Nuclear magnetic resonance. FAQ: Nuclear magnetic resonance

All elementary particles, that is, everything that we are made of, are small magnets - this is a proton, a neutron, and an electron. Thus, nuclei made up of protons and neutrons can also have a magnetic moment.

1. Characteristics of the magnetic moment of the nucleus

The nature of the magnetic moment is quantum. But if you try to illustrate it in a more understandable classical expression, the behavior of the nucleus is similar to the behavior of a small spinning magnet. Thus, if we do not have an external magnetic field, then such a magnet can be oriented in any direction. As soon as we apply an external magnetic field, the nucleus, which has a magnetic moment, like any magnet, begins to feel this magnetic field, and if its spin number is ½, then two directions of its predominant orientation appear: in the direction and against the direction of the magnetic field. These two states differ in energy, and a nucleus, such as a proton, can go from one state to another. Such a change in its orientation relative to the external magnetic field is accompanied by the absorption or release of an energy quantum.

This energy is very small. The quantum of energy lies in the field of radio frequency radiation. And it is precisely this smallness of energy that is one of the unpleasant properties of the nuclear magnetic resonance method, since it determines the closeness of the populations of the lower and upper levels. But nevertheless, if we look at an ensemble of such nuclei, that is, at a substance that we have placed in a magnetic field, a sufficiently large number of magnetic moments appear, directed upwards and downwards, and transitions occur between them. Thus, we can register these transitions and measure the properties associated with them.

2. Properties of the magnetic moment of the nucleus

Since the energy quantum during the transition from one level to another depends only on the magnetic properties of the nucleus under study and on the magnitude of the external magnetic field, the so-called magnetic precession frequency, or Larmor frequency, is a factor of these two components.

However, in fact, the magnetic field that surrounds a particular nucleus is not equal to the magnetic field that we applied to it by placing the object under study in the magnet of our spectrometer. In addition to the external magnetic field, it is necessary to take into account local magnetic fields, which are induced, for example, by the movement of electrons around nuclei, the action of neighboring nuclei, the same magnets capable of inducing local magnetic fields, and the like. Thus, each nucleus located in a different part of the molecule has a completely different effective magnetic field that surrounds this nucleus. As a result, we can register not a single resonance, but a set of them, that is, the spectrum of nuclear magnetic resonance.

The relative resonant frequency is expressed, as a rule, in parts per million with respect to the magnitude of the external magnetic field. This parameter is a stable value, independent of the value of the external magnetic field, but determined by the electronic properties of the molecule under study.

So, if we consider some chemical compound: in different positions, for example, protons feel a completely different magnetic field, then in this way it is possible to identify, say, a proton signal of an aromatic residue, a proton signal of some group –CH3, and so on. And in itself, this information is extremely important from a structural point of view.

3. Interaction of nuclei with a magnetic moment

Due to the fact that the magnetic moments interact with each other, there is another layer of information that we can extract. This is information that is related to the interaction of two different nuclei with each other. If, for example, one nucleus interacts with another through a system of electrons involved in the formation of chemical bonds, then this is called an indirect, or spin-spin, interaction. The values ​​of the spin-spin interaction of nuclei are extremely sensitive to the geometry of the molecule, to its electronic properties, for example, to the electron density surrounding certain nuclei. Thus, we can obtain a number of very important structural parameters already from the magnitude of the interaction.

In addition, two nuclei that have a magnetic moment can interact with each other simply through space. This is called a "direct dipole-dipole interaction" and, again, these sorts of interactions are extremely structurally informative. For example, the interaction vector of two nuclei can give us information about the spatial proximity of nuclei, about the orientation of a pair of interacting nuclei with respect to an external magnetic field.

Thus, if we measure the nuclear magnetic resonance spectrum of a compound, we can obtain very detailed information about its structure. If, for example, we are able to measure the internuclear distance - and this can be done by determining the properties associated with the dipole-dipole interaction of nuclei, because its value is determined by this internuclear distance - then NMR actually becomes a structural method.

4. History of the discovery of the NMR method

NMR spectroscopy as a method for studying the properties of molecules appeared in the mid-40s of the XX century and in a very short time - by the mid-1950s - became one of the key methods for studying organic compounds.

But the real pioneers of NMR in liquids are Bloch and Purcell, American scientists who received the Nobel Prize in the 1950s for a discovery they made in 1945-1946. It should be noted that our compatriot Evgeny Konstantinovich Zavoisky in 1944 published a work on the detection of the magnetic resonance of an electron. The electron, as mentioned above, also has a magnetic moment, and the magnitude of this magnetic moment is even greater than the magnetic moment of the nuclei. The physical principles of the nuclear magnetic resonance method and the electron paramagnetic resonance method are very similar.

