Solid-state nuclear magnetic resonance (ssNMR) is a spectroscopy technique used to characterize atomic-level structure and dynamics in solid materials. ssNMR spectra are broader due to nuclear spin interactions which can be categorized as dipolar coupling, chemical shielding, quadrupolar interactions, and j-coupling. These interactions directly affect the lines shapes of experimental ssNMR spectra which can be seen in powder and dipolar patterns. There are many essential solid-state techniques alongside advanced ssNMR techniques that may be applied to elucidate the fundamental aspects of solid materials. ssNMR is often combined with magic angle spinning (MAS) to remove anisotropic interactions and improve the sensitivity of the technique. The applications of ssNMR further extend to biology and medicine.
The resonance frequency of a nuclear spin depends on the strength of the magnetic field at the nucleus, which can be modified by isotropic (e.g. chemical shift, isotropic J-coupling) and anisotropic interactions (e.g. chemical shift anisotropy, dipolar interactions). In a classical liquid-state NMR experiment, molecular tumbling coming from Brownian motion averages anisotropic interactions to zero and they are therefore not reflected in the NMR spectrum. However, in media with no or little mobility (e.g. crystalline powders, glasses, large membrane vesicles, molecular aggregates), anisotropic local fields or interactions have substantial influence on the behaviour of nuclear spins, which results in the line broadening of the NMR spectra.
Chemical shielding is a local property of each nuclear site in a molecule or compound, and is proportional to the applied external magnetic field. The external magnetic field induces currents of the electrons in molecular orbitals. These induced currents create local magnetic fields that lead to characteristic changes in resonance frequency. These changes can be predicted from molecular structure using empirical rules or quantum-chemical calculations.
In general, the chemical shielding is anisotropic because of the anisotropic distribution of molecular orbitals around the nuclear sites. Under sufficiently fast magic angle spinning, or under the effect of molecular tumbling in solution-state NMR, the anisotropic dependence of the chemical shielding is time-averaged to zero, leaving only the isotropic chemical shift.
Nuclear spins exhibit a magnetic dipole moment, which generates a magnetic field that interacts with the dipole moments of other nuclei (dipolar coupling). The magnitude of the interaction is dependent on the gyromagnetic ratio of the spin species, the internuclear distance r, and the orientation, with respect to the external magnetic field B, of the vector connecting the two nuclear spins (see figure). The maximum dipolar coupling is given by the dipolar coupling constant d,
where γ1 and γ2 are the gyromagnetic ratios of the nuclei, is the reduced Planck constant, and is the vacuum permeability. In a strong magnetic field, the dipolar coupling depends on the angle θ between the internuclear vector and the external magnetic field B (figure) according to
D becomes zero for . Consequently, two nuclei with a dipolar coupling vector at an angle of θm = 54.7° to a strong external magnetic field have zero dipolar coupling. θm is called the magic angle. Magic angle spinning is typically used to remove dipolar couplings weaker than the spinning rate.
Nuclei with a spin quantum number >1/2 have a non-spherical charge distribution and a quadrupole moment. [2] The quadrupole moment is a second rank tensor that couples to the surrounding electric field gradient, another second rank tensor. [2] [3] Nuclear quadrupole coupling is typically the second largest interaction in NMR, comparable in size to the largest interaction called Zeeman interactions. [4] When the nuclear quadrupole coupling is not negligible relative to the Zeeman coupling, higher order corrections are needed to describe the NMR spectrum correctly. In such cases, the first-order correction to the NMR transition frequency leads to a strong anisotropic line broadening of the NMR spectrum. However, all symmetric transitions, between and levels are unaffected by the first-order frequency contribution. The second-order frequency contribution depends on the P4 Legendre polynomial, which has zero points at 30.6° and 70.1°. These anisotropic broadenings can be removed using DOR (DOuble angle Rotation) where you spin at two angles at the same time, or DAS (Double Angle Spinning) [5] where you switch quickly between the two angles. Both techniques were developed in the late 1980s, and require specialized hardware (probe). Multiple quantum magic angle spinning (MQMAS) NMR was developed in 1995 and has become a routine method for obtaining high resolution solid-state NMR spectra of quadrupolar nuclei. [6] [7] A similar method to MQMAS is satellite transition magic angle spinning (STMAS) NMR developed in 2000.
