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Some Key Developments in NMR Spectroscopy
Chapter 9
Some Key Developments in NMR Spectroscopy Chapter Outline 9.1 Development of New Hardware 9.1.1 Bench-Top NMR Spectrometers 9.1.2 Earth’s Field NMR Spectrometer 9.1.3 Ultra-High Field NMR Spectrometers 9.1.4 New Liquid Nitrogen Cooled Cryogenic Probe 9.1.5 Multiple Receiver Technology
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9.2 Key Developments in Solid-State NMR Spectroscopy 422 9.2.1 Solid-State Cryogenic MAS Probes 422 9.2.2 DNP-MAS NMR Spectroscopy 424 9.3 Innovative Pulse Sequences and Novel Applications of NMR Spectroscopy 425 9.3.1 Diffusion Order Spectroscopy (DOSY) 425 9.3.2 Other Key Developments 427 References 429
NMR spectroscopy continues to evolve, in terms of improvements in hardware, as well as in designing of innovative pulse sequences for new applications. A dazzling array of recent publications, referred to in this chapter, provide an overview of the key developments in this field. Some of the topics, such as DOSY, which were not included in the earlier chapters, are also presented here. The discussion is divided into three broad categories, i.e., key progress made in solid-state NMR spectroscopy, developments in NMR hardware, and advent of innovative pulse sequences for new applications (Edger, 2013).
9.1 DEVELOPMENT OF NEW HARDWARE 9.1.1 Bench-Top NMR Spectrometers NMR spectroscopy is routinely used to support and manage a variety of academic and research activities in chemistry laboratories. Conventional NMR spectrometers are large, expensive, and often difficult to maintain. The advent of low-field bench-top NMR spectrometers, therefore, is changing the way NMR spectroscopy has been practiced for many years. With a small bench top NMR, Solving Problems with NMR Spectroscopy. http://dx.doi.org/10.1016/B978-0-12-411589-7.00009-7 Copyright © 2016 Elsevier Inc. All rights reserved.
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FIGURE 9.1 Oxford Instrument’s Pulsar™ - 60 MHz (1.4 T) NMR spectrometer, fitted with cryogen-free permanent magnet. (Reprinted with the permission of Oxford Instruments).
there is no longer any need to line up at the core NMR facility, for instance to run a quick check on the purity of a natural product or determine the status of a synthesis. Whether it is for identifying a compound quantitation, reaction monitoring, structure elucidation, or kinetics measurements, bench-top NMR enables researchers, within limits, to carry out the tasks at hand. The 45, 60, 80, and 90 MHz (∼1–2 T) bench top NMR spectrometers can provide a quick look at the NMR data, with the option to do a detailed analysis on a more powerful instrument later, if required (Fig. 9.1). Generally, a resolution of between 1 and 3 Hz can be achieved by these robust, easy to handle, and affordable spectrometers (Metz and Mäder, 2008). Bench-top NMR spectrometers are equipped with cryogen-free stable permanent magnets, made from some rare earth metals. Samarium–cobalt and neodymium magnets are particularly suitable for instruments up to 90 MHz. They operate at temperatures from room temperature to 45oC and their designs allow the instruments to be made small enough to fit on a laboratory bench, and safe to be transported for on-site analysis. They require only single phase local electrical power, fitted with a UPS system, making them especially suitable for small research laboratories in developing countries. These spectrometers are also ideal for teaching and training of NMR spectroscopy in places where funding is a major constraint. Bench-top NMR spectrometers normally use traditional 3–5 mm NMR sample tubes. The samples are dissolved in deuterated solvents, and the spectrometers can be shimmed and locked at ambient room temperature without any hassle. Because of the lower field magnets and corresponding low sensitivities, the sample requirements are relatively high, that is often not an issue in medicinal and synthetic chemistry studies. With a concentrated sample, a good signal-to-noise ratio can be obtained, and 1H-NMR spectra may be recorded in
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FIGURE 9.2 1H-NMR spectrum of ibuprofen (200 mM in CDCl3), recorded in 10 seconds on a 43 MHz bench-top NMR spectrometer (Reprinted with the permission of Magritek Limited).
a few minutes (Fig. 9.2). Recently, a large number of on-site NMR studies on archeological samples have been reported with such magnets in the literature (Capitani et al., 2012).
9.1.2 Earth’s Field NMR Spectrometer The earth’s field NMR spectrometers (EF-NMR) (Fig. 9.3) are ultra-low field NMR instruments. These spectrometers use the globally available homogenous earth’s magnetic field (0.05 T) for the detection of NMR active nuclei. The resulting Larmor frequency of 1H in the earth’s field magnet is only a couple of kHz, as compared to 60 MHz in a 1.41 T low field 60 MHz NMR spectrometer. The EF-NMR signals are also affected by both the magnetic noises in the
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FIGURE 9.3 An earth’s field NMR spectrometer (Terranova EF-NMR system by Magritek) (Reprinted with the permission of Magritek Limited).
laboratory environment, and the natural variations in the earth’s field. The very low sensitivity, the dominance of scalar-couplings, and the lack of chemical-shift dispersion have been the major bottlenecks in the use of EF-NMR spectrometers. However, such disadvantages have been overcome, to an extent, with the introduction several new methods including: 1. Introduction of electronic equipment which can compensate for the changes in ambient magnetic fields. 2. Use of a pulse sequence with only DC fields. 3. Pre-polarization of the sample by the use of a crude electromagnet with the field 350 times larger than the initial magnetization. The Pre-polarization methodology significantly increases the sensitivity and makes it possible to perform a large range of modern and sophisticated NMR experiments. While chemical shifts are important in NMR, they are normally not of significance in the earth’s field NMR instrument because of very low dispersion which results in most of them, if not all, overlapping together as one broad signal. In the absence of chemical shifts, several other spectral features, such as spin–spin multiplets (that are separated by high fields) are observed in the EFNMR. The EF-NMR spectra are, therefore, dominated by spin–spin coupling
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(J-coupling) effects. Software optimized for analyzing these spectra can provide useful information about the structure of the molecules in the sample. Efforts are being made to develop methods for chemical shift based NMR signals in the EF-NMR (Katz et al., 2012). The EF-NMR spectrometer has certain advantages, including the portability of the machine, the ability to analyze samples on-site, and its low cost. In highfield NMR spectrometers, the magnetic field inhomogeneity is compensated in many ways but this is not an issue in EF-NMR spectrometers due to the excellent inherent homogeneity of the earth’s magnetic field (at a particular location). EF-NMR spectrometers are good educational tools due to their relatively low cost and simplicity, but their research applications are severely limited because of their poor sensitivity and dispersion. Compounds containing hydrogen nuclei such as water, hydrocarbons (natural gas and petroleum), and carbohydrates (plants and animals) may be analyzed by 1H-EF-NMR.