But, unfortunately, for one reason or another - reasons more of a political nature - the work of Evgeny Konstantinovich Zavoisky was not awarded the Nobel Prize, although, of course, he should have been among those people who received the prize for discovering the phenomenon of magnetic resonance.

A little earlier, Isaac Rabi received the Nobel Prize for his work in the 1930s of the XX century, for the discovery of the magnetic properties of nuclei in gas beams. And in fact, these works served as an impetus for the creation of NMR methods in liquids and solids.

Nobel Prizes were often given for discoveries related to the NMR method. One cannot fail to note, for example, the prize awarded to Richard Ernst, who created the basic methodology of NMR spectroscopy, for example, FT-IR NMR spectroscopy, methods of two-dimensional NMR spectroscopy; as well as such a scientist as Kurt Wüthrich, a Swiss colleague of Richard Ernst, who created a methodology for studying the structure of protein molecules using nuclear magnetic resonance.

5. Practical application of the NMR method

The NMR method, after its creation, began to be actively used to study organic compounds. But magnetic moments are inherent not only to those nuclei that are part of, that is, a proton, carbon or its isotope C-13 and nitrogen or its isotope N-15. In fact, the entire periodic system is, to one degree or another, covered by certain stable isotopes of nuclei that have magnetic moments. This method is completely unrelated to any radioactive properties of nuclei - only to their magnetic properties. Almost every element of the periodic system has certain isotopes that have properties convenient for nuclear magnetic resonance.

And soon after mastering NMR techniques for simple organic compounds, it began to be actively used to study various inorganic compounds. At present, the method of nuclear magnetic resonance is, according to most estimates, the most powerful physical method for studying compounds of the most diverse nature.

MAGNETIC RESONANCE
resonant (selective) absorption of radio frequency radiation by certain atomic particles placed in a constant magnetic field. Most elementary particles, like tops, rotate around their 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 SPECTRUM ; 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 activity. 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…

NMR or in English NMR imaging is an abbreviation for the phrase "nuclear magnetic resonance". This method of research entered medical practice in the 80s of the last century. It is different from X-ray tomography. The radiation used in NMR includes the radio wave range with a wavelength from 1 to 300 m. By analogy with CT, nuclear magnetic tomography uses automatic control of computer scanning with processing of a layered image of the structure of internal organs.

What is the essence of MRI

When processing information from the scanned points, images of all organs and systems are obtained in various planes, in a cut, a high-resolution three-dimensional image of tissues and organs is formed. The technology of magnetic - nuclear tomography is very complex, it is based on the principle of resonant absorption of electromagnetic waves by atoms. A person is placed in an apparatus with a strong magnetic field. The molecules there turn in the direction of the magnetic field. Then an electric wave is scanned, the change in molecules is first recorded on a special matrix, and then transferred to a computer and all data is processed.

Applications of NMRI

NMR tomography has a fairly wide range of applications, so it is much more often used as an alternative to computed tomography. The list of diseases that can be detected using MRI is very voluminous.

• Brain.

Most often, such a study is used to scan the brain for injuries, tumors, dementia, epilepsy, and problems with the vessels of the brain.

• The cardiovascular system.

In the diagnosis of the heart and blood vessels, NMR complements methods such as angiography and CT.
MRI can detect cardiomyopathy, congenital heart disease, vascular changes, myocardial ischemia, dystrophy and tumors in the area of ​​the heart and blood vessels.

• Musculoskeletal system.

NMR tomography is widely used in the diagnosis of problems with the musculoskeletal system. With this diagnostic method, ligaments, tendons and bone structures are very well differentiated.

• Internal organs.

In the study of the gastrointestinal tract and liver using nuclear magnetic resonance imaging, you can get complete information about the spleen, kidneys, liver, pancreas. If you additionally introduce a contrast agent, then it becomes possible to track the functional ability of these organs and their vascular system. And additional computer programs allow you to create images of the intestines, esophagus, biliary tract, bronchi.

Nuclear magnetic resonance imaging and MRI: is there a difference

Sometimes you can get confused in the names of MRI and MRI. Is there a difference between these two procedures? You can definitely answer no.
Initially, at the time of its discovery of magnetic resonance imaging, its name contained another word “nuclear”, which disappeared over time, leaving only the abbreviation MRI.

