Spin Properties of Nuclei

1. NMR Spectroscopy Nuclear Magnetic Resonance Spectroscopy Spin Properties of Nuclei Nuclear spin may be related to the nucleon composition of a nucleus in the following manner: Odd mass nuclei (i.e. those having an odd number of nucleons) have fractional spins. Examples are  I = 1/2 ( 1H, 13C, 19F ), I = 3/2 ( 11B ) & I = 5/2 ( 17O ).  Even mass nuclei composed of odd numbers of protons and neutrons have integral spins. Examples are  I = 1 ( 2H, 14N ).  Even mass nuclei composed of even numbers of protons and neutrons have zero spin ( I = 0 ). Examples are  I = 0 ( 2C, and 16O). • Spin 1/2 nuclei have a spherical charge distribution, and their nmr behavior is the easiest to understand. • Other spin nuclei have non-spherical charge distributions and may be analyzed as prolate or oblate spinning bodies. • All nuclei with non-zero spins have magnetic moments (μ), but the non-spherical nuclei also have an electric quadruple moment (eQ). • Some characteristic properties of selected nuclei are given in the following table.

2. 1.A spinning charge generates a magnetic field, as shown by the animation on the right. The resulting spin-magnet has a magnetic moment (μ) proportional to the spin. 2. In the presence of an external magnetic field (B0), two spin states exist, +1/2 and-1/2.The magnetic moment of the lower energy +1/2 state is alligned with the external field, but that of the higher energy -1/2 spin state is opposed to the external field. Note that the arrow representing the external field points North. 3. The difference in energy between the two spin states is dependent on the external magnetic field strength, and is always very small. The following diagram illustrates that the two spin states have the same energy when the external field is zero, but diverge as the field increases. At a field equal to Bx a formula for the energy difference is given (remember I = 1/2 and μ is the magnetic moment of the nucleus in the field).

3. magnetic moment μ The model of a spinning nuclear magnet aligned with or against an external magnetic field (for I = 1/2 nuclei) must be refined for effective interpretation of nmr phenomena. Just as a spinning mass will precess in a gravitational field (a gyroscope), the magnetic moment μ associated with a spinning spherical charge will precess in an external magnetic field. In the following illustration, the spinning nucleus has been placed at the origin of a cartesian coordinate system, and the external field is oriented along the z-axis. The frequency of precession is proportional to the strength of the magnetic field, as noted by the equation: ωo = γBo. The frequency ωo is called the Larmor frequency and has units of radians per second. The proportionality constant γ is known as the gyromagnetic ratio and is proportional to the magnetic moment (γ = 2pm/hI). Some characteristic γ's were listed in a preceding table of nuclear properties. A Spinning Gyroscopein a Gravity Field A Spinning Chargein a Magnetic Field

4. If rf energy having a frequency matching the Larmor frequency is introduced at a right angle to the external field (e.g. along the x-axis), the precessing nucleus will absorb energy and the magnetic moment will flip to its I = _1/2 state. This excitation is shown in the following diagram. Note that frequencies in radians per second may be converted to Hz (cps) by dividing by 2π. Net Macroscopic Magnetization of a Sample in an External Magnetic Field Bo Excitation by RF Energy and Subsequent Relaxation

5. THE MAGNET and rf SOURCE • The strength of the magnetic field, Ho, determines the Larmor frequency of any nucleus. • The stronger the magnetic field, the better the line separation of chemically shifted nuclei on the frequency scale. • Since coupling constants remain unaffected by the magnetic field strength, multiplet overlapping decreases with increasing field strength, and homo-nuclear couplings become small compared with chemical shift differences. • The relative population of the lower-energy spin level increases with increasing field, leading to a corresponding increase in the sensitivity of the NMR experiment. • For high-resolution work the magnetic field over the entire sample volume must be maintained uniform in space and time. • Effective homogeneity of the field is promoted by • the use of large pole pieces composed of a very homogeneous alloy, • (2) The polishing of pole faces to optical tolerances, and • (3) The use of a narrow pole gap—that is, a smaller cross section and consequently a • compromise with decreased sensitivity. • Shim coils are also used to iron out field inhomogeneities. • These shim coils are manufactured of special material without soldering and encased in epoxy resin. • They are mounted permanently in the pole cap covers of the magnet. • Since inhomogeneities of a magnetic field take the form of gradients, corrections are made by creating small corrective fields that oppose these gradients. Corrections are available along the x-, y-, and z-axes, in the xy- and yz-planes, and for curvature in the z direction.