The J-coupling or indirect nuclear spin-spin coupling (sometimes also called "scalar" coupling despite the fact that J is a tensor quantity) describes the interaction of nuclear spins through chemical bonds. J-couplings are not always resolved in solids owing to the typically large linewdiths observed in solid state NMR.
Paramagnetic substances are subject to the Knight shift.
A powder pattern arises in powdered samples where crystallites are randomly oriented relative to the magnetic field so that all molecular orientations are present. In presence of a chemical shift anisotropy interaction, each orientation with respect to the magnetic field gives a different resonance frequency. If enough crystallites are present, all the different contributions overlap continuously and lead to a smooth spectrum.
Fitting of the pattern in a static ssNMR experiment gives information about the shielding tensor, which are often described by the isotropic chemical shift , the chemical shift anisotropy parameter , and the asymmetry parameter . [8]
The dipolar powder pattern (also Pake pattern) has a very characteristic shape that arises when two nuclear spins are coupled together within a crystallite. The splitting between the maxima (the "horns") of the pattern is equal to the dipolar coupling constant .: [8]
where γ1 and γ2 are the gyromagnetic ratios of the dipolar-coupled nuclei, is the internuclear distance, is the reduced Planck constant, and is the vacuum permeability.
Magic angle spinning (MAS) is a technique routinely used in ssNMR to improve ssNMR spectra resolution. [9] After applying the MAS technique, NMR spectra will be sharper and narrower. [9] [10] This improved resolution results from manipulating a sample's spin interactions with the applied magnetic field. This is achieved by rotating the sample at a certain angle to the magnetic field to fully or partially average out anisotropic nuclear interactions such as dipolar, chemical shift anisotropy, and quadrupolar interactions. [9] This rotation angle is called the magic angle θm (ca. 54.74°, where cos2θm = 1/3). To achieve the complete averaging of these interactions, the sample needs to be spun at a rate that is at least higher than the largest anisotropy. [9] [10]
Spinning a powder sample at a slower rate than the largest component of the chemical shift anisotropy results in an incomplete averaging of the interaction, and produces a set of spinning sidebands in addition to the isotropic line, centred at the isotropic chemical shift. [10] Spinning sidebands are sharp lines separated from the isotropic frequency by a multiple of the spinning rate. Although spinning sidebands can be used to measure anisotropic interactions, they are often undesirable and removed by spinning the sample faster or by recording the data points synchronously with the rotor period.
Cross-polarization (CP) if a fundamental (Radiofrequency) RF pulse sequence and a building-block in many solid-state NMR. It is typically used to enhance the signal of a dilute nuclei with a low gyromagnetic ratio (e.g. 13
C, 15
N) by magnetization transfer from an abundant nuclei with a high gyromagnetic ratio (e.g. 1
H), or as a spectral editing method to get through space information (e.g. directed 15
N→13
C CP in protein spectroscopy). [9]
To establish magnetization transfer, RF pulses ("contact pulses") are simultaneously applied on both frequency channels to produce fields whose strength fulfil the Hartmann–Hahn condition: [11] [12]
where are the gyromagnetic ratios, is the spinning rate, and is an integer. In practice, the pulse power, as well as the length of the contact pulse are experimentally optimised. The power of one contact pulse is typically ramped to achieve a more broadband and efficient magnetisation transfer.
Spin interactions can be removed (decoupled) to increase the resolution of NMR spectra during the detection, or to extend the lifetime of the nuclear magnetization.