There are several research methods: angiography, perfusion, diffusion, spectroscopy. Nuclear magnetic resonance imaging is one of the best research methods, as it allows you to get a three-dimensional image of the state of organs and tissues, which means that the diagnosis will be established more accurately and the correct treatment will be chosen. NMR examination of the internal organs of a person is exactly images, not real tissues. Patterns appear on photosensitive film when x-rays are absorbed when an x-ray is taken.

The main advantages of NMR imaging

The advantages of NMR tomography over other research methods are many-sided and significant.

Cons of MRI

But of course, this method is not without its drawbacks.

• Big energy consumption. The operation of the chamber requires a lot of electricity and expensive technology for normal superconductivity. But magnets with high power do not have a negative impact on human health.
• Process duration. Nuclear magnetic resonance imaging is less sensitive than X-ray. Therefore, more time is required for transillumination. In addition, image distortion can occur due to respiratory movements, which distorts the data when conducting studies of the lungs and heart.
• In the presence of a disease such as claustrophobia, it is a contraindication for research using MRI. Also, it is impossible to diagnose using MRI tomography if there are large metal implants, pacemakers, artificial pacemakers. During pregnancy, diagnosis is carried out only in exceptional cases.

Every tiny object in the human body can be examined with NMR imaging. Only in some cases should the distribution of the concentration of chemical elements in the body be included. In order to make measurements more sensitive, a rather large number of signals should be accumulated and summed. In this case, a clear image of high quality is obtained, which adequately conveys reality. This is also related to the duration of a person's stay in the chamber for NMR imaging. You will have to lie still for a long time.

In conclusion, we can say that nuclear magnetic resonance imaging is a fairly safe and absolutely painless diagnostic method, which allows you to completely avoid exposure to x-rays. Computer programs allow you to process the resulting scans with the formation of virtual images. The limits of NMR are truly limitless.

Even now, this diagnostic method is a stimulus for its rapid development and wide application in medicine. The method is distinguished by its low harm to human health, but at the same time it allows you to carefully examine the structure of organs, both in a healthy person and in existing diseases.

The term "magnetic resonance" refers to the selective (resonant) absorption of the energy of an alternating electromagnetic field by an electronic or nuclear subsystem of a substance subjected to a constant magnetic field. The absorption mechanism is associated with quantum transitions in these subsystems between discrete energy levels that occur in the presence of a magnetic field.

Magnetic resonances are usually divided into five types: 1) cyclotron resonance (CR); 2) electron paramagnetic resonance (EPR); 3) nuclear magnetic resonance (NMR); 4) electronic ferromagnetic resonance; 5) electronic antiferromagnetic resonance.

Cyclotron resonance. With CR, selective absorption of electromagnetic field energy in semiconductors and metals in a constant magnetic field is observed, due to quantum transitions of electrons between Landau energy levels. The quasi-continuous energy spectrum of conduction electrons in an external magnetic field is split into such equidistant levels.

The essence of the physical mechanism of CR can also be understood within the framework of the classical theory. A free electron moves in a constant magnetic field (directed along the axis) along a spiral trajectory around magnetic induction lines with a cyclotron frequency

where and are, respectively, the magnitude of the charge and the effective mass of the electron. Let us now turn on the radio frequency field with a frequency and with a vector perpendicular to (for example, along the axis ). If the electron is in the right phase for its helix motion, then since its rotational frequency matches the external field's frequency, it will accelerate and the helix will expand. The acceleration of an electron means an increase in its energy, which occurs due to its transfer from the radio frequency field. Thus, resonant absorption is possible under the following conditions:

the frequency of the external electromagnetic field, the energy of which is absorbed, must coincide with the cyclotron frequency of electrons;

the vector of the electric field strength of the electromagnetic wave must have a component normal to the direction of the constant magnetic field ;

the mean free path of electrons in a crystal must exceed the period of cyclotron oscillations.

The CR method is used to determine the effective mass of carriers in semiconductors. From the half-width of the CR line, one can determine the characteristic scattering times and, thereby, determine the carrier mobility. The area of ​​the line can be used to determine the concentration of charge carriers in the sample.

Electron paramagnetic resonance. The EPR phenomenon consists in the resonant absorption of electromagnetic field energy in paramagnetic samples placed in a constant magnetic field, normal to the magnetic vector of the electromagnetic field. The physical essence of the phenomenon is as follows.