6. Permanent magnets are simple and inexpensive to operate but require extensive shielding and must be thermostatted to ± 0.001 °C. • Commercial units that use electromagnets or permanent magnets operate at 14.09, 21.14, or 23.49 kG. • An electromagnet requires elaborate power supplies and cooling systems, but these disadvantages are offset by the opportunity to use different field strengths to disentangle chemical shifts from multiplet structures and to study different nuclei. • Cryogenic superconducting solenoids produce homogeneous fields at 51.7 and 70.5 kG and, as a result, make possible high-resolution experiments with 220- and 300-MHz rf fields, respectively. • Some instruments are now available with 93.9- and 117.4-kG or even higher fields and operate at 400 MHz and 500 MHz and higher frequencies for proton resonances. • In the cryogenic solenoids many turns of copper-clad, niobium- tantalum superconducting wire are immersed in a Dewar flask that holds liquid helium. • The Dewar is surrounded by another that holds liquid nitrogen. • Aided by shim coils positioned in the probe, the 220-MHz spectrometer achieves a resolution of 1.1 Hz and a 55:1 signal-to-noise ratio for the ethyl-benzene quartet in a 100 solution. • The 300-MHz instrument achieves a resolution of 1.5 Hz and a 65:1 ratio under the same conditions.

7. PROBE UNIT • The probe unit is the exciting and sensing element of the spectrometer system. • It is inserted between the pole faces of the magnet in the xy-plane of the magnet-air gap by an adjustable probe holder. • The probe unit houses the sample, the rf transmitter(s), output attenuator, receiver, and phase-sensitive detector. • The sample is contained in a cylindrical, thin-walled, precision-bore glass tube that has an outer diameter of 5 mm. • To average small magnetic field inhomogeneities in the xz-plane, an air-bearing turbine rotates the sample tube at a rate of 20 - 40 or so revolutions per sec. • This spinning produces sidebands in the spectrum because the NMR peaks are modulated at the spinning frequency. • Since most NMR samples, except for 13C solids and for the wide-line technique, are liquid solutions, the sample tube is filled until the length/diameter ratio is about five, which approximates that of an infinite cylinder. • For minute samples, a tube with a capillary bore that widens to a spherical cavity at the position of the rf coil is used.

8. Two probe designs are used. (1) A single-coil probe has one coil that not only supplies the rf radiation to the sample but also serves as a part of the detector circuit for the NMR absorption signal. To detect the resonance absorption and to separate the NMR signal from the imposed rf field, a rf bridge is used. The exciting signal is balanced against an equal-amplitude reference, with the modulation appearing as bridge unbalance and extractable in that form. Alternatively, the signal can be amplified and then subjected to diode detection to extract the resonance spectrum. (2) Crossed-coil (nuclear induction) probes have two coils, one for irradiating the sample and a second coil mounted orthogonally for signal detection. The irradiating coil is split into halves with the sample inserted between. This coil is oriented with its axis perpendicular to the magnetic field (that is, along the x-axis). The detector coil is wound around the sample tube with its axis (the y-axis) perpendicular to both the field H0 (z-axis) and the rf field (H1) axis. Since magnetic resonance produces a net magnetization in the xy-plane, a current is generated at resonance in the receiver coils from an indirect coupling between the rf field and receiver coils, with the coupling produced by the sample itself. This design permits selective pickup of the resonance signal while virtually excluding the applied rf field.

9. If the 1H spectrum is being studied, an amount between a few micrograms and a few milligrams of sample is dissolved in a solvent such as CCL, or CS2 or a solvent that has had all the protons replaced by deuterium atoms. • Chloroform-d, acetone-d6, and benzene-d6 are commonly used. • The deuterium serves two purposes. • (1)It replaces hydrogen nuclei that would otherwise generate a background solvent signal and would overwhelm the signals from the sample. • In addition, the magnetic resonance response of the deuterium nuclei is used to lock the ratio of the magnetic field and frequency of the instrument over long periods of time. • If the 13C spectrum is desired, an amount of sample between a few milligrams and a few hundred milligrams is dissolved in one of the same deuterated solvents used for proton NMR. • The deuterium in this case serves only the purpose of locking the spectrometer.