Heteronuclear decoupling is achieved by radio-frequency irradiation on at the frequency of the nucleus to be decoupled, which is often 1H. The irradiation can be continuous (continuous wave decoupling [13] ), or a series of pulses that extend the performance and the bandwidth of the decoupling (TPPM, [14] SPINAL-64, [15] SWf-TPPM [16] )
Homonuclear decoupling is achieved with multiple-pulse sequences (WAHUHA, [17] MREV-8, [18] BR-24, [19] BLEW-12, [19] FSLG [20] ), or continuous wave modulation (DUMBO, [21] eDUMBO [22] ). Dipolar interactions can also be removed with magic angle spinning. Ultra fast MAS (from 60 kHz up to above 111 kHz) is an efficient way to average all dipolar interactions, including 1H–1H homonuclear dipolar interactions, which extends the resolution of 1H spectra and enables the usage of pulse sequences used in solution state NMR. [23] [24]
Rotational Echo DOuble Resonance (REDOR) experiments, [25] [26] are a type of heteronuclear dipolar recoupling experiment which enables the re-introduction of heteronuclear dipolar couplings averaged by MAS. The reintroduction of such dipolar coupling reduces the intensity of the NMR signal compared to a reference spectrum where no dephasing pulse is used. REDOR can be used to measure heteronuclear distances, and are the basis of NMR crystallographic studies.
The strong 1H-1H homonuclear dipolar interactions associated with broad NMR lines and short T2 relaxation time effectively relegate proton for bimolecular NMR. Fast MAS and reduction of dipolar interactions by deuteration have made proton ssNMR as versatile as in solution. This includes spectral dispersion in multi-dimensional experiments [27] and structurally valuable restraints and parameters important for studying material dynamics. [28]
Ultra-fast NMR and the sharpening of the NMR lines enable NMR pulse sequences to capitalize on proton-detection to improve the sensitivity of the experiments compared to the direct detection of a spin-1/2 system (X). Such enhancement factor is given by:
where are the gyromagnetic ratios, represent the NMR line widths, and represent the quality factor of the probe resonances. [29]
Magic angle spinning dynamic nuclear polarization (MAS-DNP) is a technique that increases the sensitivity of NMR experiments by several orders of magnitude. [30] [31] It involves transferring the very high electron polarisation from unpaired electrons to nearby nuclei. This is achieved at cryogenic temperatures thorugh a continuous microwave irradiation from a klystron or a gyrotron, with a frequency close to the corresponding electron paramagnetic resonance (EPR) frequency.
The development in the MAS-DNP instrumentation, and the improvement of polarising agents (TOTAPOL, AMUPOL, TEKPOL, etc. [31] ) to achieve a more efficient transfer of polarisation has dramatically reduced experiments times which enabled the observation of surfaces, [32] insensitive isotopes, [33] and multidimensional experiments on low natural abundance nuclei, [34] and diluted species. [35]
Beta-detected nuclear magnetic resonance (β-NMR) is specialized technique that has working principles similar to muon spin spectroscopy. [36] It is used in domains such as chemistry, materials science, condensed matter physics, and biology as a powerful probe. [36] [37] [38] β-NMR is practiced at facilities such as TRIUMF and ISOLDE as well as research groups in Osaka and Moscow. [38]
What makes β-NMR different than conventional NMR is firstly, where and when the spin polarization of the nuclei occurs and secondly, how the signal is produced. [36] [39] To conduct a β-NMR experiment, optical pumping is performed on a radioactive beam of particles, such as 8Li and 31Mg, to polarize their nuclear spin to nearly one-hundred percent. [36] [40] The isotopes are subsequently implanted into a sample in vacuum in the dilute-limit to eliminate homonuclear probe interactions. The spin–lattice relaxation of the probe is monitored by the parity-violating beta-decay of the radioactive isotope. [36] [40] This anisotropic decay is where the signal originates for β-NMR experiment. [36] [39] This technique allows for investigation of the local magnetic and electronic environment within a material . [36] [40]
ssNMR spectroscopy serves as an effective analytical tool in biological, organic, and inorganic chemistry due to its close resemblance to liquid-state spectra while providing additional insights into anisotropic interactions. [41]
It is used to characterize chemical composition, structure, local motions, kinetics, and thermodynamics, with the special ability to assign the observed behavior to specific sites in a molecule. It is also crucial in the area of surface and interfacial chemistry. [42]
ssNMR is used to study insoluble proteins and proteins such as membrane proteins [43] and amyloid fibrils. [44] Using the principles of MAS, protein tertiary structure information can be determined. [45] This includes the assessment of protein dynamics. [46]
ssNMR is used to study biomaterials such as bone, [47] [48] teeth, [49] [50] hair, [51] silk, [52] wood, [53] as well as viruses, [54] [55] plants, [56] [57] cells, [58] [59] and collected biopsies. [60]
ssNMR is used in pharmaceutical research for the characterization of drug polymorphs and solid dispersions. [61]
ssNMR spectroscopy is used in materials science to analyze solid samples. [62] Here, molecules have restricted motion which leads to complex magnetic interactions, such as dipole-dipole coupling, chemical shift anisotropy, and quadrupolar interactions. [63] These interactions can provide more detailed information than X-ray diffraction or solution NMR spectroscopy about the material's structure to elucidate information on the solid's (crystalline and non-crystalline) local structure and dynamics.