The magnetic moment of an atom with unpaired electrons is determined by expression (5.35). In a magnetic field, the energy levels of an atom, due to the interaction of the magnetic moment with the magnetic field, are split into sublevels with the energy

where is the magnetic quantum number of the atom and takes the value

It can be seen from (5.52) that the number of sublevels is , and the distance between sublevels is

Transitions of atoms from low to higher levels can occur under the action of an external electromagnetic field. According to the quantum mechanical selection rules, allowed transitions are those in which the magnetic quantum number changes by one, that is, . Therefore, the energy quantum of such a field must be equal to the distance between the sublevels

Relation (5.55) is the EPR condition. An alternating magnetic field of resonant frequency with the same probability will cause transitions from the lower magnetic sublevels to the upper ones (absorption) and vice versa (radiation). In a state of thermodynamic equilibrium, the relationship between the populations and two neighboring levels is determined by the Boltzmann law

It can be seen from (5.56) that states with lower energy have a larger population (). Therefore, the number of atoms absorbing electromagnetic field quanta under these conditions will prevail over the number of emitting atoms; as a result, the system will absorb the energy of the electromagnetic field, which leads to an increase in . However, due to the interaction with the lattice, the absorbed energy in the form of heat is transferred to the lattice, and usually so quickly that, at the frequencies used, the ratio differs very little from its equilibrium value (5.56).

The EPR frequencies can be determined from (5.55). Substituting the value and counting (pure spin moment), we obtain for the resonant frequency

From (5.57) it can be seen that in fields from to 1 T, the resonant frequencies lie in the range of Hz, that is, in the radio frequency and microwave regions.

The resonance condition (5.55) applies to isolated atoms that have magnetic moments. However, it remains valid for a system of atoms, if the interaction between the magnetic moments is negligibly small. Such a system is a paramagnetic crystal, in which magnetic atoms are located at large distances from one another.

The EPR phenomenon was predicted in 1923. Ya.G. Dorfman and experimentally discovered in 1944. E.K.Zavoisky. Currently, EPR is used as one of the most powerful methods for studying solids. Based on the interpretation of EPR spectra, information is obtained on defects, impurities in solids and electronic structure, on the mechanisms of chemical reactions, etc. Paramagnetic amplifiers and generators have been built on the basis of the EPR phenomenon.

Nuclear magnetic resonance. Heavy elementary particles are protons and neutrons (nucleons), and, consequently, atomic nuclei built from them have their own magnetic moments, which serve as a source of nuclear magnetism. The role of the elementary magnetic moment, by analogy with the electron, is played here by the Bohr nuclear magneton

The atomic nucleus has a magnetic moment

where is the factor of the nucleus, is the spin number of the nucleus, which takes half-integer and integer values:

0, 1/2, 1, 3/2, 2, ... . (5.60)

Projection of the nuclear magnetic moment on the axis z of an arbitrarily chosen coordinate system is determined by the relation

Here, the magnetic quantum number, when known, takes the values:

In the absence of an external magnetic field, all states with different states have the same energy, and therefore are degenerate. An atomic nucleus with a nonzero magnetic moment placed in an external constant magnetic field experiences spatial quantization, and its -fold degenerate level splits into a Zeeman multiplet whose levels have energies

If after that the nucleus is affected by an alternating field, the energy quantum of which is equal to the distance between the levels (5.63)

then there is a resonant absorption of energy by atomic nuclei, which is called nuclear paramagnetic resonance or simply nuclear magnetic resonance.

Due to the fact that is much smaller, the NMR resonant frequency is noticeably lower than the EPR frequency. So NMR in fields of the order of 1 T is observed in the radio frequency region.

NMR as a method for studying nuclei, atoms and molecules has received various applications in physics, chemistry, biology, medicine, technology, in particular, for measuring the strength of magnetic fields.

The traditional method of NMR spectroscopy has many disadvantages. First, it takes a lot of time to build each spectrum. Secondly, it is very picky about the absence of external interference, and, as a rule, the resulting spectra have significant noise. Thirdly, it is unsuitable for creating high-frequency spectrometers. Therefore, in modern NMR instruments, the so-called pulsed spectroscopy method is used, based on the Fourier transform of the received signal.

At present, all NMR spectrometers are built on the basis of powerful superconducting magnets with a constant magnetic field.

The essence of NMR introscopy (or magnetic resonance imaging) is the implementation of a special kind of quantitative analysis of the amplitude of the nuclear magnetic resonance signal. In the methods of NMR introscopy, the magnetic field is created by a deliberately inhomogeneous field. Then there is reason to expect that the frequency of nuclear magnetic resonance at each point of the sample has its own value, different from the values ​​in other parts. By specifying some code for NMR signal amplitude gradations (brightness or color on the monitor screen), one can obtain a conditional image (tomogram) of sections of the object's internal structure.