10. DETECTORS • Detectors are of two kinds: • the single coil type and • the cross coil type. Figure shows the former type. • The capacitor is varied to tune the rf with the external magnetic field so that the condition of resonance is Produced for making the absorption energy possible. • This effectively measures the coil resistance and the voltage drop across the capacitor provides a measure of the power absorbed by the sample from the source. a • The detector can detect the resistive part or reactive part of the coil in parallel with the capacitor. • These are shown in Figs (a) and (b). The curve in Fig. (a) is known as the resonance absorption curve and that in Fig. (b) is the resonance dispersion curve. b

11. The basic principle of obtaining a resonance spectrum in the crossed coil type NMR spectrometer is evident from the set-up shown in Fig.(a). • The rf generator coils are split into two halves to hold the sample in between so that rf field falls perpendicular to the coil planes. • Over the steady magnetic field an alternating one with sweeping flux density ranging from mG to G ranges is superimposed. This is allowed periodicaliy to change the field. • Another technique of quantitative study with the resonance absorption system is to use an integrator before the recorder. • Actually, two recorders one with and the other without an integrator are provided and the frequency is sweeped over a wide range to obtain both the absorption curves of all, or most of the major parts of the constituents and the integral curves. • From the absorption curves the identifications are made and from the integral curve the percentages with a reference are evaluated. An example is shown in Fig.(b). • The method is shown to be accurate to within 10 parts in a million. • As mentioned already, to ascertain homogeneity of the magnetic field, it is customary to spin the sample, however, at a large revolution rate of about 4000 rpm by driving it by air-driven turbine. • When an alternate magnetic field is used for scanning, the detection is shown to be more precise. a b

12. Continuous wave (CW) spectroscopy • In its first few decades, nuclear magnetic resonance spectrometers used a technique known as continuous-wave spectroscopy (CW spectroscopy). • Although NMR spectra could be, and have been, obtained using a fixed magnetic field and sweeping the frequency of the electromagnetic radiation, this more typically involved using a fixed frequency source and varying the current (and hence magnetic field) in an electromagnet to observe the resonant absorption signals. • This is the origin of the anachronistic, but still common, "high" and "low" field terminology for low frequency and high frequency regions respectively of the NMR spectrum. Block diagram of continuous wave NMR spectermeter • CW spectroscopy is inefficient in comparison to Fourier techniques as it probes the NMR response at individual frequencies in succession. • As the NMR signal is intrinsically weak, the observed spectra suffer from a poor signal-to-noise ratio. • This can be mitigated by signal averaging i.e. adding the spectra from repeated measurements. • While the NMR signal is constant between scans and so adds linearly, the random noise adds more slowly - as the square-root of the number of spectra. • Hence the overall ratio of the signal to the noise increases as the square-root of the number of spectra measured.

13. Fourier transform spectroscopy • Most applications of NMR involve full NMR spectra, that is, the intensity of the NMR signal as a function of frequency. • Early attempts to acquire the NMR spectrum more efficiently than simple CW methods involved irradiating simultaneously with more than one frequency. • A revolution in NMR occurred when short pulses of radio-frequency were used (centered at the middle of the NMR spectrum). Block diagram of pulsed Fourier transform NMR spectrometer. • In simple terms, a short square pulse of a given "carrier" frequency "contains" a range of frequencies centered about the carrier frequency, with the range of excitation (bandwidth) being inversely proportional to the pulse duration • (the Fourier transform (FT) of an approximate square wave contains contributions from all the frequencies in the neighborhood of the principal frequency). • The restricted range of the NMR frequencies made it relatively easy to use short (millisecond to microsecond) radiofrequency (RF) pulses to excite the entire NMR spectrum. • Applying such a pulse to a set of nuclear spins simultaneously excites all the single-quantum NMR transitions. • In terms of the net magnetization vector, this corresponds to tilting the magnetization vector away from its equilibrium position (aligned along the external magnetic field). • The out-of-equilibrium magnetization vector precesses about the external magnetic field vector at the NMR frequency of the spins.