9.1.3 Ultra-High Field NMR Spectrometers Efforts to build ultra-high field NMR spectrometers (over 1000 MHz) are underway. These include the development of a 1.2 GHz (1200 MHz) NMR spectrometer through the financial support (Euro 10 million) of the Netherlands government. Many other laboratories in US and Europe are also involved in the development of ultra-high field NMR spectrometers of 1.2 GHz or beyond. Some laboratories already have such machines, mostly custom made at site for specific purposes. Magnetic field stability with very low field drift rates, suppression of external magnetic field disturbances, and minimization of stray fields are the key challenges in the development of ultra-high field NMR spectrometers (Harrison et al., 2008).
9.1.4 New Liquid Nitrogen Cooled Cryogenic Probe The first-generation cryogenically cooled probes, as discussed in Chapters 1 and 3 (Sections 1.2.2 and 3.3.2.1), require gaseous helium as the cryogen, and they are often difficult to maintain. A new range of cryogenically cooled probes have been recently developed which use liquid nitrogen to cool the Rf coils and the preamplifier, and they provide a sensitivity gain of a factor of 2 or more over equivalent room temperature (RT) probes. The probe assembly comprises, in addition to the probe, a control unit, an automatic tuning accessory, and a liquid nitrogen vessel. These probes are now available in various settings, such as broad-band and triple resonance excitation. Their simple design, the requirement of minimum infrastructure, ease of handling and maintenance, low running cost, and most importantly affordable price make them an excellent choice for routine analysis (Ramaswamy et al., 2013). The liquid nitrogen cooled probes (Fig. 9.4) deliver a significant signal-tonoise (sensitivity) enhancement over the room temperature (RT) probes of a
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FIGURE 9.4 Liquid nitrogen cooled cryogenic Prodigy probe by Bruker Spectrospin (Reprinted with the permission of Bruker corporation).
factor of 2–3 for X-nuclei (such as 13C, 15N, 31P, etc.), although it is not as much as that obtainable from helium cooled probes that provide a sensitivity enhancement by a factor of 4. Liquid nitrogen cooled probes enable time-demanding heteronuclear correlated NMR experiments to be performed up to 10 times faster than with RT probes. They are specially suited for the study of small molecules in academia, chemical industries, and pharmaceutical sector, and can enable such laboratories to substantially increase their sample throughput.
9.1.5 Multiple Receiver Technology The use of dual receivers has been in practice in solid state NMR spectroscopy since long, as it allows rapid parallel data acquisition in each dimension. In conventional high resolution solution state NMR spectroscopy, the direct detection of only single nucleus at a time is commonly carried out. For example in COSY and TOCSY, as well as in HSQC and HMBC, 1H is detected, while in INADEQUATE, 13C nucleus is detected. As a result, NMR experiments are performed
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in sequence, one after the other. If we have the capacity to detect two or more nuclei at the same time, then we can record many experiments, parallel to each other. This has now become possible with spectrometers that incorporate two or more receivers so that they can tune into different nuclei at the same time. The availability of NMR spectrometers that incorporate multiple receivers, tuned to different nuclei, has led to the development of a new class of NMR experiments, commonly called Parallel Acquisition NMR SpectroscopY (PANSY), and Parallel Acquisition of NMR, an All-in-one Combination of Experimental Applications (PANACEA). PANSY is now used to simultaneously detect signals from up to four nuclear species, such as 1H, 2H, 13C, 15N, 19F, and 31P. Conventional COSY, TOCSY, HSQC, HMQC, and modified HMBC pulse sequences have been modified for the PANSY applications (Kupcˇ e et al., 2006). PANACEA, however, combines several standard NMR pulse sequences into a single entity. For example, PANACEA experiments with INADEQUATE, HSQC, and HMBC are designed for getting information about 13C–13C couplings, as well as direct 1H–13C and longrange 1H/13C correlations. Such parallel experiments offer a direct route to molecular structure (Kupcˇ e and Freeman, 2010). Figure 9.5 shows the PANACEA spectra, consisting of (1) INADEQUATE, and (2) multiplicity-edited HSQC spectra of cholesterol, recorded in parallel in 23-min measurements.
FIGURE 9.5 A PANACEA experiment exhibiting (a) INADEQUATE spectrum, and (b) multiplicity-edited HSQC spectrum (concatenated DEPT-HSQC) in CDCl3 (1 M), recorded in parallel in 23 minutes. Two responses in (a) are correlated with signals that lie well outside this spectral window. (b) Multiplicity-edited HSQC spectrum shows red peaks that belong to CH2 groups, while the black peaks represent CH and CH3 groups. Four-fold aliasing was used to reduce the experimental duration. Sweep width = 20 kHz; sweep width (1) = 5.2 kHz with 4 k data points and two scans per increment (Reprinted from Kupcˇ e and Freeman (2010). J. Magn. Reson. 206, 147–153, with permission from Academic Press, Inc.).
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The PANSY and PANACEA experiments allow unambiguous structure elucidation of small organic molecules from a single measurement, and include an internal field/frequency correction routine. These experiments do not require the conventional NMR lock system, and can be obtained in pure liquids. Furthermore, long-range spin–spin couplings can be extracted from the PANACEA spectra, and used for three-dimensional structure refinement. These quick NMR measurements are especially suitable for samples where long-term stability is an issue. Similarly, the recording of several NMR spectra in parallel, rather than in series, leads to a major saving in instrument time. The PANSY and PANACEA experiments are currently not optimized for structure determination of large molecules. However, this is one area in which these innovative pulse sequences are likely to have impact in the future, simply because conventional biomolecular NMR spectroscopic measurements in series, one by one, can take days if not weeks of NMR instrument time.
9.2 KEY DEVELOPMENTS IN SOLID-STATE NMR SPECTROSCOPY Major recent developments in NMR spectroscopy have largely taken place in solid-state NMR and in NMR-based imaging. Both subjects are beyond the scope of this book. However, some key developments in the field of solid-state NMR spectroscopy are presented here as we feel that they will soon find extensive uses in structural chemistry, structural biology, and pharmaceutical industrial applications.
