1.1. From the history of magnetic resonance spectroscopy.
Until recently, our ideas about the structure of atoms and molecules were based on studies using optical spectroscopy methods. In connection with the improvement of spectral methods, which have advanced the field of spectroscopic measurements into the range of ultrahigh (approximately 10 3 - 10 6 MHz; microradio waves) and high frequencies (approximately 10 -2 - 10 2 MHz; radio waves), new sources of information about the structure of matter have appeared. During the absorption and emission of radiation in this frequency range, the same basic process occurs as in other ranges of the electromagnetic spectrum, namely, when moving from one energy level to another, the system absorbs or emits a quantum of energy.
The energy difference between the levels and the energy of the quanta participating in these processes are about 10 -7 eV for the radio frequency region and about 10 -4 eV for microwave frequencies.
The existence of nuclear moments was first discovered when studying the hyperfine structure of the electronic spectra of some atoms using high-resolution optical spectrometers.
The hyperfine structure of atomic spectra led Pauli in 1924 to the idea that some nuclei have a moment of momentum (angular momentum) and, consequently, a magnetic moment that interacts with atomic orbital electrons. Subsequently, this hypothesis was confirmed by spectroscopic measurements, which made it possible to determine the values of the angular and magnetic moments for many nuclei.
Under the influence of an external magnetic field, the magnetic moments of the nuclei are oriented in a certain way, and it becomes possible to observe transitions between nuclear energy levels associated with these different orientations: transitions that occur under the action of radiation of a certain frequency. The quantization of the energy levels of the nucleus is a direct consequence of the quantum nature of the angular momentum of the nucleus receiving 2 I+ 1 values. Spin quantum number (spin) I can take any value multiple of 1/2; 176 71 Lu has the highest known I value (≥7). measurable highest value angular momentum (the largest value of the projection of the momentum on the selected direction) is equal to Iħ, where ħ=h/2π, and h is Planck's constant.
Values I it is impossible to predict for specific nuclei, but it has been observed that isotopes in which both mass number and atomic number are even have I= 0, and isotopes with odd mass numbers have half-integer spins. Such a situation, when the numbers of protons and neutrons in the nucleus are even and equal ( I= 0) can be considered as a state with "complete pairing", similar to the complete pairing of electrons in a diamagnetic molecule.
In 1921 Stern and Gerlach showed by the atomic beam method that the measurable values of the magnetic moment of an atom are discrete, corresponding to the spatial quantization of an atom in an inhomogeneous magnetic field. In subsequent experiments, passing a beam of hydrogen molecules through a constant magnetic field, it was possible to measure the small magnetic moment of the hydrogen nucleus. Further development of the method consisted in the fact that the beam was affected by an additional magnetic field, oscillating with a frequency at which transitions between nuclear energy levels corresponding to the quantum values of the nuclear magnetic moment are induced.
If the nuclear spin number is I, then the nucleus has (2I+1) equally spaced energy levels; in a constant magnetic field with strength H, the distance between the highest and lowest of these levels is 2mH, where m is the maximum measurable value of the magnetic moment of the nucleus. Hence, the distance between adjacent levels is equal to mH/I, and the frequency of the oscillating magnetic field, which can cause transitions between these levels, is equal to mH/Ih.
In an experiment with a molecular beam, those molecules reach the detector whose energy does not change. The frequency at which resonant transitions between levels occur is determined by successively changing (sweeping) the frequency in a certain range. At a certain frequency, there is a sudden decrease in the number of molecules reaching the detector.
The first successful NMR observations of this kind were made with fundamental magnetic fields of the order of several kilooersteds, which correspond to frequencies of an oscillating magnetic field in the range of 10 5 -10 8 Hz. Resonance energy exchange can occur not only in molecular beams; it can be seen in all states of aggregation substances.
In 1936 Horner tried to discover the resonance of Li 7 nuclei in lithium fluoride and H 1 nuclei in potassium alum. Another unsuccessful attempt was made by Gortner and Brur in 1942. Registration of the absorption of high-frequency energy at resonance in these experiments was supposed to be carried out, respectively, by the calorimetric method and by anomalous dispersion. The main reason for the failure of these experiments was the choice of unsuitable objects. Only at the end of 1945, two groups of American physicists led by F. Bloch and E.M. Purcell were the first to receive nuclear magnetic resonance signals. Bloch observed resonant absorption by protons in water, and Purcell was successful in discovering nuclear resonance by protons in paraffin. For this discovery they were awarded in 1952 Nobel Prize.
1.2. Technological applications of NMR (the main advantages of the NMR method).
The NMR method, although it is called the nuclear magnetic resonance method, has nothing to do with nuclear physics, which, as is known, studies the processes of transformation of nuclei, i.e. radioactive processes. In this case, the magnetic energy (and the NMR phenomenon takes place when the sample under study is placed in a constant magnetic field) does not affects on the thermodynamic properties of matter, because it is many times (more precisely, several orders of magnitude) less than the thermal energy characteristic of processes occurring under normal conditions, including biological ones.