14. This oscillating magnetization vector induces a current in a nearby pickup coil, creating an electrical signal oscillating at the NMR frequency. This signal is known as the free induction decay (FID) and contains the vector-sum of the NMR responses from all the excited spins. In order to obtain the frequency-domain NMR spectrum(NMR absorption intensity vs. NMR frequency) this time-domain signal (intensity vs. time) must be FTed. Fortunately the development of FT NMR coincided with the development of digital computers and Fast Fourier Transform algorithms. FT methods can be applied to many types of spectroscopy; see the general article on Fourier transform spectroscopy. Richard R. Ernst was one of the pioneers of pulse (FT) NMR and won a Nobel Prize in chemistry in 1991 for his work on FT NMR and his development of multi-dimensional NMR. Multi-dimensional NMR Spectroscopy The use of pulses of different shapes, frequencies and durations in specifically-designed patterns or pulse sequences allows the spectroscopist to extract many different types of information about the molecule. Multi-dimensional nuclear magnetic resonance spectroscopy is a kind of FT NMR in which there are at least two pulses and, as the experiment is repeated, the pulse sequence is varied. In multidimensional nuclear magnetic resonance there will be a sequence of pulses and, at least, one variable time period.

15. In three dimensions, two time sequences will be varied. • In four dimensions, three will be varied. • There are many such experiments. In one, these time intervals allow (amongst other things) magnetization transfer between nuclei and, therefore, the detection of the kinds of nuclear-nuclear interactions that allowed for the magnetization transfer. • Interactions that can be detected are usually classified into two kinds. They are  • through-bond interactions • through-space interactions, • The latter usually being a consequence of the nuclear Overhauser effect. Experiments of the nuclear Overhauser variety may be employed to establish distances between atoms, as for example by 2D-FT NMR of molecules in solution. • Although the fundamental concept of 2D-FT NMR was proposed by Professor Jean Jeener from the Free University of Brussels at an International Conference, this idea was largely developed by Richard Ernst who won the 1991 Nobel prize in Chemistry for his work in FT NMR, including multi-dimensional FT NMR, and especially 2D-FT NMR of small molecules. • Multi-dimensional FT NMR experiments were then further developed into powerful methodologies for studying bio-molecules in solution, in particular for the determination of the structure of biopolymers such as proteins or even small nucleic acids. • Kurt Wüthrich shared (with John B. Fenn) in 2002 the Nobel Prize in Chemistry for his work in protein FT NMR in solution.

16. Solid-state NMR spectroscopy • This technique complements biopolymer X-ray crystallography in that it is frequently applicable to biomolecules in a liquid or liquid crystal phase, whereas crystallography, as the name implies, is performed on molecules in a solid phase. • Though nuclear magnetic resonance is used to study solids, extensive atomic-level bio-molecular structural detail is especially challenging to obtain in the solid state. • There is little signal averaging by thermal motion in the solid state, where most molecules can only undergo restricted vibrations and rotations at room temperature, each in a slightly different electronic environment, therefore exhibiting a different NMR absorption peak. • Such a variation in the electronic environment of the resonating nuclei results in a blurring of the observed spectra—which is often only a broad Gaussian band for non-quadrupolar spins in a solid- thus making the interpretation of such "dipolar" and "chemical shift anisotropy" (CSA) broadened spectra either very difficult or impossible. • Professor Raymond Andrew at Nottingham University in UK pioneered the development of high-resolution solid-state nuclear magnetic resonance. • He was the first to report the introduction of the MAS (magic angle sample spinning; MASS) technique that allowed him to achieve spectral resolution in solids sufficient to distinguish between chemical groups with either different chemical shifts or distinct Knight shifts. • In MASS, the sample is spun at several kilohertz around an axis that makes the so-called magic angleθm (which is ~54.74°, where cos2θm = 1/3) with respect to the direction of the static magnetic field B0; as a result of such magic angle sample spinning, the chemical shift anisotropy bands are averaged to their corresponding average (isotropic) chemical shift values.

17. The above expression involving cos2θm has its origin in a calculation that predicts the magnetic dipolar interaction effects to cancel out for the specific value of θm called the magic angle. • One notes that correct alignment of the sample rotation axis as close as possible to θm is essential for cancelling out the dipolar interactions whose strength for angles sufficiently far from θm is usually greater than ~10 kHz for C-H bonds in solids, for example, and it is thus greater than their CSA values. • There are different angles for the sample spinning relative to the applied field for the averaging of quadrupole interactions and paramagnetic interactions, correspondingly ~30.6° and ~70.1° • A concept developed by Sven Hartmann and Erwin Hahn was utilized in transferring magnetization from protons to less sensitive nuclei (popularly known as cross-polarization) by M.G. Gibby, Alex Pines and John S. Waugh. • Then, Jake Schaefer and Ed Stejskal demonstrated also the powerful use of cross-polarization under MASS conditions which is now routinely employed to detect low-abundance and low-sensitivity nuclei.

