NMR spectroscopy

Nuclear magnetic resonance spectroscopy, NMR spectroscopy- a spectroscopic method for studying chemical objects, using the phenomenon of nuclear magnetic resonance. The most important for chemistry and practical applications are proton magnetic resonance spectroscopy (PMR spectroscopy), as well as NMR spectroscopy on carbon-13 ( 13 C NMR spectroscopy), fluorine-19 (infrared spectroscopy, NMR reveals information about the molecular structure of chemicals However, it provides more complete information than IS, allowing one to study dynamic processes in a sample - to determine the rate constants of chemical reactions, the value of energy barriers to intramolecular rotation. These features make NMR spectroscopy a convenient tool both in theoretical organic chemistry and for the analysis of biological objects.

Basic NMR technique

A sample of a substance for NMR is placed in a thin-walled glass tube (ampule). When it is placed in a magnetic field, NMR active nuclei (such as 1 H or 13 C) absorb electromagnetic energy. The resonant frequency, absorption energy and intensity of the emitted signal are proportional to the strength of the magnetic field. So in a field of 21 Tesla, a proton resonates at a frequency of 900 MHz.

Chemical shift

Depending on the local electronic environment, different protons in a molecule resonate at slightly different frequencies. Since both this frequency shift and the fundamental resonant frequency are directly proportional to the strength of the magnetic field, this displacement is converted into a dimensionless quantity independent of the magnetic field known as a chemical shift. Chemical shift is defined as a relative change relative to some reference samples. The frequency shift is extremely small compared to the main NMR frequency. The typical frequency shift is 100 Hz, whereas the base NMR frequency is on the order of 100 MHz. Thus, the chemical shift is often expressed in parts per million (ppm). In order to detect such a small frequency difference, the applied magnetic field must be constant inside the sample volume.

Since a chemical shift depends on the chemical structure of a substance, it is used to obtain structural information about the molecules in a sample. For example, the spectrum for ethanol (CH 3 CH 2 OH) gives 3 distinctive signals, that is, 3 chemical shifts: one for the CH 3 group, the second for the CH 2 group and the last for OH. The typical shift for a CH 3 group is approximately 1 ppm, for a CH 2 group attached to OH-4 ppm and OH is approximately 2-3 ppm.

Due to molecular motion at room temperature, the signals of the 3 methyl protons are averaged out during the NMR process, which lasts only a few milliseconds. These protons degenerate and form peaks at the same chemical shift. The software allows you to analyze the size of the peaks in order to understand how many protons contribute to these peaks.

Spin-spin interaction

The most useful information for determining structure in a one-dimensional NMR spectrum is provided by the so-called spin-spin interaction between active NMR nuclei. This interaction results from transitions between different spin states of nuclei in chemical molecules, resulting in splitting of the NMR signals. This splitting can be simple or complex and, as a consequence, can be either easy to interpret or can be confusing to the experimenter.

This binding provides detailed information about the bonds of atoms in the molecule.

Second order interaction (strong)

Simple spin-spin coupling assumes that the coupling constant is small compared to the difference in chemical shifts between the signals. If the shift difference decreases (or the interaction constant increases), the intensity of the sample multiplets becomes distorted and becomes more difficult to analyze (especially if the system contains more than 2 spins). However, in high-power NMR spectrometers the distortion is usually moderate and this allows associated peaks to be easily interpreted.

Second-order effects decrease as the frequency difference between multiplets increases, so a high-frequency NMR spectrum shows less distortion than a low-frequency spectrum.

Application of NMR spectroscopy to the study of proteins

Most of the recent innovations in NMR spectroscopy are made in the so-called NMR spectroscopy of proteins, which is becoming a very important technique in modern biology and medicine. The overall goal is to obtain the 3-dimensional structure of a protein in high resolution, similar to the images obtained in X-ray crystallography. Due to the presence of more atoms in a protein molecule compared to a simple organic compound, the basic 1D spectrum is crowded with overlapping signals, making direct analysis of the spectrum impossible. Therefore, multidimensional techniques have been developed to solve this problem.

To improve the results of these experiments, the tagged atom method is used, using 13 C or 15 N. In this way, it becomes possible to obtain a 3D spectrum of a protein sample, which has become a breakthrough in modern pharmaceuticals. Recently, techniques (which have both advantages and disadvantages) for obtaining 4D spectra and spectra of higher dimensions, based on nonlinear sampling methods with subsequent restoration of the free induction decay signal using special mathematical techniques, have become widespread.