The main advantages of the NMR method.
- High resolution - on ten orders more than optical spectroscopy.
Ability to lead quantitative accounting (counting) of resonating nuclei. This opens up opportunities for quantitative analysis substances.
The NMR spectra depend on the nature of the processes occurring in the substance under study. Therefore, these processes can be studied by the indicated method. Moreover, it is available temporary the scale is very wide - from many hours to small fractions of a second.
Modern radio-electronic equipment and computers make it possible to obtain parameters characterizing the phenomenon, in comfortable for researchers and users of the NMR method form. This circumstance is especially important when it comes to the practical use of experimental data.
The main advantage of NMR in comparison with other types of spectroscopy is the possibility of transforming and modifying the nuclear spin Hamiltonian at the will of the experimenter practically without any restrictions and fitting it to the special requirements of the problem being solved. Due to the great complexity of the pattern of incompletely resolved lines, many infrared and ultraviolet spectra cannot be deciphered. However, in NMR, transforming the Hamiltonian so that the spectrum can be analyzed in detail can in many cases simplify complex spectra.
The ease with which it is possible to transform the nuclear spin Hamiltonian is due to certain reasons. Due to the fact that nuclear interactions are weak, it is possible to introduce strong perturbations sufficient to suppress unwanted interactions. In optical spectroscopy, the corresponding interactions have a much higher energy, and such transformations are virtually impossible.
The modification of the spin Hamiltonian plays an essential role in many applications of one-dimensional NMR spectroscopy. At present, the simplification of spectra or the increase in their informativeness by means of spin decoupling, coherent averaging by multipulse sequences, rotation of samples, or partial orientation in liquid crystal solvents has become widespread.
Speaking about the advantages of NMR instruments, it is necessary to proceed from the real possibilities in the acquisition and operation of NMR spectrometers. In this regard, the following should be noted.
Operator duties when working on these spectrometers can be performed by any Human. But maintenance and repair require high qualifications.
NMR experiments are reduced to the following. The test sample is placed in a constant magnetic field, which is created by a permanent magnet or, most often, an electromagnet.
In this case, radio frequency radiation, usually in the meter range, is applied to the sample. The resonance is detected by the corresponding electronic devices, processed by them and issued in the form of a spectrogram, which can be output to an oscilloscope or recorder, in the form of a series of numbers and tables obtained using a printing device. The resonant output signal can also be introduced into a particular process to control this process or cycle.
Usually, when it comes to the study of monomeric compounds on hydrogen nuclei under stationary conditions with molecular weight several hundred units (and such substances are the majority in the study), the mass of the test sample should be from several milligrams to one hundred milligrams. The sample is usually dissolved in one or another solvent, and the volume of the solution is 0.7¸1 mm 3 . When detecting NMR signals from nuclei other than H1, the mass of the sample can reach two grams. If the substance under study is a liquid, then, naturally, it is not necessary to prepare a solution in this case - it all depends on the goals of the experiment.
MAGNETIC RESONANCE
resonant (selective) absorption of radio frequency radiation by certain atomic particles placed in a constant magnetic field. Majority elementary particles like spinning tops revolve around own axis. If a particle has an electric charge, then when it rotates, a magnetic field arises, i.e. it behaves like a tiny magnet. When this magnet interacts with an external magnetic field, phenomena occur that make it possible to obtain information about nuclei, atoms or molecules, which include this elementary particle. The magnetic resonance method is a universal research tool used in such diverse fields of science as biology, chemistry, geology and physics. There are two main types of magnetic resonances: electron paramagnetic resonance and nuclear magnetic resonance.
See also
MAGNETS AND MAGNETIC PROPERTIES OF SUBSTANCE;
ELEMENTARY PARTICLES.
Electron paramagnetic resonance (EPR). EPR was discovered in 1944 by the Russian physicist E.K. Zavoisky. Electrons in substances behave like microscopic magnets. In different substances, they are reoriented in different ways if the substance is placed in a constant external magnetic field and exposed to a radio frequency field. The return of electrons to their original orientation is accompanied by a radio frequency signal that carries information about the properties of the electrons and their environment. This method, which is one of the types of spectroscopy, is used in the study of the crystal structure of elements, the chemistry of living cells, chemical bonds in substances, etc.
see also RANGE ; SPECTROSCOPY.
Nuclear magnetic resonance (NMR). NMR was discovered in 1946 by the American physicists E. Purcell and F. Bloch. Working independently of each other, they found a way of resonant "tuning" in magnetic fields of the own rotations of the nuclei of some atoms, such as hydrogen and one of the isotopes of carbon. When a sample containing such nuclei is placed in a strong magnetic field, their nuclear moments "line up" like iron filings near a permanent magnet. This general orientation can be disturbed by an RF signal. When the signal is turned off, the nuclear moments return to their original state, and the speed of such recovery depends on their energy state, the type of surrounding nuclei, and a number of other factors. The transition is accompanied by the emission of a radio frequency signal. The signal is sent to a computer that processes it. In this way (the method of computed NMR tomography), images can be obtained. (When the external magnetic field is changed in small steps, the effect of a three-dimensional image is achieved.) The NMR method provides a high contrast of different soft tissues in the image, which is extremely important for identifying diseased cells against the background of healthy ones. NMR tomography is considered safer than X-ray, because it does not cause any destruction or tissue irritation.