18. Key Components of an NMR Spectrometer There is more to an NMR Spectrometer than meets the eye. It is a complex system integrating several technologies into an analytically powerful, information-rich system. Key parts of the system are the computer workstation and control software, the NMR console with its advanced radio frequency (RF) electronics, the superconducting magnet, and the NMR probe.

19. An NMR spectrometer consists of several parts or components, each of which is critical to its operation. Depending on the specific experiment, the process can take weeks and even months, but this will give a general idea of how it all comes together: • The test sample is put in a long slender glass tube with 1" of liquid. • Samples can be less than 1/1000 cubic inch of the gas, liquid, or solid. • The sample is lowered into the bore, a hollow tube at the center of the magnet. • The sample tube is spun with an air jet, to produce a more uniform sample for scanning. • The computer workstation and software direct the experiment from start to finish. • The experiment begins with the computer sending directions to the spectrometer console. • The NMR console triggers split-second bursts, or pulses, of RF energy that are precisely sequenced to excite the sample in the probe and cause the atomic nuclei to resonate. • The high-power RF energy pulses are sent to the NMR probe, the “antenna” that provides the radio frequency (RF) link between the sample and the instrument electronics. • The superconducting magnet provides a strong, homogenous magnetic field that can be as much as 200,000 times stronger than the earth’s magnetic field. • The natural magnets in the nuclei line up with the powerful NMR magnet, similar to iron filings aligning with the magnetic field of a toy magnet. • The RF pulses from the NMR console excite the atoms in the sample, making the nuclei “wobble” or resonate. • As soon as the RF signal stops, the nuclei return to their natural and more comfortable state. • As the nuclei relax, the NMR probe receives a very weak RF resonance response back from the sample and transmits it to the NMR console for amplification.

20. The NMR console amplifies the faint returning signals over 1,000,000 times before sending them to the computer workstation for analysis. • The computer workstation stores and processes the NMR data using complex software and creates a unique “spectrum” for the sample, showing the atomic structure of each molecule as well as the dynamics of its chemical environment for interpretation by the scientist. NMR Magnet  The superconducting magnet provides a strong, extremely homogenous magnetic field into which the sample (liquid or solid) is placed. Depending on the magnet, the field strength can range from 200 MHz to 900 MHz. This strength is typically specified in terms of the resonance frequency for the hydrogen atom expressed in megahertz (MHz). The 900 MHz NMR magnet is 10 times stronger than the most powerful magnetic resonance imaging (MRI) system used in hospitals and 200,000 times stronger than the earth’s magnetic field The NMR magnet is cooled to –452°F with liquid helium. That’smore than six times colder than Antarctica. The helium is kept cold by liquid nitrogen, which “weighs in” at -321°F.

21. NMRConsole • The NMR console’s job is multi-faceted and includes acting as transmitter, receiver, and amplifier. • It’s sophisticated RF electronics send split-second pulses that are precisely delivered to excite the sample in the probe. • The NMR console also receives the faint return signals from the sample and then amplifies them one million times so they can be read by the computer workstation. NMR Probe • The NMR probe is the “antenna” that provides the RF link between the sample and the instrument electronics. • Inserted in the magnet, the probe holds the sample at the center of the magnetic field. • It bombards the sample with RF energy and then receives the very weak RF responses from the sample, which it sends on to the console. • In recent years, Varian engineers have developed a unique NMR probe where the internal electronics of the probe are chilled to approximately -250 degrees C (comparable to temperatures on the surface of Pluto).   • These Cold Probes offer radically improved sensitivity and thus reduced time to collect NMR data.  An NMR spectrometer requires less than1/1000 cubic inch of sample, whether it is a gas, liquid or solid.

22. Computer Workstation and Control Software • It is the “brain” of the operations, the computer workstation and complex software directs the NMR experiment from start to finish. • NMR spectrometers make heavy use of computers and software, both for the control of the various RF pulses, as well as for storing and processing the NMR data. • NMR signals are subjected to complex digital signal processing algorithms, including the Fourier Transform, to convert the NMR information into a form that is easily interpreted by the end user. • The signals are displayed as a series of peaks, or spectrum, on the workstation’s monitor.