9.2.1 Solid-State Cryogenic MAS Probes Magic angle spinning (MAS) probes have long been in use in solid-state NMR spectroscopy. Most of these probes function at room temperature. Recently, cryogenic MAS solid-state probes have been constructed which provide a sensitivity enhancement by cooling the sample to a temperature of 30 K. They also have excellent spinning stability which allows the recording of two-dimensional (2D) NMR spectra. This development should not be confused with the cryogenically cooled probes in solution state NMR spectroscopy, as in those probes, the sensitivity gain is based on cooling of the detection coil and circuitry, and not of the sample itself. In cryogenic MAS probes, the solid sample is cooled with liquid gas, while room temperature nitrogen is used as the MAS bearing and drive gas (Thurber and Tycko, 2008). This probe can provide only medium speed magic angle spinning due to the relatively large sample volume (40–80 mL). The gain in sensitivity in these probes is based on the fact that by lowering the temperature of the sample, one can increase the spin polarization. At a temperature 1 K, the nuclear spin polarization at thermal equilibrium is proportional to 1/T, where T is the temperature of the sample. Thus if we cool down the sample to
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25 K, from the room temperature (∼295 K), a 12-fold increase in spin polarization is expected, leading to a 12-fold increase in sensitivity (Fernandez and Pruski, 2012). Substantial lowering of temperature in solution-state NMR spectroscopy for sensitivity enhancement is not practical due to a number of factors, including temperature gradient issues, precipitation of solute particles, and solidification or icing of deuterated solvents. It is, therefore, more logical to use other methods, including lowering of the temperature of the detection circuits to reduce the noise level and thus increase the S/N ratio. However, in solid-state NMR spectroscopy, modest lowering of the temperature of the sample in MAS probe is an excellent approach for the enhancement of S/N ratio. Figure 9.6 shows 13 C-NMR (CP-MAS) proton decoupled spectra of powdered 13C-labeled valine at 300 and 31 K, recorded using a cryogenic MAS probe (revolution NMR LLC, USA). Over 10-fold enhancement in sensitivity is clearly seen (Polenova et al., 2015; Thurber and Tycko, 2008).
FIGURE 9.6 13C CP-MAS spectra of powdered 13C-labeled valine powder (9 mg) in one scan, (a) 13C CP-MAS NMR spectrum at 300 K, and (b) 13C CP-MAS NMR spectrum at 31 K (Reprinted with the permission of Revolution NMR LLC, USA).
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9.2.2 DNP-MAS NMR Spectroscopy The development of dynamic-MAS NMR spectrometers represents one of the most important advances in the field of NMR spectroscopy. The process of Dynamic Nuclear Polarization (DNP) provides a dramatic enhancement in sensitivity, both in liquid- and solid-state NMR studies. However, it is in the solidstate where DNP has found the greatest use. As described in Section 3.6.1, the large electron polarization of radical molecules can be transferred to a nuclear spin by using in situ microwaves for low-temperature state NMR spectroscopy. These polarization transfers between electrons and coupled nuclei can occur spontaneously through electron-nuclear cross relaxation, and/or spin-state mixing among the electrons and nuclei. The DNP-MAS NMR spectrometers are now available in the range of 200–800 MHz, and they involve major developments in probe design and microwave technologies (gyrotron) (Barnes et al., 2012). The DNP-enhanced solid-state NMR experiments at high field can yield sensitivity enhancements of up to 80-fold (Fig. 9.7). These systems are equipped with unique high power microwave sources, i.e., an easy-to-use software-controlled high-power gyrotron (a vacuum electronic device, capable of generating
FIGURE 9.7 A DNP solid-state NMR spectrometer (Reprinted with the permission of Bruker Corporation).
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high-power, high-frequency THz electromagnetic radiations, which serves as the microwave source). Optimum beam propagation to the sample is ensured by microwave transmission lines. Low-temperature (cryogenic) MAS probe technology (∼100 K), with cold spinning gas and built-in waveguide, are now available for a range of NMR systems, such as for 400, 600, and 800 MHz spectrometers.
9.3 INNOVATIVE PULSE SEQUENCES AND NOVEL APPLICATIONS OF NMR SPECTROSCOPY 9.3.1 Diffusion Order Spectroscopy (DOSY) The introduction of second frequency dimension gave birth to 2D NMR spectroscopy, as described previously. A logical extension of the idea of the second dimension in NMR spectroscopy was the advent of additional NMR dimensions that depend on other molecular properties, such as size, shape, mass, and charge. In DOSY spectroscopy, the mixtures of organic molecules are analyzed based on molecular diffusion of each component. Very often chemists need to handle mixtures of organic compounds. They can be mixtures of compounds in extracts of medicinal plants, reaction mixtures containing several products, etc. Historically, mixture analysis in organic chemistry was generally performed in two steps. In the first step, the various constituents are separated using chromatographic techniques, including high performance liquid chromatography or gas chromatography, while in the second step, these constituents are individually characterized by using various spectroscopic techniques. The advent of so-called hyphenated techniques such as LC-MS and GC-MS as well as LC-NMR made it possible to separate and analyze the individual constituents of complex mixtures in one step in a timeefficient manner. The LC-NMR technique suffers from a number of limitations, such as the need of setting up of ideal separation conditions for HPLC and issues of solubility in the appropriate deuterated solvent. In the last decade, the Diffusion Order SpectroscopY (DOSY) NMR technique has been developed for the analysis of mixtures of organic compounds. The method relies on the different rates of chemicals through a solution. These rates are directly related to the physical properties of the various components making up the mixture. DOSY NMR does not require the physical separation of the components in a mixture, and provides chemical shift information in situ. This has made DOSY NMR a powerful technique with wide applications. The DOSY spectrum (Fig. 9.9) separates the NMR signals of different compounds in a mixture according to their diffusion coefficients. It is a 2D NMR experiment in which the information is spread in two dimensions. A series of spin echo spectra is recorded with different pulsed field gradient strengths, and the resulting signal decays are analyzed to extract a set of diffusion coefficients
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with which to synthesize the diffusion domain (diffusion dimension) of the DOSY spectrum. The molecules, as a liquid or in the solution state, are in constant motion. This includes translational motion (Brownian motion) which provides a mechanism by which the diffusion process takes place. Brownian motion is that random motion of molecules that occurs as a consequence of their absorption of heat. Molecules will diffuse from volumes of high concentration to low concentration; this happens because the molecules are in constant random (Brownian) motion, and they tend to bump into each other more if they are in more concentrated areas. Hence, they tend to move away from each other through diffusion. The diffusion depends on many physical parameters, including molecular weight, size and shape of the molecule, and the temperature and viscosity of solution. Assuming a spherical size of the molecule, the diffusion coefficient D is described by Eq. 9.1. kT (9.1) D= 6πη rS where k is the Boltzmann constant, T is the temperature, η is the viscosity of the liquid, and rS is the (hydrodynamic) radius of the molecule. Pulse gradient NMR spectroscopy is used to measure the translational diffusion of molecules. By the use of gradients, the molecules can be spatially labeled (marked depending on their position in the sample tube). If they move, after the initial labeling, during the diffusion time ∆, their new position is again decoded by using the second gradient. This difference in diffusion time directly affects the NMR signal intensity or signal strength. This simple theory forms the basis of differentiating different solutes in a mixture. The basic pulse sequence of DOSY involves the characterization of molecular diffusion by pulsed field gradient spin-echo (PGSE). The PGSE constructs a 2D plot where the x-axis contains the chemical shifts of 1H (in ppm), while the y-axis gives the diffusion coefficient D of each signal. Since diffusion coefficients of various components are different, one can record separate 1H-NMR spectra for each compound in a mixture. There are several basic sequences developed to map the diffusion rates of individual components in a complex mixture. In its simplest form, the magnetization is excited with a 90° Rf pulse, and then dispersed using a magnetic field gradient pulse. After a period of ∆/2, an 180° Rf pulse is applied to invert the dispersed magnetization. After a period ∆, a second gradient pulse is applied to refocus the signal. The PGSE pulse sequence is presented in Fig. 9.8. The DOSY NMR experiments are also used for the measurement of diffusion rates of different constituents in a mixture. Figure 9.9 shows the DOSY spectrum of a mixture of three components, cholesterol, aspirin, and phenol. Each line, horizontal to the diffusion axis, presents a separate 1H-NMR spectrum of each component, which is also shown in 1D slices.