(see also X-RAY RADIATION). NMR also makes it possible to study living cells without disturbing their vital functions. Therefore, it should be expected that the use of NMR in clinical medicine will expand. See also SURGERY.
Collier Encyclopedia. - Open society. 2000 .
See what "MAGNETIC RESONANCE" is in other dictionaries:
elect. absorption by a substance. magn. waves of a certain frequency w, due to a change in the orientation of the magnetic. moments of particles of matter (electrons, at. nuclei). Energy levels of a particle with a magnetic moment m, in ext. magn. field H… … Physical Encyclopedia
elect. absorption in vom el. magn. waves defined. frequency w, due to a change in the orientation of the magnetic. moments h c in va (el new, at. nuclei). Energy levels h tsy, which has a magnet. moment m, in ext. magn. field H is split into magnetic. ... ... Physical Encyclopedia
magnetic resonance- — [Ya.N. Luginsky, M.S. Fezi Zhilinskaya, Yu.S. Kabirov. English Russian Dictionary of Electrical Engineering and Power Engineering, Moscow, 1999] Electrical engineering topics, basic concepts EN magnetic resonance ... Technical Translator's Handbook
Selective absorption by a substance of electromagnetic waves of a certain wavelength, due to a change in the orientation of the magnetic moments of electrons or atomic nuclei. Energy levels of a particle with a magnetic moment (See ... ... Great Soviet Encyclopedia
elect. absorption of email magn. radiation of a certain frequency with a PTO located in the external. magn. field. Due to transitions between magnetic sublevels of the same energy level of the atom, nucleus, and other quantum systems. Naib. important examples of such resonances ... ... Natural science. encyclopedic Dictionary
magnetic resonance- selective absorption by a substance of electromagnetic waves of a certain frequency, due to a change in the orientation of the magnetic moments of the particles of the substance; See also: Resonance nuclear magnetic resonance (NMR) ... Encyclopedic Dictionary of Metallurgy
magnetic resonance- magnetinis rezonansas statusas T sritis chemija apibrėžtis Tam tikro dažnio elektromagnetinių bangų atrankioji sugertis medžiagoje. atitikmenys: engl. magnetic resonance. magnetic resonance... Chemijos terminų aiskinamasis žodynas
- (NMR), selective absorption of email. magn. energy in vom due to nuclear paramagnetism. NMR is one of the methods of radiospectroscopy; it is observed when mutually perpendicular magnetic fields act on the sample under study. fields: strong constant H0 ... Physical Encyclopedia
Image of the human brain on a medical NMR tomograph Nuclear magnetic resonance (NMR) resonant absorption or emission of electromagnetic energy by a substance containing nuclei with non-zero spin in an external magnetic field, at a frequency ν ... ... Wikipedia
- (NAM), selective absorption of acoustic energy. vibrations (phonons), due to the reorientation of the magnetic. moments at. cores in tv. body placed in a permanent magnet. field. For most nuclei, resonant absorption is observed in the ultrasonic region ... ... Physical Encyclopedia
Books
- Magnetic resonance in chemistry and medicine, R. Freeman, The monograph of the well-known scientist in the field of NMR spectroscopy R. Freeman combines the visibility of the consideration of the basic principles of magnetic resonance in chemistry and medicine (biology) with a high ... Category: Physics Publisher: KRASAND, Manufacturer: KRASAND,
- Nuclear magnetic resonance in inorganic and coordination chemistry , M. A. Fedotov , The monograph introduces the possibilities of the nuclear magnetic resonance (NMR) method in the study inorganic substances. The features of the NMR phenomenon in the liquid phase and measurement techniques are considered ... Category:
- The essence of the phenomenon
First of all, it should be noted that although the word “nuclear” is present in the name of this phenomenon, NMR has nothing to do with nuclear physics and has nothing to do with radioactivity. If we talk about a strict description, then without laws quantum mechanics can't get by. According to these laws, the interaction energy of a magnetic core with an external magnetic field can take only a few discrete values. If magnetic nuclei are irradiated with an alternating magnetic field, the frequency of which corresponds to the difference between these discrete energy levels, expressed in frequency units, then the magnetic nuclei begin to move from one level to another, while absorbing the energy of the alternating field. This is the phenomenon of magnetic resonance. This explanation is formally correct, but not very clear. There is another explanation, without quantum mechanics. The magnetic core can be thought of as an electrically charged ball rotating around its axis (although, strictly speaking, this is not the case). According to the laws of electrodynamics, the rotation of a charge leads to the appearance of a magnetic field, i.e., the magnetic moment of the nucleus, which is directed along the axis of rotation. If this magnetic moment is placed in a constant external field, then the vector of this moment begins to precess, i.e., rotate around the direction of the external field. In the same way, the spinning wheel axis precesses (rotates) around the vertical, if it is unwound not strictly vertically, but at a certain angle. In this case, the role of the magnetic field is played by the gravitational force.