Literature

• Gunther X. Introduction to NMR spectroscopy course. - Per. from English - M., 1984.

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See what “NMR spectroscopy” is in other dictionaries:

Nuclear magnetic resonance spectroscopy on carbon nuclei 13, 13C NMR spectroscopy is one of the methods of NMR spectroscopy using nuclei of the carbon isotope 13C. The 13C nucleus has a spin of 1/2 in its ground state, its content in nature... ... Wikipedia

Image of a human brain on a medical NMR tomograph Nuclear magnetic resonance (NMR) resonant absorption of electromagnetic energy by a substance containing nuclei with non-zero spin in an external magnetic field, caused by reorientation ... ... Wikipedia

NMR spectroscopy

NMR spectroscopy

magnetic resonance spectroscopy- magnetinio branduolių rezonanso spektroskopija statusas T sritis Standartizacija ir metrologija apibrėžtis Spektroskopija, pagrįsta kietųjų, skystųjų ir dujinių medžiagų magnetinio branduolių rezonanso reiškiniu. atitikmenys: engl. NMR... ... Penkiakalbis aiškinamasis metrologijos terminų žodynas

nuclear magnetic resonance spectroscopy- branduolinio magnetinio rezonanso spektroskopija statusas T sritis fizika atitikmenys: engl. NMR spectroscopy; nuclear magnetic resonance spectroscopy vok. magnetische Kernresonanzspektroskopie, f; NMR Spektroskopie, f rus. spectroscopy of nuclear… Fizikos terminų žodynas

Magnetinio branduolių rezonanso spektroskopija statusas T sritis Standartizacija ir metrologija apibrėžtis Spektroskopija, pagrįsta kietųjų, skystųjų ir dujinių medžiagų magnetinio branduolių rezonanso reiškiniu. atitikmenys: engl. NMR... ... Penkiakalbis aiškinamasis metrologijos terminų žodynas

nuclear resonance spectroscopy- branduolinio magnetinio rezonanso spektroskopija statusas T sritis fizika atitikmenys: engl. NMR spectroscopy; nuclear magnetic resonance spectroscopy vok. magnetische Kernresonanzspektroskopie, f; NMR Spektroskopie, f rus. spectroscopy of nuclear… Fizikos terminų žodynas

A set of research methods. in VA according to the absorption spectra of their atoms, ions and molecules. mag. radio waves. Radiation includes electron paramagnetic methods. resonance (EPR), nuclear magnetic. resonance (NMR), cyclotron resonance, etc... Natural science. encyclopedic Dictionary

Image of a 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

Nuclear magnetic resonance spectroscopy is one of the most common and very sensitive methods for determining the structure of organic compounds, allowing one to obtain information not only about the qualitative and quantitative composition, but also the location of atoms relative to each other. Various NMR techniques have many possibilities for determining the chemical structure of substances, confirmation states of molecules, effects of mutual influence, and intramolecular transformations.

The nuclear magnetic resonance method has a number of distinctive features: in contrast to optical molecular spectra, the absorption of electromagnetic radiation by a substance occurs in a strong uniform external magnetic field. Moreover, to conduct an NMR study, the experiment must meet a number of conditions reflecting the general principles of NMR spectroscopy:

1) recording NMR spectra is possible only for atomic nuclei with their own magnetic moment or so-called magnetic nuclei, in which the number of protons and neutrons is such that the mass number of isotope nuclei is odd. All nuclei with an odd mass number have spin I, the value of which is 1/2. So for nuclei 1 H, 13 C, l 5 N, 19 F, 31 R the spin value is equal to 1/2, for nuclei 7 Li, 23 Na, 39 K and 4 l R the spin is equal to 3/2. Nuclei with an even mass number either have no spin at all if the nuclear charge is even, or have integer spin values ​​if the charge is odd. Only those nuclei whose spin is I 0 can produce an NMR spectrum.

The presence of spin is associated with the circulation of atomic charge around the nucleus, therefore, a magnetic moment arises μ . A rotating charge (for example, a proton) with angular momentum J creates a magnetic moment μ=γ*J . The angular nuclear momentum J and the magnetic moment μ arising during rotation can be represented as vectors. Their constant ratio is called the gyromagnetic ratio γ. It is this constant that determines the resonant frequency of the core (Fig. 1.1).