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FIGURE 9.8 Basic pulse sequences for the measurement of molecular diffusion based on spinecho. (a) Pulse field gradient spin-echo, and (b) PFG stimulated-echo. The molecular diffusions are measured during the delay period ∆ with increasing gradient strengths.
9.3.2 Other Key Developments In addition to developments mentioned above, there are several other interesting techniques that have been developed which are presented below: 1. Development of multiple-sample solid-state magic-angle spinning (MAS) probes which can simultaneously acquire NMR spectra of seven or more samples. They can substantially increase throughput of solid-state NMR spectroscopy (Nelson et al., 2006). 2. A “moving-tube” method has been developed to circumvent problems associated with long T1 relaxation times. This utilizes a 5 mm NMR tube (1.5 m long), filled with a large quantity of sample solution. A motor physically moves this long NMR tube vertically after each pulse-acquisition (scan), and thus removes (moves away) the excited spins from the coil, and introduce relaxed spins (so called “fresh sample”) in front of the detection coil. This method has been applied in recording of variant COSY spectrum (Donovan et al., 2012). 3. The methodology for quantitative 1H-NMR (qHNMR) is continuously evolving, expanding its applications to various fields. A major challenge is to accurately and most comprehensively analyze complex mixtures (natural
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FIGURE 9.9 (a) DOSY NMR spectrum of a mixture of three compounds. (b) Each slice corresponds to the regular 1H-NMR spectrum of one component of the mixture (cholesterol, aspirin, and phenol in this case). The diffusion rate of various constituents can be obtained from the DOSY spectrum.
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products, environmental pollutants, biological fluids, toxins, etc.) containing low-level analytes. For this purpose, standardization, calibration, rapid identification, and computational methods are regularly reported in a large number of publications (Pauli et al., 2012).
REFERENCES Barnes, A.B., Markhasin, E., Daviso, E., Michaelis, V.K., Nanni, E.A., Jawla, S.K., Mena, E.L., DeRocher, R., Thakkar, A., Woskov, P.P., Herzfeld, J., Temkin, R.J., Griffin, R.G., 2012. Dynamic nuclear polarization at 700 MHz/460 GHz. J. Magn. Reson. 224, 1–7. Capitani, D., Di Tullio, V.D., Porietti, N., 2012. Nuclear magnetic resonance to characterize and monitor cultural heritage. Prog. Nucl. Magn. Reson. Spectrosc. 64, 29–69. Donovan, K.J., Allen, M., Martin, R.W., Shaka, A.J., 2012. Improving the double quantum filtered COSY experiment by “Moving Tube” NMR. J. Magn. Reson. 219, 41–45. Edger, M., 2013. Physical methods and techniques: NMR spectroscopy. Ann. Rep. Prog. Chem. Sect. B: Org. Chem. 109, 256–274. Fernandez, C., Pruski, M., 2012. Solid State NMR. Probing quadrupolar nuclei by solid-state NMR spectroscopy: recent advances. Top. Curr. Chem. 306, 119–188. Harrison, R., Bateman, R., Brown, J., Domptail, F., Friend, C.M., Ghoshal, P., King, C., Van der Linden, A., Melhem, Z., Noonan, P., Twin, A., Field, M., Hong, S., Parrell, J., Zhang, Y., 2008. Development trends in high field magnet technology. IEEE. Trans. Appl. Supercond. 18 (2), 540–543. Katz, I., Shtirberg, L., Shakour, G., Black, A., 2012. Earth field NMR with chemical shift spectral resolution: Theory and proof of concept. J. Magn. Reson. 219, 13–24. Kupcˇe, E., Freeman, R., 2010a. High-resolution NMR correlation experiments in a single measurement (HR-PANACEA). Magn. Reson. Chem. 48 (5), 333–336. Kupcˇe, E., Freeman, R., 2010b. Molecular structure from a single NMR sequence (fast-PANACEA). J. Magn. Reson. 206 (1), 147–153. Kupcˇe, E., Freeman, R., John, B.K., 2006. Parallel acquisition of two-dimensional NMR spectra of several nuclear species. J. Am. Chem. Soc. 128 (30), 9606–9607. Metz, H., Mäder, K., 2008. Benchtop-NMR and MRI- a new analytical tool in drug delivery research. Int. J. Pharm. 364 (2), 170–175. Nelson, B.N., Schieber, L.J., Barich, D.H., Lubach, J.W., Offerdahl, T.J., Lewis, D.H., Heinrich, J.P., Munson, E.J., 2006. Multiple-sample probe for solid-state NMR studies of pharmaceuticals. Solid State Nucl. Magn. Reson. 29, 204–213. Pauli, G.F., Godecke, T., Jaki, B.U., Lankin, D.C., 2012. Quantitative 1H NMR. Development and potential of an analytical methods: an update. J. Nat. Prod. 75 (4), 834–851. Polenova, T., Gupta, R., Goldbourt, A., 2015. Magic angle spinning NMR spectroscopy: a versatile technique for structural and dynamic analysis of solid-phase systems. Anal. Chem. 87 (11), 5458–5469. Ramaswamy, V., Hooker, J.W., Withers, R.S., Nast, R.E., Edison, A.S., Brey, W.W., 2013. Microsample cryogenic probes: technology and applications. eMagRes. 2 (2). Thurber, K.R., Tycko, R., 2008. Biomolecular solid state NMR with magic-angle spinning at 25 K. J. Magn. Reson. 195 (2), 179–186.