The precession frequency is determined both by the properties of the nucleus and by the strength of the magnetic field: the stronger the field, the higher the frequency. Then, if, in addition to a constant external magnetic field, an alternating magnetic field acts on the nucleus, then the nucleus begins to interact with this field - it, as it were, swings the nucleus more strongly, the precession amplitude increases, and the nucleus absorbs the energy of the alternating field. However, this will occur only under the condition of resonance, i.e., the coincidence of the precession frequency and the frequency of the external alternating field. It looks like a classic example from high school physics - soldiers marching across a bridge. If the step frequency coincides with the natural frequency of the bridge, then the bridge sways more and more. Experimentally, this phenomenon manifests itself in the dependence of the absorption of an alternating field on its frequency. At the moment of resonance, the absorption increases sharply, and the simplest magnetic resonance spectrum looks like this:
- Fourier spectroscopy
The first NMR spectrometers worked exactly as described above - the sample was placed in a constant magnetic field, and RF radiation was continuously applied to it. Then either the frequency of the alternating field or the intensity of the constant magnetic field changed smoothly. The energy absorption of the alternating field was recorded by a radio frequency bridge, the signal from which was output to a recorder or an oscilloscope. But this method of signal registration has not been used for a long time. In modern NMR spectrometers, the spectrum is recorded using pulses. The magnetic moments of the nuclei are excited by a short powerful pulse, after which a signal is recorded, which is induced in the RF coil by freely precessing magnetic moments. This signal gradually decreases to zero as the magnetic moments return to equilibrium (this process is called magnetic relaxation). The NMR spectrum is obtained from this signal using a Fourier transform. This is a standard mathematical procedure that allows you to decompose any signal into frequency harmonics and thus obtain the frequency spectrum of this signal. This method of recording the spectrum allows you to significantly reduce the noise level and conduct experiments much faster.
One excitation pulse to record the spectrum is the simplest NMR experiment. However, there can be many such pulses, of different durations, amplitudes, with different delays between them, etc., in the experiment, depending on what kind of manipulations the researcher needs to perform with the system of nuclear magnetic moments. However, almost all of these pulse sequences end in the same thing - recording a free precession signal followed by a Fourier transform.
- Magnetic interactions in matter
In itself, magnetic resonance would remain nothing more than an interesting physical phenomenon, if it were not for the magnetic interactions of nuclei with each other and with the electron shell of the molecule. These interactions affect the resonance parameters, and with their help, NMR can be used to obtain various information about the properties of molecules - their orientation, spatial structure (conformation), intermolecular interactions, chemical exchange, rotational and translational dynamics. Thanks to this, NMR has become a very powerful tool for studying substances at the molecular level, which is widely used not only in physics, but mainly in chemistry and molecular biology. An example of one of these interactions is the so-called chemical shift. Its essence is as follows: the electron shell of the molecule responds to an external magnetic field and tries to screen it - partial screening of the magnetic field occurs in all diamagnetic substances. This means that the magnetic field in the molecule will differ from the external magnetic field by a very small amount, which is called the chemical shift. However, the properties of the electron shell in different parts of the molecule are different, and the chemical shift is also different. Accordingly, the resonance conditions for nuclei in different parts of the molecule will also differ. This makes it possible to distinguish chemically nonequivalent nuclei in the spectrum. For example, if we take the spectrum of hydrogen nuclei (protons) of pure water, then there will be only one line in it, since both protons in the H 2 O molecule are exactly the same. But for methyl alcohol CH 3 OH, there will already be two lines in the spectrum (if we neglect other magnetic interactions), since there are two types of protons - protons of the methyl group CH 3 and a proton associated with an oxygen atom. As the molecules become more complex, the number of lines will increase, and if we take such a large and complex molecule as a protein, then in this case the spectrum will look something like this:
- Magnetic cores
NMR can be observed on different nuclei, but it must be said that not all nuclei have a magnetic moment. It often happens that some isotopes have a magnetic moment, while other isotopes of the same nucleus do not. In total, there are more than a hundred isotopes of various chemical elements having magnetic nuclei, however, no more than 1520 magnetic nuclei are usually used in research, everything else is exotic. Each nucleus has its own characteristic ratio of the magnetic field and the precession frequency, called the gyromagnetic ratio. For all nuclei these ratios are known. Using them, one can choose the frequency at which, for a given magnetic field, a signal from the nuclei needed by the researcher will be observed.
