Nuclear magnetic resonance
High-resolution nuclear magnetic resonance spectroscopy is widely used to determine the structure of organic molecules in solution and study molecular physics and crystals as well as non-crystalline materials.
[2] It is a key feature of NMR that the resonance frequency of nuclei in a particular sample substance is usually directly proportional to the strength of the applied magnetic field.
[5][6] Russell H. Varian filed the "Method and means for correlating nuclear properties of atoms and magnetic fields", U.S. patent 2,561,490 on October 21, 1948 and was accepted on July 24, 1951.
[citation needed] Originally as a research tool it was limited primarily to dynamic nuclear polarization, by the work of Anatole Abragam and Albert Overhauser, and to condensed matter physics, where it produced one of the first demonstrations of the validity of the BCS theory of superconductivity by the observation by Charles Slichter of the Hebel-Slichter effect.
Unless the local symmetry of such molecular orbitals is very high (leading to "isotropic" shift), the shielding effect will depend on the orientation of the molecule with respect to the external field (B0).
In 1949, Suryan first suggested that the interaction between a radiofrequency coil and a sample's bulk magnetization could explain why experimental observations of relaxation times differed from theoretical predictions.
[14] Building on this idea, Bloembergen and Pound further developed Suryan's hypothesis by mathematically integrating the Maxwell–Bloch equations, a process through which they introduced the concept of "radiation damping.
RD occurs when transverse bulk magnetization from the sample, following a radio frequency pulse, induces an electromagnetic field (emf) in the receiver coil of the NMR spectrometer.
This generates an oscillating current and a non-linear induced transverse magnetic field which returns the spin system to equilibrium faster than other mechanisms of relaxation.
[16] Hardware modifications including RF feed-circuit [20] and Q-factor switches [21] reduce the feedback loop between the sample magnetization and the electromagnetic field induced by the coil and function successfully.
Overall, understanding and managing radiation damping is crucial for obtaining high-quality NMR data, especially in modern high-field spectrometers where the effects can be significant due to the increased sensitivity and resolution.
This process is also called T1, "spin-lattice" or "longitudinal magnetic" relaxation, where T1 refers to the mean time for an individual nucleus to return to its thermal equilibrium state of the spins.
[citation needed] In the corresponding FT-NMR spectrum—meaning the Fourier transform of the free induction decay— the width of the NMR signal in frequency units is inversely related to the T2* time.
NMR spectroscopy can provide detailed and quantitative information on the functional groups, topology, dynamics and three-dimensional structure of molecules in solution and the solid state.
[citation needed] Additional structural and chemical information may be obtained by performing double-quantum NMR experiments for pairs of spins or quadrupolar nuclei such as 2H.
[27][clarification needed] CW spectroscopy is inefficient in comparison with Fourier analysis techniques (see below) since it probes the NMR response at individual frequencies or field strengths in succession.
Early attempts to acquire the NMR spectrum more efficiently than simple CW methods involved illuminating the target simultaneously with more than one frequency.
In two-dimensional nuclear magnetic resonance spectroscopy (2D-NMR), there will be one systematically varied time period in the sequence of pulses, which will modulate the intensity or phase of the detected signals.
[30] Multi-dimensional FT NMR experiments were then further developed into powerful methodologies for studying molecules in solution, in particular for the determination of the structure of biopolymers such as proteins or even small nucleic acids.
[31] In 2002 Kurt Wüthrich shared the Nobel Prize in Chemistry (with John Bennett Fenn and Koichi Tanaka) for his work with protein FT NMR in solution.
Then, Jake Schaefer and Ed Stejskal demonstrated the powerful use of cross polarization under MAS conditions (CP-MAS) and proton decoupling, which is now routinely employed to measure high resolution spectra of low-abundance and low-sensitivity nuclei, such as carbon-13, silicon-29, or nitrogen-15, in solids.
The technique is also used to measure the ratio between water and fat in foods, monitor the flow of corrosive fluids in pipes, or to study molecular structures such as catalysts.
Biochemical information can also be obtained from living tissue (e.g. human brain tumors) with the technique known as in vivo magnetic resonance spectroscopy or chemical shift NMR microscopy.
As one of the two major spectroscopic techniques used in metabolomics, NMR is used to generate metabolic fingerprints from biological fluids to obtain information about disease states or toxic insults.
Because the nuclear magnetic resonance timescale is rather slow, compared to other spectroscopic methods, changing the temperature of a T2* experiment can also give information about fast reactions, such as the Cope rearrangement or about structural dynamics, such as ring-flipping in cyclohexane.
[35] Nuclear Magnetic Resonance (NMR) is a powerful analytical tool for investigating the local structure and ion dynamics in battery materials.
Typically this standard will have a high molecular weight to facilitate accurate weighing, but relatively few protons so as to give a clear peak for later integration e.g. 1,2,4,5-tetrachloro-3-nitrobenzene.
For example, various expensive biological samples, such as nucleic acids, including RNA and DNA, or proteins, can be studied using nuclear magnetic resonance for weeks or months before using destructive biochemical experiments.
Time-domain NMR (TD-NMR) spectrometers operating at low field (2–20 MHz for 1H) yield free induction decay data that can be used to determine absolute hydrogen content values, rheological information, and component composition.
Under those circumstances the observed spectra are no-longer dictated by chemical shifts but primarily by J-coupling interactions which are independent of the external magnetic field.