Some Key Developments in NMR Spectroscopy Chapter Outline 9.1 Development of New Hardware 9.1.1 Bench-Top NMR Spectrometers 9.1.2 Earth’s Field NMR Spectrometer 9.1.3 Ultra-High Field NMR Spectrometers 9.1.4 New Liquid Nitrogen Cooled Cryogenic Probe 9.1.5 Multiple Receiver Technology
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9.2 Key Developments in Solid-State NMR Spectroscopy 422 9.2.1 Solid-State Cryogenic MAS Probes 422 9.2.2 DNP-MAS NMR Spectroscopy 424 9.3 Innovative Pulse Sequences and Novel Applications of NMR Spectroscopy 425 9.3.1 Diffusion Order Spectroscopy (DOSY) 425 9.3.2 Other Key Developments 427 References 429
NMR spectroscopy continues to evolve, in terms of improvements in hardware, as well as in designing of innovative pulse sequences for new applications. A dazzling array of recent publications, referred to in this chapter, provide an overview of the key developments in this field. Some of the topics, such as DOSY, which were not included in the earlier chapters, are also presented here. The discussion is divided into three broad categories, i.e., key progress made in solid-state NMR spectroscopy, developments in NMR hardware, and advent of innovative pulse sequences for new applications (Edger, 2013).
9.1 DEVELOPMENT OF NEW HARDWARE 9.1.1 Bench-Top NMR Spectrometers NMR spectroscopy is routinely used to support and manage a variety of academic and research activities in chemistry laboratories. Conventional NMR spectrometers are large, expensive, and often difficult to maintain. The advent of low-field bench-top NMR spectrometers, therefore, is changing the way NMR spectroscopy has been practiced for many years. With a small bench top NMR, Solving Problems with NMR Spectroscopy. http://dx.doi.org/10.1016/B978-0-12-411589-7.00009-7 Copyright © 2016 Elsevier Inc. All rights reserved.
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FIGURE 9.1 Oxford Instrument’s Pulsar™ - 60 MHz (1.4 T) NMR spectrometer, fitted with cryogen-free permanent magnet. (Reprinted with the permission of Oxford Instruments).
there is no longer any need to line up at the core NMR facility, for instance to run a quick check on the purity of a natural product or determine the status of a synthesis. Whether it is for identifying a compound quantitation, reaction monitoring, structure elucidation, or kinetics measurements, bench-top NMR enables researchers, within limits, to carry out the tasks at hand. The 45, 60, 80, and 90 MHz (∼1–2 T) bench top NMR spectrometers can provide a quick look at the NMR data, with the option to do a detailed analysis on a more powerful instrument later, if required (Fig. 9.1). Generally, a resolution of between 1 and 3 Hz can be achieved by these robust, easy to handle, and affordable spectrometers (Metz and Mäder, 2008). Bench-top NMR spectrometers are equipped with cryogen-free stable permanent magnets, made from some rare earth metals. Samarium–cobalt and neodymium magnets are particularly suitable for instruments up to 90 MHz. They operate at temperatures from room temperature to 45oC and their designs allow the instruments to be made small enough to fit on a laboratory bench, and safe to be transported for on-site analysis. They require only single phase local electrical power, fitted with a UPS system, making them especially suitable for small research laboratories in developing countries. These spectrometers are also ideal for teaching and training of NMR spectroscopy in places where funding is a major constraint. Bench-top NMR spectrometers normally use traditional 3–5 mm NMR sample tubes. The samples are dissolved in deuterated solvents, and the spectrometers can be shimmed and locked at ambient room temperature without any hassle. Because of the lower field magnets and corresponding low sensitivities, the sample requirements are relatively high, that is often not an issue in medicinal and synthetic chemistry studies. With a concentrated sample, a good signal-to-noise ratio can be obtained, and 1H-NMR spectra may be recorded in
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FIGURE 9.2 1H-NMR spectrum of ibuprofen (200 mM in CDCl3), recorded in 10 seconds on a 43 MHz bench-top NMR spectrometer (Reprinted with the permission of Magritek Limited).
a few minutes (Fig. 9.2). Recently, a large number of on-site NMR studies on archeological samples have been reported with such magnets in the literature (Capitani et al., 2012).
9.1.2 Earth’s Field NMR Spectrometer The earth’s field NMR spectrometers (EF-NMR) (Fig. 9.3) are ultra-low field NMR instruments. These spectrometers use the globally available homogenous earth’s magnetic field (0.05 T) for the detection of NMR active nuclei. The resulting Larmor frequency of 1H in the earth’s field magnet is only a couple of kHz, as compared to 60 MHz in a 1.41 T low field 60 MHz NMR spectrometer. The EF-NMR signals are also affected by both the magnetic noises in the
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FIGURE 9.3 An earth’s field NMR spectrometer (Terranova EF-NMR system by Magritek) (Reprinted with the permission of Magritek Limited).
laboratory environment, and the natural variations in the earth’s field. The very low sensitivity, the dominance of scalar-couplings, and the lack of chemical-shift dispersion have been the major bottlenecks in the use of EF-NMR spectrometers. However, such disadvantages have been overcome, to an extent, with the introduction several new methods including: 1. Introduction of electronic equipment which can compensate for the changes in ambient magnetic fields. 2. Use of a pulse sequence with only DC fields. 3. Pre-polarization of the sample by the use of a crude electromagnet with the field 350 times larger than the initial magnetization. The Pre-polarization methodology significantly increases the sensitivity and makes it possible to perform a large range of modern and sophisticated NMR experiments. While chemical shifts are important in NMR, they are normally not of significance in the earth’s field NMR instrument because of very low dispersion which results in most of them, if not all, overlapping together as one broad signal. In the absence of chemical shifts, several other spectral features, such as spin–spin multiplets (that are separated by high fields) are observed in the EFNMR. The EF-NMR spectra are, therefore, dominated by spin–spin coupling
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(J-coupling) effects. Software optimized for analyzing these spectra can provide useful information about the structure of the molecules in the sample. Efforts are being made to develop methods for chemical shift based NMR signals in the EF-NMR (Katz et al., 2012). The EF-NMR spectrometer has certain advantages, including the portability of the machine, the ability to analyze samples on-site, and its low cost. In highfield NMR spectrometers, the magnetic field inhomogeneity is compensated in many ways but this is not an issue in EF-NMR spectrometers due to the excellent inherent homogeneity of the earth’s magnetic field (at a particular location). EF-NMR spectrometers are good educational tools due to their relatively low cost and simplicity, but their research applications are severely limited because of their poor sensitivity and dispersion. Compounds containing hydrogen nuclei such as water, hydrocarbons (natural gas and petroleum), and carbohydrates (plants and animals) may be analyzed by 1H-EF-NMR.