The most important nuclei for NMR are protons. They are most abundant in nature, and they have a very high sensitivity. For chemistry and biology, the nuclei of carbon, nitrogen and oxygen are very important, but scientists were not very lucky with them: the most common isotopes of carbon and oxygen, 12 C and 16 O, do not have a magnetic moment, the natural nitrogen isotope 14 N has a moment, but it for a number of reasons it is very inconvenient for experiments. There are 13 C, 15 N and 17 O isotopes that are suitable for NMR experiments, but their natural abundance is very low and the sensitivity is very low compared to protons. Therefore, special isotopically enriched samples are often prepared for NMR studies, in which the natural isotope of one or another nucleus is replaced by the one needed for experiments. In most cases, this procedure is very difficult and expensive, but sometimes it is the only way to get the necessary information.
- Electron paramagnetic and quadrupole resonance
Speaking of NMR, one cannot fail to mention two other related physical phenomena- electron paramagnetic resonance (EPR) and nuclear quadrupole resonance (NQR). EPR is essentially similar to NMR, the difference lies in the fact that the resonance is observed on the magnetic moments not of atomic nuclei, but of the electron shell of the atom. EPR can be observed only in those molecules or chemical groups whose electron shell contains the so-called unpaired electron, then the shell has a non-zero magnetic moment. Such substances are called paramagnets. EPR, like NMR, is also used to study various structural and dynamic properties of substances at the molecular level, but its scope is much narrower. This is mainly due to the fact that most molecules, especially in living nature, do not contain unpaired electrons. In some cases, you can use the so-called paramagnetic probe, i.e. a chemical group with unpaired electron that binds to the molecule under study. But this approach has obvious drawbacks that limit the possibilities of this method. In addition, in EPR there is no such high spectral resolution (ie, the ability to distinguish one line from another in the spectrum) as in NMR.
It is most difficult to explain the nature of NQR "on the fingers". Some nuclei have a so-called electric quadrupole moment. This moment characterizes the deviation of the distribution electric charge kernels from spherical symmetry. Interaction of this moment with the gradient electric field, created by the crystalline structure of the substance, leads to a splitting of the energy levels of the nucleus. In this case, resonance can be observed at a frequency corresponding to transitions between these levels. Unlike NMR and EPR, NQR does not require an external magnetic field, since level splitting occurs without it. NQR is also used to study substances, but its scope is even narrower than that of EPR.
- Advantages and disadvantages of NMR
NMR is the most powerful and informative method for studying molecules. Strictly speaking, this is not one method, but a large number of different types of experiments, i.e., pulse sequences. Although they are all based on the NMR phenomenon, but each of these experiments is designed to obtain some specific specific information. The number of these experiments is measured by many tens, if not hundreds. Theoretically, NMR can, if not everything, then almost everything that all other experimental methods for studying the structure and dynamics of molecules can, although in practice this is, of course, far from always feasible. One of the main advantages of NMR is that, on the one hand, its natural probes, i.e., magnetic nuclei, are distributed over the entire molecule, and, on the other hand, it makes it possible to distinguish these nuclei from each other and obtain spatially selective data on properties of the molecule. Almost all other methods provide information either averaged over the entire molecule, or only about one of its parts.
There are two main disadvantages of NMR. First, this is a low sensitivity compared to most other experimental methods (optical spectroscopy, fluorescence, EPR, etc.). This leads to the fact that in order to average the noise, the signal must be accumulated for a long time. In some cases, the NMR experiment can be carried out for even several weeks. Secondly, it is its high cost. NMR spectrometers are among the most expensive scientific instruments, costing at least hundreds of thousands of dollars, with the most expensive spectrometers costing several million. Not all laboratories, especially in Russia, can afford to have such scientific equipment.
- Magnets for NMR spectrometers
One of the most important and expensive parts of a spectrometer is the magnet, which creates a constant magnetic field. The stronger the field, the higher the sensitivity and spectral resolution, so scientists and engineers are constantly trying to get the highest possible fields. The magnetic field is created electric shock in a solenoid - the stronger the current, the greater the field. However, it is impossible to increase the current indefinitely; at a very high current, the solenoid wire will simply begin to melt. Therefore, superconducting magnets, i.e., magnets in which the solenoid wire is in the superconducting state, have been used for a very long time for high-field NMR spectrometers. In this case electrical resistance wire is zero, and no energy is released at any amount of current. The superconducting state can only be obtained at very low temperatures, just a few degrees Kelvin - this is the temperature of liquid helium. (High-temperature superconductivity is still only purely pure fundamental research.) It is with the maintenance of such a low temperature that all the technical difficulties in the design and production of magnets are associated, which cause their high cost. The superconducting magnet is built on the principle of a thermos matryoshka. The solenoid is in the center, in the vacuum chamber. It is surrounded by a shell containing liquid helium. This shell is surrounded by a shell of liquid nitrogen through a vacuum layer. The temperature of liquid nitrogen is minus 196 degrees Celsius, nitrogen is needed so that helium evaporates as slowly as possible. Finally, the nitrogen shell is isolated from room temperature by an outer vacuum layer. Such a system is able to maintain the desired temperature of the superconducting magnet for a very long time, although this requires regular pouring of liquid nitrogen and helium into the magnet. The advantage of such magnets, in addition to the ability to obtain high magnetic fields, is also that they do not consume energy: after the start of the magnet, the current runs through the superconducting wires with virtually no loss for many years.