ssNMR has been successfully used to study metal organic frameworks, [64] solid-state batteries, [65] surfaces of nanoporous materials, [66] and polymers. [67]
The nuclear Overhauser effect (NOE) is the transfer of nuclear spin polarization from one population of spin-active nuclei to another via cross-relaxation. A phenomenological definition of the NOE in nuclear magnetic resonance spectroscopy (NMR) is the change in the integrated intensity of one NMR resonance that occurs when another is saturated by irradiation with an RF field. The change in resonance intensity of a nucleus is a consequence of the nucleus being close in space to those directly affected by the RF perturbation.
Nuclear quadrupole resonance spectroscopy or NQR is a chemical analysis technique related to nuclear magnetic resonance (NMR). Unlike NMR, NQR transitions of nuclei can be detected in the absence of a magnetic field, and for this reason NQR spectroscopy is referred to as "zero Field NMR". The NQR resonance is mediated by the interaction of the electric field gradient (EFG) with the quadrupole moment of the nuclear charge distribution. Unlike NMR, NQR is applicable only to solids and not liquids, because in liquids the electric field gradient at the nucleus averages to zero. Because the EFG at the location of a nucleus in a given substance is determined primarily by the valence electrons involved in the particular bond with other nearby nuclei, the NQR frequency at which transitions occur is unique for a given substance. A particular NQR frequency in a compound or crystal is proportional to the product of the nuclear quadrupole moment, a property of the nucleus, and the EFG in the neighborhood of the nucleus. It is this product which is termed the nuclear quadrupole coupling constant for a given isotope in a material and can be found in tables of known NQR transitions. In NMR, an analogous but not identical phenomenon is the coupling constant, which is also the result of an internuclear interaction between nuclei in the analyte.
Dynamic nuclear polarization (DNP) is one of several hyperpolarization methods developed to enhance the sensitivity of nuclear magnetic resonance (NMR) spectroscopy. While an essential analytical tool with applications in several fields, NMR’s low sensitivity poses major limitations to analyzing samples with low concentrations and limited masses and volumes. This low sensitivity is due to the relatively low nuclear gyromagnetic ratios (γn) of NMR active nuclei (1H, 13C, 15N, etc.) as well as the low natural abundance of certain nuclei. Several techniques have been developed to address this limitation, including hardware adjustments to NMR instruments and equipment (e.g., NMR tubes), improvements to data processing methods, and polarization transfer methods to NMR active nuclei in a sample—under which DNP falls.
In nuclear magnetic resonance (NMR) spectroscopy, the chemical shift is the resonant frequency of an atomic nucleus relative to a standard in a magnetic field. Often the position and number of chemical shifts are diagnostic of the structure of a molecule. Chemical shifts are also used to describe signals in other forms of spectroscopy such as photoemission spectroscopy.