9.1.3 Ultra-High Field NMR Spectrometers Efforts to build ultra-high field NMR spectrometers (over 1000 MHz) are underway. These include the development of a 1.2 GHz (1200 MHz) NMR spectrometer through the financial support (Euro 10 million) of the Netherlands government. Many other laboratories in US and Europe are also involved in the development of ultra-high field NMR spectrometers of 1.2 GHz or beyond. Some laboratories already have such machines, mostly custom made at site for specific purposes. Magnetic field stability with very low field drift rates, suppression of external magnetic field disturbances, and minimization of stray fields are the key challenges in the development of ultra-high field NMR spectrometers (Harrison et al., 2008).
9.1.4 New Liquid Nitrogen Cooled Cryogenic Probe The first-generation cryogenically cooled probes, as discussed in Chapters 1 and 3 (Sections 1.2.2 and 3.3.2.1), require gaseous helium as the cryogen, and they are often difficult to maintain. A new range of cryogenically cooled probes have been recently developed which use liquid nitrogen to cool the Rf coils and the preamplifier, and they provide a sensitivity gain of a factor of 2 or more over equivalent room temperature (RT) probes. The probe assembly comprises, in addition to the probe, a control unit, an automatic tuning accessory, and a liquid nitrogen vessel. These probes are now available in various settings, such as broad-band and triple resonance excitation. Their simple design, the requirement of minimum infrastructure, ease of handling and maintenance, low running cost, and most importantly affordable price make them an excellent choice for routine analysis (Ramaswamy et al., 2013). The liquid nitrogen cooled probes (Fig. 9.4) deliver a significant signal-tonoise (sensitivity) enhancement over the room temperature (RT) probes of a
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FIGURE 9.4 Liquid nitrogen cooled cryogenic Prodigy probe by Bruker Spectrospin (Reprinted with the permission of Bruker corporation).
factor of 2–3 for X-nuclei (such as 13C, 15N, 31P, etc.), although it is not as much as that obtainable from helium cooled probes that provide a sensitivity enhancement by a factor of 4. Liquid nitrogen cooled probes enable time-demanding heteronuclear correlated NMR experiments to be performed up to 10 times faster than with RT probes. They are specially suited for the study of small molecules in academia, chemical industries, and pharmaceutical sector, and can enable such laboratories to substantially increase their sample throughput.
9.1.5 Multiple Receiver Technology The use of dual receivers has been in practice in solid state NMR spectroscopy since long, as it allows rapid parallel data acquisition in each dimension. In conventional high resolution solution state NMR spectroscopy, the direct detection of only single nucleus at a time is commonly carried out. For example in COSY and TOCSY, as well as in HSQC and HMBC, 1H is detected, while in INADEQUATE, 13C nucleus is detected. As a result, NMR experiments are performed
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in sequence, one after the other. If we have the capacity to detect two or more nuclei at the same time, then we can record many experiments, parallel to each other. This has now become possible with spectrometers that incorporate two or more receivers so that they can tune into different nuclei at the same time. The availability of NMR spectrometers that incorporate multiple receivers, tuned to different nuclei, has led to the development of a new class of NMR experiments, commonly called Parallel Acquisition NMR SpectroscopY (PANSY), and Parallel Acquisition of NMR, an All-in-one Combination of Experimental Applications (PANACEA). PANSY is now used to simultaneously detect signals from up to four nuclear species, such as 1H, 2H, 13C, 15N, 19F, and 31P. Conventional COSY, TOCSY, HSQC, HMQC, and modified HMBC pulse sequences have been modified for the PANSY applications (Kupcˇ e et al., 2006). PANACEA, however, combines several standard NMR pulse sequences into a single entity. For example, PANACEA experiments with INADEQUATE, HSQC, and HMBC are designed for getting information about 13C–13C couplings, as well as direct 1H–13C and longrange 1H/13C correlations. Such parallel experiments offer a direct route to molecular structure (Kupcˇ e and Freeman, 2010). Figure 9.5 shows the PANACEA spectra, consisting of (1) INADEQUATE, and (2) multiplicity-edited HSQC spectra of cholesterol, recorded in parallel in 23-min measurements.
FIGURE 9.5 A PANACEA experiment exhibiting (a) INADEQUATE spectrum, and (b) multiplicity-edited HSQC spectrum (concatenated DEPT-HSQC) in CDCl3 (1 M), recorded in parallel in 23 minutes. Two responses in (a) are correlated with signals that lie well outside this spectral window. (b) Multiplicity-edited HSQC spectrum shows red peaks that belong to CH2 groups, while the black peaks represent CH and CH3 groups. Four-fold aliasing was used to reduce the experimental duration. Sweep width = 20 kHz; sweep width (1) = 5.2 kHz with 4 k data points and two scans per increment (Reprinted from Kupcˇ e and Freeman (2010). J. Magn. Reson. 206, 147–153, with permission from Academic Press, Inc.).
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The PANSY and PANACEA experiments allow unambiguous structure elucidation of small organic molecules from a single measurement, and include an internal field/frequency correction routine. These experiments do not require the conventional NMR lock system, and can be obtained in pure liquids. Furthermore, long-range spin–spin couplings can be extracted from the PANACEA spectra, and used for three-dimensional structure refinement. These quick NMR measurements are especially suitable for samples where long-term stability is an issue. Similarly, the recording of several NMR spectra in parallel, rather than in series, leads to a major saving in instrument time. The PANSY and PANACEA experiments are currently not optimized for structure determination of large molecules. However, this is one area in which these innovative pulse sequences are likely to have impact in the future, simply because conventional biomolecular NMR spectroscopic measurements in series, one by one, can take days if not weeks of NMR instrument time.
9.2 KEY DEVELOPMENTS IN SOLID-STATE NMR SPECTROSCOPY Major recent developments in NMR spectroscopy have largely taken place in solid-state NMR and in NMR-based imaging. Both subjects are beyond the scope of this book. However, some key developments in the field of solid-state NMR spectroscopy are presented here as we feel that they will soon find extensive uses in structural chemistry, structural biology, and pharmaceutical industrial applications.