- Tomography
In conventional NMR spectrometers, they try to make the magnetic field as uniform as possible, this is necessary to improve the spectral resolution. But if the magnetic field inside the sample, on the contrary, is made very inhomogeneous, this opens up fundamentally new possibilities for using NMR. The inhomogeneity of the field is created by the so-called gradient coils, which are paired with the main magnet. In this case, the magnitude of the magnetic field in different parts of the sample will be different, which means that the NMR signal can be observed not from the entire sample, as in a conventional spectrometer, but only from its narrow layer, for which the resonance conditions are met, i.e., the desired ratio of magnetic field and frequency. By changing the magnitude of the magnetic field (or, which is essentially the same thing, the frequency of observing the signal), you can change the layer that will give the signal. Thus, it is possible to "scan" the sample throughout its volume and "see" its internal three-dimensional structure without destroying the sample in any mechanical way. To date, a large number of techniques have been developed that make it possible to measure various NMR parameters (spectral characteristics, magnetic relaxation times, self-diffusion rate, and some others) with spatial resolution inside a sample. The most interesting and important, from a practical point of view, the use of NMR tomography was found in medicine. In this case, the "sample" being examined is the human body. NMR imaging is one of the most effective and safe (but also expensive) diagnostic tools in various fields of medicine, from oncology to obstetrics. It is curious to note that doctors do not use the word "nuclear" in the name of this method, because some patients associate it with nuclear reactions and the atomic bomb.
- Discovery history
The year of the discovery of NMR is considered to be 1945, when the Americans Felix Bloch from Stanford and independently Edward Parcell and Robert Pound from Harvard first observed the NMR signal on protons. By that time, much was already known about the nature of nuclear magnetism, the NMR effect itself was theoretically predicted, and several attempts were made to observe it experimentally. It is important to note that a year earlier in the Soviet Union, in Kazan, the EPR phenomenon was discovered by Evgeny Zavoisky. It is now well known that Zavoisky also observed the NMR signal, this was before the war, in 1941. However, he had a poor quality magnet with poor field uniformity at his disposal, the results were poorly reproducible and therefore remained unpublished. In fairness, it should be noted that Zavoisky was not the only one who observed NMR before its "official" discovery. In particular, the American physicist Isidore Rabi (Nobel Prize winner in 1944 for the study of the magnetic properties of nuclei in atomic and molecular beams) also observed NMR in the late 1930s, but considered this to be an instrumental artifact. One way or another, but our country remains a priority in the experimental detection of magnetic resonance. Although Zavoisky himself soon after the war began to deal with other problems, his discovery for the development of science in Kazan played a huge role. Kazan is still one of the world's leading research centers for EPR spectroscopy.
- Nobel Prizes in Magnetic Resonance
In the first half of the 20th century, several Nobel Prizes were awarded to scientists without whose work the discovery of NMR could not have taken place. Among them are Peter Szeeman, Otto Stern, Isidor Rabi, Wolfgang Pauli. But there were four Nobel Prizes directly related to NMR. In 1952, Felix Bloch and Edward Purcell received the prize for the discovery of NMR. This is the only "NMR" Nobel Prize in physics. In 1991, the Swiss Richard Ernst, who worked at the famous ETH Zurich, won the Chemistry Prize. He was awarded it for the development of multidimensional NMR spectroscopy methods, which made it possible to radically increase the information content of NMR experiments. In 2002, the prize winner, also in chemistry, was Kurt Wüthrich, who worked with Ernst in neighboring buildings at the same Technical School. He received the award for developing methods for determining the three-dimensional structure of proteins in solution. Prior to this, the only method that allowed determining the spatial conformation of large biomacromolecules was only X-ray diffraction analysis. Finally, in 2003, the American Paul Lauterbur and the Englishman Peter Mansfield received the Medical Prize for the invention of NMR imaging. The Soviet discoverer of the EPR E.K. Zavoisky, alas, did not receive the Nobel Prize.
Nuclear magnetic resonance (NMR) is a nuclear spectroscopy that is widely used in all physical sciences and industry. In NMR for probing intrinsic spin properties of atomic nuclei using a large magnet. Like any spectroscopy, to create a transition between energy levels (resonance), it uses electromagnetic radiation(radio frequency waves in the VHF band). In chemistry, NMR helps determine the structure of small molecules. Nuclear magnetic resonance in medicine has found application in magnetic resonance imaging (MRI).