Figure 1.1 - A rotating charge with an angular moment J creates a magnetic moment μ=γ*J.

2) the NMR method examines the absorption or emission of energy under unusual conditions of spectrum formation: in contrast to other spectral methods. The NMR spectrum is recorded from a substance located in a strong uniform magnetic field. Such nuclei in an external field have different potential energy values ​​depending on several possible (quantized) orientation angles of the vector μ relative to the external magnetic field strength vector H 0 . In the absence of an external magnetic field, the magnetic moments or spins of nuclei do not have a specific orientation. If magnetic nuclei with spin 1/2 are placed in a magnetic field, then some of the nuclear spins will be located parallel to the magnetic field lines, the other part antiparallel. These two orientations are no longer energetically equivalent and the spins are said to be distributed at two energy levels.

Spins with a magnetic moment oriented along the +1/2 field are designated by the symbol | α >, with an orientation antiparallel to the external field -1/2 - symbol | β > (Fig. 1.2) .

Figure 1.2 - Formation of energy levels when an external field H 0 is applied.

1.2.1 NMR spectroscopy on 1 H nuclei. Parameters of PMR spectra.

To decipher the data of 1H NMR spectra and assign signals, the main characteristics of the spectra are used: chemical shift, spin-spin interaction constant, integrated signal intensity, signal width [57].

A) Chemical shift (C.C). H.S. scale Chemical shift is the distance between this signal and the signal of the reference substance, expressed in parts per million of the external field strength.

Tetramethylsilane [TMS, Si(CH 3) 4], containing 12 structurally equivalent, highly shielded protons, is most often used as a standard for measuring the chemical shifts of protons.

B) Spin-spin interaction constant. In high-resolution NMR spectra, signal splitting is observed. This splitting or fine structure in high-resolution spectra results from spin-spin interactions between magnetic nuclei. This phenomenon, along with the chemical shift, serves as the most important source of information about the structure of complex organic molecules and the distribution of the electron cloud in them. It does not depend on H0, but depends on the electronic structure of the molecule. The signal of a magnetic nucleus interacting with another magnetic nucleus is split into several lines depending on the number of spin states, i.e. depends on the spins of nuclei I.

The distance between these lines characterizes the spin-spin coupling energy between nuclei and is called the spin-spin coupling constant n J, where n-the number of bonds that separate interacting nuclei.

There are direct constants J HH, geminal constants 2 J HH , vicinal constants 3 J HH and some long-range constants 4 J HH , 5 J HH .

- geminal constants 2 J HH can be both positive and negative and occupy the range from -30 Hz to +40 Hz.

The vicinal constants 3 J HH occupy the range 0 20 Hz; they are almost always positive. It has been established that vicinal interaction in saturated systems very strongly depends on the angle between carbon-hydrogen bonds, that is, on the dihedral angle - (Fig. 1.3).

Figure 1.3 - Dihedral angle φ between carbon-hydrogen bonds.

Long-range spin-spin interaction (4 J HH , 5 J HH ) - interaction of two nuclei separated by four or more bonds; the constants of such interaction are usually from 0 to +3 Hz.

Table 1.1 – Spin-spin interaction constants

B) Integrated signal intensity. The area of ​​the signals is proportional to the number of magnetic nuclei resonating at a given field strength, so that the ratio of the areas of the signals gives the relative number of protons of each structural variety and is called the integrated signal intensity. Modern spectrometers use special integrators, the readings of which are recorded in the form of a curve, the height of the steps of which is proportional to the area of ​​the corresponding signals.

D) Width of lines. To characterize the width of lines, it is customary to measure the width at a distance of half the height from the zero line of the spectrum. The experimentally observed line width consists of the natural line width, which depends on the structure and mobility, and the broadening due to instrumental reasons

The usual line width in PMR is 0.1-0.3 Hz, but it can increase due to the overlap of adjacent transitions, which do not exactly coincide, but are not resolved as separate lines. Broadening is possible in the presence of nuclei with a spin greater than 1/2 and chemical exchange.