9.2.1 Solid-State Cryogenic MAS Probes Magic angle spinning (MAS) probes have long been in use in solid-state NMR spectroscopy. Most of these probes function at room temperature. Recently, cryogenic MAS solid-state probes have been constructed which provide a sensitivity enhancement by cooling the sample to a temperature of 30 K. They also have excellent spinning stability which allows the recording of two-dimensional (2D) NMR spectra. This development should not be confused with the cryogenically cooled probes in solution state NMR spectroscopy, as in those probes, the sensitivity gain is based on cooling of the detection coil and circuitry, and not of the sample itself. In cryogenic MAS probes, the solid sample is cooled with liquid gas, while room temperature nitrogen is used as the MAS bearing and drive gas (Thurber and Tycko, 2008). This probe can provide only medium speed magic angle spinning due to the relatively large sample volume (40–80 mL). The gain in sensitivity in these probes is based on the fact that by lowering the temperature of the sample, one can increase the spin polarization. At a temperature 1 K, the nuclear spin polarization at thermal equilibrium is proportional to 1/T, where T is the temperature of the sample. Thus if we cool down the sample to
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25 K, from the room temperature (∼295 K), a 12-fold increase in spin polarization is expected, leading to a 12-fold increase in sensitivity (Fernandez and Pruski, 2012). Substantial lowering of temperature in solution-state NMR spectroscopy for sensitivity enhancement is not practical due to a number of factors, including temperature gradient issues, precipitation of solute particles, and solidification or icing of deuterated solvents. It is, therefore, more logical to use other methods, including lowering of the temperature of the detection circuits to reduce the noise level and thus increase the S/N ratio. However, in solid-state NMR spectroscopy, modest lowering of the temperature of the sample in MAS probe is an excellent approach for the enhancement of S/N ratio. Figure 9.6 shows 13 C-NMR (CP-MAS) proton decoupled spectra of powdered 13C-labeled valine at 300 and 31 K, recorded using a cryogenic MAS probe (revolution NMR LLC, USA). Over 10-fold enhancement in sensitivity is clearly seen (Polenova et al., 2015; Thurber and Tycko, 2008).
FIGURE 9.6 13C CP-MAS spectra of powdered 13C-labeled valine powder (9 mg) in one scan, (a) 13C CP-MAS NMR spectrum at 300 K, and (b) 13C CP-MAS NMR spectrum at 31 K (Reprinted with the permission of Revolution NMR LLC, USA).
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9.2.2 DNP-MAS NMR Spectroscopy The development of dynamic-MAS NMR spectrometers represents one of the most important advances in the field of NMR spectroscopy. The process of Dynamic Nuclear Polarization (DNP) provides a dramatic enhancement in sensitivity, both in liquid- and solid-state NMR studies. However, it is in the solidstate where DNP has found the greatest use. As described in Section 3.6.1, the large electron polarization of radical molecules can be transferred to a nuclear spin by using in situ microwaves for low-temperature state NMR spectroscopy. These polarization transfers between electrons and coupled nuclei can occur spontaneously through electron-nuclear cross relaxation, and/or spin-state mixing among the electrons and nuclei. The DNP-MAS NMR spectrometers are now available in the range of 200–800 MHz, and they involve major developments in probe design and microwave technologies (gyrotron) (Barnes et al., 2012). The DNP-enhanced solid-state NMR experiments at high field can yield sensitivity enhancements of up to 80-fold (Fig. 9.7). These systems are equipped with unique high power microwave sources, i.e., an easy-to-use software-controlled high-power gyrotron (a vacuum electronic device, capable of generating
FIGURE 9.7 A DNP solid-state NMR spectrometer (Reprinted with the permission of Bruker Corporation).
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high-power, high-frequency THz electromagnetic radiations, which serves as the microwave source). Optimum beam propagation to the sample is ensured by microwave transmission lines. Low-temperature (cryogenic) MAS probe technology (∼100 K), with cold spinning gas and built-in waveguide, are now available for a range of NMR systems, such as for 400, 600, and 800 MHz spectrometers.
9.3 INNOVATIVE PULSE SEQUENCES AND NOVEL APPLICATIONS OF NMR SPECTROSCOPY 9.3.1 Diffusion Order Spectroscopy (DOSY) The introduction of second frequency dimension gave birth to 2D NMR spectroscopy, as described previously. A logical extension of the idea of the second dimension in NMR spectroscopy was the advent of additional NMR dimensions that depend on other molecular properties, such as size, shape, mass, and charge. In DOSY spectroscopy, the mixtures of organic molecules are analyzed based on molecular diffusion of each component. Very often chemists need to handle mixtures of organic compounds. They can be mixtures of compounds in extracts of medicinal plants, reaction mixtures containing several products, etc. Historically, mixture analysis in organic chemistry was generally performed in two steps. In the first step, the various constituents are separated using chromatographic techniques, including high performance liquid chromatography or gas chromatography, while in the second step, these constituents are individually characterized by using various spectroscopic techniques. The advent of so-called hyphenated techniques such as LC-MS and GC-MS as well as LC-NMR made it possible to separate and analyze the individual constituents of complex mixtures in one step in a timeefficient manner. The LC-NMR technique suffers from a number of limitations, such as the need of setting up of ideal separation conditions for HPLC and issues of solubility in the appropriate deuterated solvent. In the last decade, the Diffusion Order SpectroscopY (DOSY) NMR technique has been developed for the analysis of mixtures of organic compounds. The method relies on the different rates of chemicals through a solution. These rates are directly related to the physical properties of the various components making up the mixture. DOSY NMR does not require the physical separation of the components in a mixture, and provides chemical shift information in situ. This has made DOSY NMR a powerful technique with wide applications. The DOSY spectrum (Fig. 9.9) separates the NMR signals of different compounds in a mixture according to their diffusion coefficients. It is a 2D NMR experiment in which the information is spread in two dimensions. A series of spin echo spectra is recorded with different pulsed field gradient strengths, and the resulting signal decays are analyzed to extract a set of diffusion coefficients
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with which to synthesize the diffusion domain (diffusion dimension) of the DOSY spectrum. The molecules, as a liquid or in the solution state, are in constant motion. This includes translational motion (Brownian motion) which provides a mechanism by which the diffusion process takes place. Brownian motion is that random motion of molecules that occurs as a consequence of their absorption of heat. Molecules will diffuse from volumes of high concentration to low concentration; this happens because the molecules are in constant random (Brownian) motion, and they tend to bump into each other more if they are in more concentrated areas. Hence, they tend to move away from each other through diffusion. The diffusion depends on many physical parameters, including molecular weight, size and shape of the molecule, and the temperature and viscosity of solution. Assuming a spherical size of the molecule, the diffusion coefficient D is described by Eq. 9.1. kT (9.1) D= 6πη rS where k is the Boltzmann constant, T is the temperature, η is the viscosity of the liquid, and rS is the (hydrodynamic) radius of the molecule. Pulse gradient NMR spectroscopy is used to measure the translational diffusion of molecules. By the use of gradients, the molecules can be spatially labeled (marked depending on their position in the sample tube). If they move, after the initial labeling, during the diffusion time ∆, their new position is again decoded by using the second gradient. This difference in diffusion time directly affects the NMR signal intensity or signal strength. This simple theory forms the basis of differentiating different solutes in a mixture. The basic pulse sequence of DOSY involves the characterization of molecular diffusion by pulsed field gradient spin-echo (PGSE). The PGSE constructs a 2D plot where the x-axis contains the chemical shifts of 1H (in ppm), while the y-axis gives the diffusion coefficient D of each signal. Since diffusion coefficients of various components are different, one can record separate 1H-NMR spectra for each compound in a mixture. There are several basic sequences developed to map the diffusion rates of individual components in a complex mixture. In its simplest form, the magnetization is excited with a 90° Rf pulse, and then dispersed using a magnetic field gradient pulse. After a period of ∆/2, an 180° Rf pulse is applied to invert the dispersed magnetization. After a period ∆, a second gradient pulse is applied to refocus the signal. The PGSE pulse sequence is presented in Fig. 9.8. The DOSY NMR experiments are also used for the measurement of diffusion rates of different constituents in a mixture. Figure 9.9 shows the DOSY spectrum of a mixture of three components, cholesterol, aspirin, and phenol. Each line, horizontal to the diffusion axis, presents a separate 1H-NMR spectrum of each component, which is also shown in 1D slices.