Opening
NMR was discovered in 1946 by Harvard University scientists Purcell, Pound, and Torrey, and Stanford's Bloch, Hansen, and Packard. They noticed that the 1 H and 31 P nuclei (proton and phosphorus-31) are able to absorb radio frequency energy when exposed to a magnetic field, the strength of which is specific to each atom. When absorbed, they began to resonate, each element at its own frequency. This observation allowed a detailed analysis of the structure of the molecule. Since then, NMR has found application in kinetic and structural studies of solids, liquids and gases, resulting in 6 Nobel Prizes.
Spin and magnetic properties
The nucleus is made up of elementary particles called neutrons and protons. They have their own angular momentum, called spin. Like electrons, the spin of a nucleus can be described by quantum numbers I and m in a magnetic field. Atomic nuclei with an even number of protons and neutrons have zero spin, and all the rest - non-zero. In addition, molecules with non-zero spin have a magnetic moment μ = γ I, where γ is the gyromagnetic ratio, the constant of proportionality between the magnetic dipole moment and the angular moment, which is different for each atom.
The magnetic moment of the core makes it behave like a tiny magnet. In the absence of an external magnetic field, each magnet is randomly oriented. During the NMR experiment, the sample is placed in an external magnetic field B 0 , which causes the low energy bar magnets to align in the direction of B 0 and the high energy in the opposite direction. In this case, the orientation of the spin of the magnets changes. To understand this rather abstract concept, one must consider the energy levels of the nucleus during an NMR experiment.
Energy levels
A spin flip requires an integer number of quanta. For any m, there are 2m + 1 energy levels. For a nucleus with spin 1/2 there are only 2 of them - low, occupied by spins aligned with B 0 , and high, occupied by spins directed against B 0 . Each energy level is defined by E = -mℏγВ 0 , where m is the magnetic quantum number, in this case +/- 1/2. The energy levels for m > 1/2, known as quadrupole nuclei, are more complex.
The energy difference between the levels is: ΔE = ℏγB 0 , where ℏ is Planck's constant.
As can be seen, the strength of the magnetic field is of great importance, since in its absence the levels degenerate.
Energy transitions
For nuclear magnetic resonance to occur, a spin flip must occur between energy levels. The energy difference between the two states corresponds to the energy of electromagnetic radiation, which causes the nuclei to change their energy levels. For most NMR spectrometers At 0 it has the order of 1 Tesla (T), and γ - 10 7 . Therefore, the required electromagnetic radiation is of the order of 10 7 Hz. The photon energy is represented by the formula E = hν. Therefore, the frequency required for absorption is: ν= γВ 0 /2π.
Nuclear shielding
The physics of NMR is based on the concept of nuclear shielding, which makes it possible to determine the structure of matter. Each atom is surrounded by electrons that revolve around the nucleus and act on its magnetic field, which in turn causes small changes in energy levels. This is called shielding. Nuclei that experience different magnetic fields associated with local electronic interactions are called non-equivalent. Changing the energy levels for a spin flip requires a different frequency, which creates a new peak in the NMR spectrum. Screening allows the structural determination of molecules by analyzing the NMR signal using the Fourier transform. The result is a spectrum consisting of a set of peaks, each corresponding to a different chemical environment. The peak area is directly proportional to the number of nuclei. Detailed structure information is retrieved by NMR interactions, which change the spectrum in different ways.
Relaxation
Relaxation refers to the phenomenon of returning nuclei to their thermodynamically stable after excitation to higher energy levels of the state. In this case, the energy absorbed during the transition from more low level to a higher one. This is a rather complex process that takes place in different time frames. The two most widespread relaxation types are spin-lattice and spin-spin.
To understand relaxation, it is necessary to consider the entire sample. If the nuclei are placed in an external magnetic field, they will create bulk magnetization along the Z axis. Their spins are also coherent and allow the signal to be detected. NMR shifts the bulk magnetization from the Z axis to the XY plane, where it manifests itself.
Spin-lattice relaxation is characterized by the time T 1 required to restore 37% of the bulk magnetization along the Z axis. more efficient process relaxation, the smaller T 1 . In solids, since the movement between molecules is limited, the relaxation time is long. Measurements are usually carried out by pulse methods.
Spin-spin relaxation is characterized by the loss of mutual coherence T 2 . It may be less than or equal to T 1 .
Nuclear magnetic resonance and its applications
The two main areas in which NMR has proved extremely important are medicine and chemistry, but new applications are being developed every day.
Nuclear magnetic resonance imaging, more commonly known as magnetic resonance imaging (MRI), is important medical diagnostic tool used to study the functions and structure of the human body. It allows you to get detailed images of any organ, especially soft tissues, in all possible planes. Used in the areas of cardiovascular, neurological, musculoskeletal and oncological imaging. Unlike alternative computed tomography, magnetic resonance imaging does not use ionizing radiation, therefore it is completely safe.
MRI can detect subtle changes that occur over time. NMR imaging can be used to identify structural abnormalities that occur during the course of the disease, how they affect subsequent development, and how their progression correlates with the mental and emotional aspects of the disorder. Since MRI does not visualize bone well, excellent intracranial and intravertebral content.