Ferro- and antiferromagnetic resonance. The physical essence of ferromagnetic resonance lies in the fact that under the influence of an external magnetic field that magnetizes a ferromagnet to saturation, the total magnetic moment of the sample begins to precess around this field with a Larmor frequency depending on the field. If a high-frequency electromagnetic field perpendicular to is applied to such a sample and its frequency is changed, then at , resonant absorption of the field energy occurs. Absorption in this case is several orders of magnitude higher than in paramagnetic resonance, because the magnetic susceptibility, and, consequently, the magnetic moment of saturation, is much higher in them than in paramagnets.

Features of resonance phenomena in ferro - and antiferromagnets are determined primarily by the fact that in such substances one deals not with isolated atoms or relatively weakly interacting ions of ordinary paramagnetic bodies, but with a complex system of strongly interacting electrons. The exchange (electrostatic) interaction creates a large resulting magnetization, and with it a large internal magnetic field, which significantly changes the resonance conditions (5.55).

Ferromagnetic resonance differs from EPR in that the energy absorption in this case is many orders of magnitude stronger and the resonance condition (relationship between the resonant frequency of the alternating field and the magnitude of the constant magnetic field) depends significantly on the shape of the samples.

Many microwave devices are based on the phenomenon of ferromagnetic resonance: resonant valves and filters, paramagnetic amplifiers, power limiters and delay lines.

Antiferromagnetic resonance ( electronic magnetic resonance in antiferromagnets) - the phenomenon of a relatively large selective response of the magnetic system of an antiferromagnet to the action of an electromagnetic field with a frequency (10-1000 GHz) close to the natural frequencies of the precession of the magnetization vectors of the magnetic sublattices of the system. This phenomenon is accompanied by a strong absorption of the energy of the electromagnetic field.

From a quantum point of view, a antiferromagnetic resonance can be considered as a resonant transformation of electromagnetic field photons into magnons with wave vector .

To observe a antiferromagnetic resonance radio spectrometers are used, similar to those used to study EPR, but allowing measurements at high (up to 1000 GHz) frequencies and in strong (up to 1 MG) magnetic fields. The most promising are spectrometers in which the frequency, rather than the magnetic field, is scanned. Optical methods for detecting a antiferromagnetic resonance.

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Nuclear magnetic resonance spectroscopy (NMR) is a method based on the absorption of radio frequency electromagnetic radiation by the nuclei of a sample with a nonzero magnetic moment placed in a constant magnetic field ( B 0). Non-zero magnetic moments have isotopes of nuclei of elements with an odd atomic mass (1 H, 13 C, 15 N, 19 F, 31 P, etc.).

General principles

A nucleus rotating around its axis has its own moment of momentum (angular momentum, or spin) P. The magnetic moment of the nucleus μ is directly proportional to the spin: μ = γ ∙P(γ is the proportionality factor or gyromagnetic ratio). The angular and magnetic moments are quantized, i.e. can be in one of 2 I+ 1 spin states ( I – spin quantum number). Different states of the magnetic moments of nuclei have the same energy if they are not affected by an external magnetic field. When nuclei are placed in an external magnetic field B 0, the energy degeneracy of the nuclei is removed and the possibility of an energy transition from one level to another arises. The process of distribution of nuclei between different energy levels proceeds in accordance with the Boltzmann distribution law and leads to the appearance of a macroscopic equilibrium longitudinal magnetization M z . The time it takes to create M z after turning on the external magnetic field AT 0 , is called time longitudinal or spin—lattice relaxation (T one). Violation of the equilibrium distribution of nuclei occurs under the action of a radio frequency magnetic field ( B 1), perpendicular B 0 , which causes additional transitions between energy levels, accompanied by energy absorption (the phenomenon nuclear magnetic resonance). Frequency ν 0 , at which the absorption of energy by nuclei occurs ( Larmorova or resonant absorption frequency), varies depending on the value of the constant field B 0: ν 0 = γ B 0 /2π. At the moment of resonance, there is an interaction between the individual nuclear magnetic moments and the field AT 1 , which outputs a vector M z from its equilibrium position along the axis z. As a result, there appears transverse magnetization M xy. Its change associated with the exchange within the spin system is characterized by the time transverse or spin-spin relaxation (T 2).

Dependence of the intensity of energy absorption by nuclei of the same type on the frequency of the radio-frequency magnetic field at a fixed value AT 0 is called one-dimensional spectrumnuclear magnetic resonance kernels of this type. An NMR spectrum can be obtained in two ways: by continuously irradiating the sample with an RF field of varying frequency, as a result of which the NMR spectrum is recorded directly (continuous exposure spectroscopy), or by exposing the sample to a short RF pulse ( pulsed spectroscopy). In pulsed NMR spectroscopy, time-decayed coherent radiation emitted by nuclei upon returning to the initial spin state ( free induction decay signal) followed by the transformation of the time scale into frequency ( Fourier transform).

