Nuclear Magnetic Resonance (NMR)

Nuclear Magnetic Resonance (NMR) spectroscopy is an analytical technique used to determine the molecular structure and chemical composition of a sample. It works by analyzing the interaction of spinning nuclei in a strong magnetic field. In NMR spectroscopy, a stationary external magnetic field causes certain nuclei in a molecule to absorb selective radiofrequencies. The energy absorbed induces a transition in nuclear spins, which is observed on an NMR spectrum.  

Principles of NMR Spectroscopy

Nuclear spin is related to the composition of an element’s nucleus. Nuclei containing an even number of both protons and neutrons have 0 nuclear spin and cannot undergo NMR (e.g., ⁴He,¹²C,¹⁶O). Nuclei with an odd number of protons and/or neutrons display nuclear spin and can experience NMR (e.g., ¹H, ²H, ¹⁴N, ¹⁷O). These nuclei behave like tiny spinning magnets and can interact with an external magnetic field. Spinning nuclei also create their own magnetic field that can interact with other nuclei having spin.

A NMR instrument measures the interaction of nuclear spin states under the influence of a powerful magnetic field. The magnetic field causes nuclei to precess (rotate) like a spinning top. A precessing nucleus selectively absorbs energy from radiofrequency waves when the frequency of the precessing nuclei matches the low external frequency of radiofrequency waves interacting with it. When this absorption occurs, the precessing nucleus and radiofrequency waves are said to be in ‘resonance’, hence the term nuclear magnetic resonance. Resonance can be produced by tuning the frequency of the nuclei to the fixed frequency of radio waves, or by tuning the frequency of radio waves to that of the nuclei.

During NMR, an applied magnetic field excites nuclei having different magnetic moments across various energy levels. After absorbing a characteristic radiofrequency, the excited-state nuclei return to lower energy states by transferring energy to the surrounding environment. When energy is transferred to other atoms or the solvent, the relaxation process is called ‘spin-lattice relaxation’. If energy is transferred to neighboring nuclei at the same energy level, the process is termed ‘spin-spin relaxation’. These two relaxation processes are characterized by time constants: spin-lattice relaxation time (T₁) and spin-spin relaxation time (T₂), which are responsible for the resulting NMR spectrum.

Characteristics of a NMR Spectrum

A NMR spectrum is a plot of the applied radiofrequency against absorption. The position on the plot at which the nuclei absorbs is called the chemical shift. Chemical shift is affected by the electron density around the nucleus. If a nucleus is surrounded by a high electron density, the nucleus is shielded from the external magnetic field, which shifts the signals upfield on the NMR spectrum. If a nucleus is surrounded by an electronegative atom, it removes electron density around the nucleus and causes a ‘deshielding’ effect. This shifts the signal ‘downfield’ on an NMR spectrum. The spin of neighboring nuclei also affects the signals seen on an NMR spectrum and may cause splitting of the NMR signal, known as ‘spin-spin coupling’.