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FIGURE 9.8 Basic pulse sequences for the measurement of molecular diffusion based on spinecho. (a) Pulse field gradient spin-echo, and (b) PFG stimulated-echo. The molecular diffusions are measured during the delay period ∆ with increasing gradient strengths.
9.3.2 Other Key Developments In addition to developments mentioned above, there are several other interesting techniques that have been developed which are presented below: 1. Development of multiple-sample solid-state magic-angle spinning (MAS) probes which can simultaneously acquire NMR spectra of seven or more samples. They can substantially increase throughput of solid-state NMR spectroscopy (Nelson et al., 2006). 2. A “moving-tube” method has been developed to circumvent problems associated with long T1 relaxation times. This utilizes a 5 mm NMR tube (1.5 m long), filled with a large quantity of sample solution. A motor physically moves this long NMR tube vertically after each pulse-acquisition (scan), and thus removes (moves away) the excited spins from the coil, and introduce relaxed spins (so called “fresh sample”) in front of the detection coil. This method has been applied in recording of variant COSY spectrum (Donovan et al., 2012). 3. The methodology for quantitative 1H-NMR (qHNMR) is continuously evolving, expanding its applications to various fields. A major challenge is to accurately and most comprehensively analyze complex mixtures (natural
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FIGURE 9.9 (a) DOSY NMR spectrum of a mixture of three compounds. (b) Each slice corresponds to the regular 1H-NMR spectrum of one component of the mixture (cholesterol, aspirin, and phenol in this case). The diffusion rate of various constituents can be obtained from the DOSY spectrum.
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products, environmental pollutants, biological fluids, toxins, etc.) containing low-level analytes. For this purpose, standardization, calibration, rapid identification, and computational methods are regularly reported in a large number of publications (Pauli et al., 2012).
REFERENCES Barnes, A.B., Markhasin, E., Daviso, E., Michaelis, V.K., Nanni, E.A., Jawla, S.K., Mena, E.L., DeRocher, R., Thakkar, A., Woskov, P.P., Herzfeld, J., Temkin, R.J., Griffin, R.G., 2012. Dynamic nuclear polarization at 700 MHz/460 GHz. J. Magn. Reson. 224, 1–7. Capitani, D., Di Tullio, V.D., Porietti, N., 2012. Nuclear magnetic resonance to characterize and monitor cultural heritage. Prog. Nucl. Magn. Reson. Spectrosc. 64, 29–69. Donovan, K.J., Allen, M., Martin, R.W., Shaka, A.J., 2012. Improving the double quantum filtered COSY experiment by “Moving Tube” NMR. J. Magn. Reson. 219, 41–45. Edger, M., 2013. Physical methods and techniques: NMR spectroscopy. Ann. Rep. Prog. Chem. Sect. B: Org. Chem. 109, 256–274. Fernandez, C., Pruski, M., 2012. Solid State NMR. Probing quadrupolar nuclei by solid-state NMR spectroscopy: recent advances. Top. Curr. Chem. 306, 119–188. Harrison, R., Bateman, R., Brown, J., Domptail, F., Friend, C.M., Ghoshal, P., King, C., Van der Linden, A., Melhem, Z., Noonan, P., Twin, A., Field, M., Hong, S., Parrell, J., Zhang, Y., 2008. Development trends in high field magnet technology. IEEE. Trans. Appl. Supercond. 18 (2), 540–543. Katz, I., Shtirberg, L., Shakour, G., Black, A., 2012. Earth field NMR with chemical shift spectral resolution: Theory and proof of concept. J. Magn. Reson. 219, 13–24. Kupcˇe, E., Freeman, R., 2010a. High-resolution NMR correlation experiments in a single measurement (HR-PANACEA). Magn. Reson. Chem. 48 (5), 333–336. Kupcˇe, E., Freeman, R., 2010b. Molecular structure from a single NMR sequence (fast-PANACEA). J. Magn. Reson. 206 (1), 147–153. Kupcˇe, E., Freeman, R., John, B.K., 2006. Parallel acquisition of two-dimensional NMR spectra of several nuclear species. J. Am. Chem. Soc. 128 (30), 9606–9607. Metz, H., Mäder, K., 2008. Benchtop-NMR and MRI- a new analytical tool in drug delivery research. Int. J. Pharm. 364 (2), 170–175. Nelson, B.N., Schieber, L.J., Barich, D.H., Lubach, J.W., Offerdahl, T.J., Lewis, D.H., Heinrich, J.P., Munson, E.J., 2006. Multiple-sample probe for solid-state NMR studies of pharmaceuticals. Solid State Nucl. Magn. Reson. 29, 204–213. Pauli, G.F., Godecke, T., Jaki, B.U., Lankin, D.C., 2012. Quantitative 1H NMR. Development and potential of an analytical methods: an update. J. Nat. Prod. 75 (4), 834–851. Polenova, T., Gupta, R., Goldbourt, A., 2015. Magic angle spinning NMR spectroscopy: a versatile technique for structural and dynamic analysis of solid-phase systems. Anal. Chem. 87 (11), 5458–5469. Ramaswamy, V., Hooker, J.W., Withers, R.S., Nast, R.E., Edison, A.S., Brey, W.W., 2013. Microsample cryogenic probes: technology and applications. eMagRes. 2 (2). Thurber, K.R., Tycko, R., 2008. Biomolecular solid state NMR with magic-angle spinning at 25 K. J. Magn. Reson. 195 (2), 179–186.