Principles of using nuclear magnetic resonance in diagnostics
During an MRI procedure, the patient lies inside a massive hollow cylindrical magnet and is exposed to a powerful, stable magnetic field. Different atoms in the scanned part of the body resonate at different frequencies of the field. MRI is used primarily to detect vibrations of hydrogen atoms, which contain a rotating proton nucleus with a small magnetic field. In MRI, the background magnetic field lines up all the hydrogen atoms in the tissue. The second magnetic field, whose orientation differs from that of the background, turns on and off many times per second. At a certain frequency, the atoms resonate and line up with the second field. When it turns off, the atoms bounce back, aligning with the background. This creates a signal that can be received and converted into an image.
Fabrics with big amount hydrogen, which is present in the human body as part of water, creates a bright image, and with a small content or absence of it (for example, bones) look dark. The brightness of the MRI is enhanced by a contrast agent such as gadodiamide, which patients take before the procedure. Although these agents can improve image quality, the sensitivity of the procedure remains relatively limited. Techniques are being developed to increase the sensitivity of MRI. The most promising is the use of parahydrogen, a form of hydrogen with unique molecular spin properties that is very sensitive to magnetic fields.
Improvements in the performance of the magnetic fields used in MRI have led to the development of highly sensitive imaging modalities such as diffusion and functional MRI, which are designed to display very specific tissue properties. In addition, a unique form of MRI technology called magnetic resonance angiography is used to image the movement of blood. It allows visualization of arteries and veins without the need for needles, catheters or contrast agents. As with MRI, these techniques have helped revolutionize biomedical research and diagnostics.
Advanced computer technology has allowed radiologists to create three-dimensional holograms from digital sections obtained by MRI scanners, which serve to determine the exact location of lesions. Tomography is especially valuable in the examination of the head and spinal cord, as well as pelvic organs such as the bladder, and cancellous bone. The method allows you to quickly and clearly accurately determine the extent of tumor damage and assess the potential damage from a stroke, allowing doctors to prescribe the appropriate treatment in a timely manner. MRI has largely supplanted arthrography, the need to inject a contrast agent into a joint to visualize cartilage or ligament damage, and myelography, the injection of a contrast agent into the spinal canal to visualize disorders of the spinal cord or intervertebral disc.
Application in chemistry
In many laboratories today, nuclear magnetic resonance is used to determine the structures of important chemical and biological compounds. In NMR spectra, different peaks provide information about the specific chemical environment and bonds between atoms. Most widespread the isotopes used to detect magnetic resonance signals are 1 H and 13 C, but many others are suitable, such as 2 H, 3 He, 15 N, 19 F, etc.
Modern NMR spectroscopy has found wide application in biomolecular systems and plays an important role in structural biology. With the development of methodology and tools, NMR has become one of the most powerful and versatile spectroscopic methods for the analysis of biomacromolecules, which makes it possible to characterize them and their complexes with sizes up to 100 kDa. Together with X-ray crystallography, this is one of the two leading technologies for determining their structure at the atomic level. In addition, NMR provides a unique and important information about the functions of the protein, which plays a crucial role in the development medicines. Some of the applications NMR spectroscopy are listed below.
- This is the only method for determining the atomic structure of biomacromolecules in aqueous solutions close to physiological conditions or membrane-simulating media.
- Molecular dynamics. This is the most powerful method for quantitative determination of the dynamic properties of biomacromolecules.
- Protein folding. NMR spectroscopy is the most powerful tool for determining the residual structures of unfolded proteins and folding mediators.
- The state of ionization. The method is effective in determining chemical properties functional groups in biomacromolecules, such as ionization states of ionizable groups of enzyme active sites.
- Nuclear magnetic resonance makes it possible to study weak functional interactions between macrobiomolecules (for example, with dissociation constants in the micromolar and millimolar ranges), which cannot be done using other methods.
- Protein hydration. NMR is a tool for detecting domestic water and its interaction with biomacromolecules.
- It's unique direct interaction detection method hydrogen bonds.
- Screening and drug development. In particular, nuclear magnetic resonance is particularly useful in identifying drugs and determining the conformations of compounds associated with enzymes, receptors, and other proteins.
- Native membrane protein. Solid state NMR has the potential determination of atomic structures of membrane protein domains in the environment of the native membrane, including those with bound ligands.
- Metabolic analysis.
- Chemical analysis. Chemical identification and conformational analysis of synthetic and natural chemicals.
- Materials Science. A powerful tool in the study of polymer chemistry and physics.
Other uses
Nuclear magnetic resonance and its applications are not limited to medicine and chemistry. The method has proven to be very useful in other areas as well, such as environmental testing, the oil industry, process control, NMR of the Earth's field, and magnetometers. Non-destructive testing saves on expensive biological samples that can be reused if more testing is needed. Nuclear magnetic resonance in geology is used to measure the porosity of rocks and the permeability of underground fluids. Magnetometers are used to measure various magnetic fields.