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique based on re-orientation of atomic nuclei with non-zero nuclear spins in an external magnetic field. This re-orientation occurs with absorption of electromagnetic radiation in the radio frequency region from roughly 4 to 900 MHz, which depends on the isotopic nature of the nucleus and increased proportionally to the strength of the external magnetic field. Notably, the resonance frequency of each NMR-active nucleus depends on its chemical environment. As a result, NMR spectra provide information about individual functional groups present in the sample, as well as about connections between nearby nuclei in the same molecule. As the NMR spectra are unique or highly characteristic to individual compounds and functional groups, NMR spectroscopy is one of the most important methods to identify molecular structures, particularly of organic compounds.
In solid-state NMR spectroscopy, magic-angle spinning (MAS) is a technique routinely used to produce better resolution NMR spectra. MAS NMR consists in spinning the sample at the magic angle θm with respect to the direction of the magnetic field.
Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a method for studying materials that have unpaired electrons. The basic concepts of EPR are analogous to those of nuclear magnetic resonance (NMR), but the spins excited are those of the electrons instead of the atomic nuclei. EPR spectroscopy is particularly useful for studying metal complexes and organic radicals. EPR was first observed in Kazan State University by Soviet physicist Yevgeny Zavoisky in 1944, and was developed independently at the same time by Brebis Bleaney at the University of Oxford.
The magic angle is a precisely defined angle, the value of which is approximately 54.7356°. The magic angle is a root of a second-order Legendre polynomial, P2(cos θ) = 0, and so any interaction which depends on this second-order Legendre polynomial vanishes at the magic angle. This property makes the magic angle of particular importance in magic angle spinning solid-state NMR spectroscopy. In magnetic resonance imaging, structures with ordered collagen, such as tendons and ligaments, oriented at the magic angle may appear hyperintense in some sequences; this is called the magic angle artifact or effect.
Nuclear magnetic resonance spectroscopy of proteins is a field of structural biology in which NMR spectroscopy is used to obtain information about the structure and dynamics of proteins, and also nucleic acids, and their complexes. The field was pioneered by Richard R. Ernst and Kurt Wüthrich at the ETH, and by Ad Bax, Marius Clore, Angela Gronenborn at the NIH, and Gerhard Wagner at Harvard University, among others. Structure determination by NMR spectroscopy usually consists of several phases, each using a separate set of highly specialized techniques. The sample is prepared, measurements are made, interpretive approaches are applied, and a structure is calculated and validated.
Nuclear magnetic resonance quantum computing (NMRQC) is one of the several proposed approaches for constructing a quantum computer, that uses the spin states of nuclei within molecules as qubits. The quantum states are probed through the nuclear magnetic resonances, allowing the system to be implemented as a variation of nuclear magnetic resonance spectroscopy. NMR differs from other implementations of quantum computers in that it uses an ensemble of systems, in this case molecules, rather than a single pure state.
In nuclear chemistry and nuclear physics, J-couplings are mediated through chemical bonds connecting two spins. It is an indirect interaction between two nuclear spins that arises from hyperfine interactions between the nuclei and local electrons. In NMR spectroscopy, J-coupling contains information about relative bond distances and angles. Most importantly, J-coupling provides information on the connectivity of chemical bonds. It is responsible for the often complex splitting of resonance lines in the NMR spectra of fairly simple molecules.
The residual dipolar coupling between two spins in a molecule occurs if the molecules in solution exhibit a partial alignment leading to an incomplete averaging of spatially anisotropic dipolar couplings.
Zero- to ultralow-field (ZULF) NMR is the acquisition of nuclear magnetic resonance (NMR) spectra of chemicals with magnetically active nuclei in an environment carefully screened from magnetic fields. ZULF NMR experiments typically involve the use of passive or active shielding to attenuate Earth’s magnetic field. This is in contrast to the majority of NMR experiments which are performed in high magnetic fields provided by superconducting magnets. In ZULF experiments the sample is moved through a low field magnet into the "zero field" region where the dominant interactions are nuclear spin-spin couplings, and the coupling between spins and the external magnetic field is a perturbation to this. There are a number of advantages to operating in this regime: magnetic-susceptibility-induced line broadening is attenuated which reduces inhomogeneous broadening of the spectral lines for samples in heterogeneous environments. Another advantage is that the low frequency signals readily pass through conductive materials such as metals due to the increased skin depth; this is not the case for high-field NMR for which the sample containers are usually made of glass, quartz or ceramic. High-field NMR employs inductive detectors to pick up the radiofrequency signals, but this would be inefficient in ZULF NMR experiments since the signal frequencies are typically much lower. The development of highly sensitive magnetic sensors in the early 2000s including SQUIDs, magnetoresistive sensors, and SERF atomic magnetometers made it possible to detect NMR signals directly in the ZULF regime. Previous ZULF NMR experiments relied on indirect detection where the sample had to be shuttled from the shielded ZULF environment into a high magnetic field for detection with a conventional inductive pick-up coil. One successful implementation was using atomic magnetometers at zero magnetic field working with rubidium vapor cells to detect zero-field NMR.