In molecules, the electrons of atoms reduce the magnitude of the acting external magnetic field B 0 at the location of the kernel, i.e. appears diamagnetic shielding:

B loc = B 0 ∙ (1 – σ),

B lok is the intensity of the resulting field;

σ is the screening constant.

The difference in the resonant frequencies of the signals of the nuclei, equal to the difference in their screening constants, is called chemical shift signals, indicated by the symbol δ , measured in parts per million (ppm). Interaction of magnetic moments of nuclei through chemical bond electrons ( spin-spin interaction) causes splitting of the NMR signal ( multiplicity, m). The number of components in multiplets is determined by the nuclear spin and the number of interacting nuclei. The measure of the spin-spin interaction is spin-spin coupling constant (J, measured in hertz, Hz). Values ​​δ, m and J do not depend on the magnitude of the constant magnetic field.

The intensity of the nuclear NMR signal in the spectrum is determined by the population of its energy levels. Of the nuclei with a natural abundance of isotopes, the most intense signals are produced by hydrogen nuclei. The intensity of NMR signals is also affected by the time of longitudinal-transverse relaxation (large T 1 lead to a decrease in signal intensity).

The width of NMR signals (difference between frequencies at half maximum of the signal) depends on T 1 and T 2. small times T 1 and T 2 cause wide and poorly interpreted spectrum signals.

The sensitivity of the NMR method (maximum detectable concentration of a substance) depends on the intensity of the nuclear signal. For 1 H nuclei, the sensitivity is 10 -9 ÷ 10 -11 mol.

Correlations of various spectral parameters (for example, chemical shifts of different nuclei within the same molecular system) can be obtained by homo- and heteronuclear methods in 2D or 3D format.

device

High resolution NMR pulse spectrometer (NMR spectrometer) consists of:

• magnet to create a constant magnetic field B 0 ;
• a temperature-controlled sensor with a sample holder for applying an RF pulse and detecting the radiation emitted by the sample;
• an electronic device for creating a radio frequency pulse, recording, amplifying and converting the free induction decay signal into digital form;
• devices for tuning and adjusting electronic circuits;
• data collection and processing devices (computer);

and may also include:

a flow cell for NMR liquid chromatography or flow-injection analysis;

• system for creating a pulsed magnetic field gradient.

A strong magnetic field is generated by a superconductivity coil in a Dewar vessel filled with liquid helium.

The proper functioning of the NMR spectrometer should be checked. For verification, appropriate tests are carried out, including, as a rule, the measurement of the spectral linewidth at half-height of certain peaks under certain conditions ( permission), signal position reproducibility and signal-to-noise ratio (the ratio between the intensity of a specific signal in the NMR spectrum and random fluctuations in the region of the spectrum that does not contain signals from the analyte, S/N) for standard mixtures. The spectrometer software contains algorithms for determining S/N. All instrument manufacturers provide specifications and measurement protocols for these parameters.

NMR Spectroscopy of Samples in Solutions

Methodology

The test sample is dissolved in a solvent to which an appropriate chemical shift calibration standard may be added as specified in the regulatory documentation. The value of the relative chemical shift of the nucleus of a substance (δ in-in) is determined by the following expression:

δ in-in \u003d (ν in-in - ν standard) / ν of the device,

ν in-in - the resonance frequency of the core of the substance, Hz;

ν etalon is the resonance frequency of the etalon core, Hz;

ν of the device is the operating frequency of the NMR spectrometer (the frequency at which the resonance conditions for hydrogen nuclei are satisfied for a given B 0, MHz).

For solutions in organic solvents, the chemical shift in the 1H and 13C spectra is measured relative to the tetramethylsilane signal, the position of which is taken as 0 ppm. The chemical shifts are counted in the direction of a weak field (to the left) from the tetramethylsilane signal (delta is the scale of chemical shifts). For aqueous solutions, sodium 2,2-dimethyl-2-silanepentane-5-sulfonate is used as a reference in the 1 H NMR spectra, the chemical shift of the protons of the methyl group of which is 0.015 ppm. For the spectra of 13 C aqueous solutions, dioxane is used as a reference, the chemical shift of which is 67.4 ppm.

When calibrating the 19 F spectra, trifluoroacetic acid or trichlorofluoromethane is used as the primary standard with zero chemical shift; spectra 31 P - 85% solution of phosphoric acid or trimethyl phosphate; spectra 15 N - nitromethane or saturated ammonia solution. In 1 H and 13 C NMR, as a rule, an internal standard is used, which is directly added to the test sample. 15 N, 19 F, and 31 P NMR often use an external standard, which is held separately in a coaxial cylindrical tube or capillary.