23. World’s First 1000 MHz NMR Spectrometer Now Offers New Research Capabilities to European Scientists, Following its Successful Installation at CRMN in Lyon, France TheAVANCE 1000 system incorporates a record-breaking 23.5 Tesla UltraStabilized™ superconducting magnet, and offers unparalleled research opportunities, both to the CRMN (Centre de Resonance Magnétique Nucléaire à Très Hauts Champs (CRMN) and to other French and European scientists who will access this unique facility. For the entire community of thousands of scientists using modern NMR around the world today, this represents a landmark moment.

24. CryoProbe • The new QNP CryoProbe, enables multipurpose NMR measurements of four different nuclei -- fluorine, phosphorus, carbon and hydrogen.  • This unique 5mm, general-purpose, cryogenically cooled probe is automated to easily switch between these nuclei, eliminating the need to change probes.  • Fluorine, phosphorus, carbon and hydrogen are present individually or together in a majority of organic, biological and inorganic compounds.  • The direct observation of these nuclei can provide quantitative information in addition to small molecule structural data.  • This new probe provides the Cryo-Probe equivalent of one of Bruker Bio-Spin’s most popular conventional probes. • The most significant advantage of this new probe is that it enables more complete characterization of molecules that contain any of these four nuclei.  • The new QNP Cryo-Probe is available for Bruker Bio-Spin Avance 400 and 500 MHz NMR Spectrometer. • Cryo-Probes improve the signal-to-noise ratio by three to four times, as compared to non-cryogenically cooled probes.  • As a result, they are capable of measuring lower concentrations and enable experiments to be completed as much as 16 times faster.  • While traditional Cryo-Probes are aimed at biological research, this new QNP CryoProbe is designed for applications in organic and inorganic chemistry.  Features : 5 mm NMR Cryo-Probe for 19F, 31P, 13C and 1H observation Automatic tune and match compatible for high throughput automation available for 400 and 500 MHz systems For organic, organo-metallic and inorganic chemistry

25. Electron paramagnetic resonance (EPR) or electron spin resonance  (ESR)  spectroscopy • It is a technique for studying chemical species that have one or more unpaired electrons, such as organic and inorganic free radicals or inorganic complexes possessing a transition metalion. • The basic physical concepts of EPR are analogous to those of nuclear magnetic resonance(NMR), but it is electron spins that are excited instead of spins of atomic nuclei. • Because most stable molecules have all their electrons paired, the EPR technique is less widely used than NMR. • This limitation to paramagnetic species also means that the EPR technique is one of great specificity, since ordinary chemical solvents and matrices do not give rise to EPR spectra.

26. EPR was first observed in Kazan State University by a Soviet physicist Yevgeny Zavoisky in 1944, and was developed independently at the same time by Brebis Bleaney at Oxford University. • Every electron has a magnetic moment and spinquantum number s = 1/2, with magnetic components ms = +1/2 and ms = -1/2. • In the presence of an external magnetic field with strengthB0, the electron's magnetic moment aligns itself either parallel (ms = -1/2) or anti-parallel (ms = +1/2) to the field, each alignment having a specific energy.

27. The parallel alignment corresponds to the lower energy state, and the separation between it and the upper state is  • ΔE = ge μBB0, • where ge is the electron's so-called g-factor or splitting factor and μB is the Bohr magneton.(9.7x10-21erg/G, and ge is around 2.0023.) • This equation implies that the splitting of the energy levels is directly proportional to the magnetic field's strength, as shown in the diagram below. • An unpaired electron can move between the two energy levels by either absorbing or emitting electromagnetic radiation of energy ε = hν such that the resonance condition, ε = ΔE, is obeyed. • Substituting in ε = hν and ΔE = geμBB0  leads to the fundamental equation of EPR spectroscopy: hν = geμBB0. • Experimentally, this equation permits a large combination of frequency and magnetic field values, but the great majority of EPR measurements are made with microwaves in the 9000 – 10000 MHz (9 – 10 GHz) region, with fields corresponding to about 3500 G (0.35 T).

28. The klystron oscillator feeds the sample in a resonator cavity through a hybrid “Tee” and crystal detection is applied. • The scheme of part of the system is shown in Fig. At resonance microwave power is fully shared by the resonator and the load and hence the crystal detector goes blank. • The magnet is a permanent magnet but Helmholtz coils provide the means for varying the field over the small range for striking the resonance. • Another phenomenon, correlated to the NMR, viz., the Nuclear Qudrupole Resonance (NQR), is also used for analysis purposes. • Here resonance occurs because of the existence of nuclear quadrupole moment in contrast to the nuclear dipole moment in the NMR case. • The rest of the system is alike and is similarly used