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field and respond by producing an electromagnetic signal with a frequency characteristic of the magnetic field at the nucleus. This process occurs near resonance, when the oscillation frequency matches the intrinsic frequency of the nuclei, which depends on the strength of the static magnetic field, the chemical environment, and the magnetic properties of the isotope involved; in practical applications with static magnetic fields up to ca. 20 tesla, the frequency is similar to VHF and UHF television broadcasts (60–1000 MHz). NMR results from specific magnetic properties of certain atomic nuclei. 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. NMR is also routinely used in advanced medical imaging techniques, such as in magnetic resonance imaging (MRI). The original application of NMR to condensed matter physics is nowadays mostly devoted to strongly correlated electron systems. It reveals large many-body couplings by fast broadband detection and should not be confused with solid state NMR, which aims at removing the effect of the same couplings by Magic Angle Spinning techniques.
Electron nuclear double resonance (ENDOR) is a magnetic resonance technique for elucidating the molecular and electronic structure of paramagnetic species. The technique was first introduced to resolve interactions in electron paramagnetic resonance (EPR) spectra. It is currently practiced in a variety of modalities, mainly in the areas of biophysics and heterogeneous catalysis.
Nuclear magnetic resonance crystallography is a method which utilizes primarily NMR spectroscopy to determine the structure of solid materials on the atomic scale. Thus, solid-state NMR spectroscopy would be used primarily, possibly supplemented by quantum chemistry calculations, powder diffraction etc. If suitable crystals can be grown, any crystallographic method would generally be preferred to determine the crystal structure comprising in case of organic compounds the molecular structures and molecular packing. The main interest in NMR crystallography is in microcrystalline materials which are amenable to this method but not to X-ray, neutron and electron diffraction. This is largely because interactions of comparably short range are measured in NMR crystallography.
Nitrogen-15 nuclear magnetic resonance spectroscopy is a version of nuclear magnetic resonance spectroscopy that examines samples containing the 15N nucleus. 15N NMR differs in several ways from the more common 13C and 1H NMR. To circumvent the difficulties associated with measurement of the quadrupolar, spin-1 14N nuclide, 15N NMR is employed in samples for detection since it has a ground-state spin of ½. Since14N is 99.64% abundant, incorporation of 15N into samples often requires novel synthetic techniques.
David Lyndon Emsley FRSC is a British chemist specialising in solid-state nuclear magnetic resonance and a professor at EPFL. He was awarded the 2012 Grand Prix Charles-Leopold Mayer of the French Académie des Sciences and the 2015 Bourke Award of the Royal Society of Chemistry.
Cross-polarization (CP), originally published as proton-enhanced nuclear induction spectroscopy (PENIS) is a solid-state nuclear magnetic resonance (ssNMR) technique to transfer nuclear magnetization from different types of nuclei via heteronuclear dipolar interactions. The 1H-X cross-polarization dramatically improves the sensitivity of ssNMR experiments of most experiments involving spin-1/2 nuclei, capitalizing on the higher 1H polarisation, and shorter T1(1H) relaxation times. It was developed by Michael Gibby, Alexander Pines and Professor John S. Waugh at the Massachusetts Institute of Technology.
Spinach is an open-source magnetic resonance simulation package initially released in 2011 and continuously updated since. The package is written in Matlab and makes use of the built-in parallel computing and GPU interfaces of Matlab.
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