When describing NMR spectra, it is necessary to indicate the solvent in which the substance is dissolved and its concentration. Easily mobile liquids are used as solvents, in which hydrogen atoms are replaced by deuterium atoms to reduce the intensity of solvent signals. The deuterated solvent is selected based on the following criteria:

• 1) the solubility of the test compound in it;
• 2) no overlap between the signals of residual protons of the deuterated solvent and the signals of the test compound;
• 3) no interaction between the solvent and the test compound, unless otherwise indicated.

Solvent atoms give signals that are easily identified by their chemical shift and can be used to calibrate the chemical shift axis (secondary standard). The chemical shifts of the residual proton signals of deuterated solvents have the following values ​​(ppm): chloroform, 7.26; benzene, 7.16; water - 4.7; methanol -3.35 and 4.78; dimethyl sulfoxide - 2.50; acetone - 2.05; the position of the signal of water and the protons of the hydroxyl groups of alcohols depends on the pH of the medium and temperature.

For quantitative analysis, solutions must be free of undissolved particles. For some assays, it may be necessary to add an internal standard to compare test and reference intensities. Appropriate standard samples and their concentrations should be specified in the normative documentation. After placing the sample in a test tube and capping, the sample is introduced into the magnet of the NMR spectrometer, the test parameters are set (settings, registration, digitization of the free induction decay signal). The main test parameters given in the regulatory documentation are recorded or stored in a computer.

To prevent spectrum drift over time, a stabilization procedure (deuterium lock) is performed using the deuterium signal induced by deuterated solvents, unless otherwise indicated. The instrument is adjusted to obtain the most optimal resonance conditions and the maximum ratio S/N(shimming).

During the test, it is possible to perform multiple sequences of cycles "impulse - data acquisition - pause" with subsequent summation of individual signals of the decay of free induction and averaging the noise level. The delay time between pulse sequences during which the system of nuclear spins restores its magnetization ( D 1), for quantitative measurements must exceed the longitudinal relaxation time T 1: D 1 ≥ 5 T one . The spectrometer software contains algorithms for determining T one . If the value T 1 is unknown, it is recommended to use the value D 1 = 25 sec.

After carrying out the Fourier transform, the signals in the frequency representation are calibrated to the selected standard and their relative intensity is measured by integration - measuring the ratio of the areas of the resonant signals. In the 13 C spectra, only signals of the same type are integrated. The signal integration accuracy depends on the ratio signal – noise (S/N):

where u(I) is the standard uncertainty of integration.

The number of free induction decay accumulations required to achieve a satisfactory ratio S/ N, should be given in the regulatory documentation.

Along with one-dimensional for analytical purposes, homo- and heteronuclear two-dimensional correlation spectra are used, based on a certain sequence of pulses (COSY, NOESY, ROESY, HSQC, HMBC, HETCOR, CIGAR, INADEQUATE, etc.). In two-dimensional spectra, the interaction between nuclei manifests itself in the form of signals called cross peaks. The position of the cross peaks is determined by the values ​​of the chemical shifts of the two interacting nuclei. Two-dimensional spectra are preferably used to determine the composition of complex mixtures and extracts, because the probability of signal superposition (cross peaks) in two-dimensional spectra is significantly lower than the probability of signal superposition in one-dimensional spectra.

To quickly obtain the spectra of heteronuclei (13 C, 15 N, etc.), methods (HSQC, HMBC) are used, which make it possible to obtain spectra of other nuclei on 1 H nuclei using the mechanisms of heteronuclear interaction.

The DOSY technique, based on recording the loss of phase coherence of nuclear spins due to translational displacements of molecules under the action of a magnetic field gradient, makes it possible to obtain spectra of individual compounds (spectral separation) in a mixture without their physical separation and to determine the sizes, degrees of aggregation, and molecular weights of molecular objects (molecules , macromolecules, molecular complexes, supramolecular systems).

Areas of use

The variety of structural and analytical information contained in nuclear magnetic resonance spectra makes it possible to use the nuclear magnetic resonance method for qualitative and quantitative analysis. The use of nuclear magnetic resonance spectroscopy in quantitative analysis is based on the direct proportionality of the molar concentration of magnetically active nuclei to the integrated intensity of the corresponding absorption signal in the spectrum.

