Nuclear Magnetic Resonance (NMR) spectroscopy is an analytical method employed for identifying the molecular structure and chemical composition of a sample. It operates by examining how spinning nuclei interact within a powerful magnetic field. When subjected to an external magnetic field, specific nuclei in a molecule absorb particular radiofrequencies during NMR spectroscopy. The absorbed energy causes a transition in nuclear spins, which is then detected and observed on an NMR spectrum.

APPLICATIONS OF NMR SPECTROSCOPY

NMR spectroscopy is a non-destructive and non-invasive method utilized for determining molecular structure and dynamics. It finds diverse applications across various research areas and industries:

– In biology, NMR is used to study macromolecules like proteins, lipids, and nucleic acids, employing nuclei such as 13C, 1H, 15N, 31P, 23Na, and 19F. These biologically relevant NMR-active nuclei help understand biochemical pathways related to amino acid, lipid, and carbohydrate metabolism.

– In chemistry, NMR is widely applied for qualitative and quantitative analysis, monitoring reactions, identifying structures, and assessing purity.

– In polymer science, it aids in analyzing monomer ratio, molecular weight, tacticity, sequencing, chain length, branching, and determining end groups.

– In the pharmaceutical industry, NMR is used to determine the purity and quantity of active ingredients, excipients, and impurities in pharmaceutical products.

– In the petroleum industry, NMR helps assess hydrocarbons in raw petroleum and its derivatives.

– In medicine, NMR is applied in magnetic resonance imaging (MRI) for soft tissue analysis, enabling the identification of damaged or diseased tissues.

PRINCIPLES OF NMR SPECTROSCOPY

Nuclear spin is linked to an element’s nucleus composition. Nuclei with an even number of protons and neutrons have 0 nuclear spin and cannot undergo NMR (e.g., 4He, 12C, 16O). However, nuclei with an odd number of protons and/or neutrons exhibit nuclear spin and can experience NMR (e.g., 1H, 2H, 14N, 17O). These spinning nuclei act like miniature magnets and interact with an external magnetic field. Moreover, they create their own magnetic field, influencing other nuclei with spin.

In an NMR instrument, nuclear spin states’ interaction is measured under the influence of a strong magnetic field. This magnetic field causes nuclei to precess, similar to a spinning top. A precessing nucleus selectively absorbs energy from radiofrequency waves when its frequency matches the low external frequency of the interacting radiofrequency waves. This absorption is called ‘resonance’, hence the term nuclear magnetic resonance. Resonance can occur by tuning the frequency of the nuclei to the fixed frequency of radio waves or vice versa.

During NMR, the applied magnetic field excites nuclei with different magnetic moments across various energy levels. After absorbing characteristic radiofrequency, the excited nuclei return to lower energy states by transferring energy to the surroundings. This relaxation process is known as ‘spin-lattice relaxation’ when energy is transferred to other atoms or the solvent and ‘spin-spin relaxation’ when energy is transferred to neighboring nuclei at the same energy level. These relaxation processes are characterized by time constants, known as spin-lattice relaxation time (T1) and spin-spin relaxation time (T2), which determine the resulting NMR spectrum.

CHARACTERISTICS OF A NMR SPECTRUM

An NMR spectrum represents a graph of applied radiofrequency versus absorption, where the position at which nuclei absorb is known as the chemical shift. The chemical shift is influenced by the electron density surrounding the nucleus. When a nucleus is surrounded by high electron density, it becomes shielded from the external magnetic field, causing signals to shift upfield on the NMR spectrum. Conversely, if a nucleus is surrounded by an electronegative atom, it removes electron density from the nucleus, leading to a ‘deshielding’ effect and shifting the signal downfield on the NMR spectrum. Additionally, the spin of neighboring nuclei affects the signals observed on an NMR spectrum and may cause splitting of the NMR signal, known as ‘spin-spin coupling’.