molecules in the sample. In NMR spectrometers, organic compounds are placed in magnetic fields ranging from about 1.4 to 18.0 tesla (T).

The NMR spectrometry procedure involves placing a sample in a strong magnetic field and then exciting the sample using pulsed Radiofrequency waves of the appropriate frequency range. At resonant frequencies, the spin-flip of various nuclei is picked up by a detector. The respective resonant frequencies and absorbance intensities are recorded as peaks in the NMR spectrum. It must be noted that NMR detects only the nuclei with magnetic moments. For example, magnetic isotopes of elements such as ¹H, ¹³C, ¹⁹F, ²⁹Si, ¹⁵N, yield high-resolution NMR spectra at their resonant frequencies. Hence, in a typical organic compound having Carbon backbone and covalent bonds with Hydrogen, NMR targets only the magnetic ¹³C nuclei and the surrounding Hydrogen atoms (Protons), to infer the structure. This process requires computer analysis, using a reference database. Proton NMR is an important form of NMR for structural analysis, since Hydrogen atoms absorb energy of different wavelengths depending on their bonding environment.

NMR can be applied to substances in liquid solution or in solid state. When NMR spectroscopy is performed on solids, it is termed Solid State NMR. Polymers and plastics are examples where Solid State NMR has been used. In such cases, magnetized Protons distort the ¹³C signal, which is resolved by heteronuclear dipolar decoupling (DD). In the case of liquid solutions, Deuterated solvents are used, in which all Protons have been replaced by Deuterium. This avoids signal distortion by magnetic Protons in the solvent.

Common Uses of Nuclear Magnetic Resonance Spectroscopy (NMR) 

• In research and Quality control for analysis of Organic compounds
• Analysis of composition and structure of coal
• Study of Protein structures
• Compositional analysis of polymers and plastics

Advantages of Nuclear Magnetic Resonance Spectroscopy (NMR) 

• NMR analyses the sample in solid or liquid state
• Does not destroy the sample enabling other tests to be conducted on the same sample
• Analyte molecules are not fragmented.

 Limitations of Nuclear Magnetic Resonance Spectroscopy (NMR) 

• Sensitivity of NMR is less compared to Mass Spectrometry
• Interference from Protons, electrons and chemical shift of Spectra need experimental adjustments

Industrial Applications of Nuclear Magnetic Resonance Spectroscopy (NMR) 

• Organic Chemistry research
• Pharmaceutical research and quality control
• Diagnostic medicine
• Polymers research and quality control
• Environmental analysis