1. Identification of the active substance. The identification of the active substance is carried out by comparing the spectrum of the test sample with the spectrum of a standard sample or with a published reference spectrum. The spectra of standard and test samples should be obtained using the same methods and conditions. The peaks in the compared spectra should coincide in position (deviations of the values δ test and standard samples within ± 0.1 ppm. for nuclear magnetic resonance 1 N and ± 0.5 ppm. for nuclear magnetic resonance 13 C), integrated intensity and multiplicity, the values ​​of which should be given when describing the spectra. In the absence of a standard sample, a pharmacopoeial standard sample can be used, the identity of which is confirmed by independent structural interpretation of the spectral data and alternative methods.

When confirming the authenticity of samples of non-stoichiometric composition (for example, natural polymers of variable composition), the peaks of the test and standard samples are allowed to differ in position and integral intensity of the signals. The spectra to be compared must be similar, i.e. contain the same characteristic regions of the signals, confirming the coincidence of the fragment composition of the test and standard samples.

To establish the authenticity of a mixture of substances (extracts), one-dimensional NMR spectra can be used as a whole, as “fingerprints” of an object, without detailing the values ​​of δ and the multiplicity of individual signals. In the case of using two-dimensional NMR spectroscopy in the description of spectra (spectrum fragments) claimed for authenticity, the values ​​of cross peaks should be given.

1. Identification of foreign matter/residual organic solvents. Identification of impurities/residual organic solvents is carried out similarly to the identification of the active substance, tightening the requirements for sensitivity and digital resolution.
2. Determination of the content of foreign impurities / residual organic solvents in relation to the active substance. The NMR method is a direct absolute method for determining the molar ratio of the active substance and the impurity compound ( n/n impurity):

where S‘ and S‘ impurity - normalized values ​​of the integral intensities of the signals of the active substance and impurity.

Normalization is carried out according to the number of nuclei in the structural fragment, which determine the measured signal.

Mass fraction of impurity / residual organic solvent relative to the active substance ( X pr) is determined by the formula:

M pr is the molecular weight of the impurity;

M is the molecular weight of the active substance;

S‘ pr is the normalized value of the integral intensity of the impurity signal;

S'– normalized value of the integral intensity of the signal of the active substance.

1. Quantitative determination of the content of the substance (active substance, impurity / residual solvent) in the pharmaceutical substance. Absolute content of matter in a pharmaceutical substance, it is determined by the internal standard method, which is chosen as a substance whose signals are close to the signals of the analyte, without overlapping with them. The signal intensities of the analyte and the standard should not differ significantly.

The percentage of the analyte in the test sample in terms of dry matter ( x,% mass) is calculated by the formula:

x,% mass = 100 ∙ ( S‘ /S‘ 0) ∙ (M ∙ a 0 /M 0 ∙ a) ∙ ,

S' is the normalized value of the integral intensity of the signal of the analyte;

S‘ 0 is the normalized value of the integrated signal intensity of the standard;

M is the molecular weight of the analyte;

M 0 – molecular weight;

a- weighing of the test sample;

a 0– weight of the standard substance;

W- moisture contents, %.

The following compounds can be used as standards: maleic acid (2H; 6.60 ppm, M= 116.07), benzyl benzoate (2H; 5.30 ppm, M= 212.25), malonic acid (2H; 3.30 ppm, M= 104.03), succinimide (4H; 2.77 ppm, M= 99.09), acetanilide (3H; 2.12 ppm, M = 135,16), tert-butanol (9H; 1.30 ppm, M = 74,12).

Relative substance content as the proportion of a component in a mixture of components of a pharmaceutical substance is determined by the method of internal normalization. molar ( X mol) and mass ( X mass) component fraction i in a mixture n substances is determined by the formulas:

1. Determination of the molecular weight of proteins and polymers. The molecular weights of proteins and polymers are determined by comparing their mobility with that of reference compounds of known molecular weight using DOSY techniques. Self-diffusion coefficients are measured ( D) of the test and standard samples, build a graph of the dependence of the logarithms of the molecular weights of the standard compounds on the logarithms D. From the graph thus obtained, the unknown molecular weights of the test samples are determined by linear regression. A full description of the DOSY experiment should be given in the regulatory documentation.

NMR spectroscopy of solids

Samples in the solid state are analyzed using specially equipped NMR spectrometers. Certain technical operations (rotation of a powdered sample in a rotor inclined at a magic angle (54.7°) to the magnetic field axis AT 0 , force depairing, polarization transfer from highly excitable nuclei to less polarizable nuclei - cross-polarization) make it possible to obtain high-resolution spectra of organic and inorganic compounds. A full description of the procedure should be given in the regulatory documentation. The main area of ​​application of this type of NMR spectroscopy is the study of polymorphism of solid drugs.