1.2.2 Application of 1 H NMR data to determine the structure of organic molecules.

When solving a number of problems of structural analysis, in addition to tables of empirical values, Kh.S. It may be useful to quantify the effects of neighboring substituents on Ch.S. according to the rule of additivity of effective screening contributions. In this case, substituents that are no more than 2-3 bonds distant from a given proton are usually taken into account, and the calculation is made using the formula:

δ=δ 0 +ε i *δ i (3)

where δ 0 is the chemical shift of protons of the standard group;

δi is the contribution of screening by the substituent.

1.3 NMR spectroscopy 13 C. Obtaining and modes of recording spectra.

The first reports of the observation of 13 C NMR appeared in 1957, but the transformation of 13 C NMR spectroscopy into a practically used method of analytical research began much later.

Magnetic resonance 13 C and 1 H have much in common, but there are also significant differences. The most common carbon isotope 12 C has I=0. The 13 C isotope has I=1/2, but its natural content is 1.1%. This is along with the fact that the gyromagnetic ratio of 13 C nuclei is 1/4 of the gyromagnetic ratio for protons. Which reduces the sensitivity of the method in experiments on observing 13 C NMR by 6000 times compared to 1 H nuclei.

a) without suppressing spin-spin interaction with protons. 13 C NMR spectra obtained in the absence of complete suppression of spin-spin resonance with protons were called high-resolution spectra. These spectra contain complete information about the 13 C - 1 H constants. In relatively simple molecules, both types of constants - direct and long-range - are found quite simply. So 1 J (C-H) is 125 - 250 Hz, however, spin-spin interaction can also occur with more distant protons with constants less than 20 Hz.

b) complete suppression of spin-spin interaction with protons. The first major progress in the field of 13 C NMR spectroscopy is associated with the use of complete suppression of spin-spin interaction with protons. The use of complete suppression of spin-spin interaction with protons leads to the merging of multiplets with the formation of singlet lines if there are no other magnetic nuclei in the molecule, such as 19 F and 31 P.

c) incomplete suppression of spin-spin interaction with protons. However, using the mode of complete decoupling from protons has its drawbacks. Since all carbon signals are now in the form of singlets, all information about the spin-spin interaction constants 13 C- 1 H is lost. A method is proposed that makes it possible to partially restore information about the direct spin-spin interaction constants 13 C- 1 H and at the same time retain more part of the benefits of broadband decoupling. In this case, splittings will appear in the spectra due to the direct constants of the spin-spin interaction 13 C - 1 H. This procedure makes it possible to detect signals from unprotonated carbon atoms, since the latter do not have protons directly associated with 13 C and appear in the spectra with incomplete decoupling from protons as singlets.

d) modulation of the CH interaction constant, JMODCH spectrum. A traditional problem in 13C NMR spectroscopy is determining the number of protons associated with each carbon atom, i.e., the degree of protonation of the carbon atom. Partial suppression by protons makes it possible to resolve the carbon signal from multiplicity caused by long-range spin-spin interaction constants and obtain signal splitting due to direct 13 C-1 H coupling constants. However, in the case of strongly coupled spin systems AB and the overlap of multiplets in the OFFR mode makes unambiguous resolution of signals difficult.

Nuclear magnetic resonance spectroscopy, NMR spectroscopy- a spectroscopic method for studying chemical objects, using the phenomenon of nuclear magnetic resonance. The NMR phenomenon was discovered in 1946 by American physicists F. Bloch and E. Purcell. The most important for chemistry and practical applications are proton magnetic resonance spectroscopy (PMR spectroscopy), as well as NMR spectroscopy on carbon-13 ( 13 C NMR spectroscopy), fluorine-19 ( 19 F NMR spectroscopy), phosphorus-31 ( 31 P NMR spectroscopy).If an element has an odd atomic number or an isotope of any (even even) element has an odd mass number, the nucleus of such an element has a spin different from zero. From an excited state to a normal state, nuclei can return, transferring excitation energy to the surrounding “lattice,” which in this case means electrons or atoms of a different type than those being studied. This energy transfer mechanism is called spin-lattice relaxation, and its efficiency can be characterized by a constant T1, called the spin-lattice relaxation time.

These features make NMR spectroscopy a convenient tool both in theoretical organic chemistry and for the analysis of biological objects.

Basic NMR technique

A sample of a substance for NMR is placed in a thin-walled glass tube (ampule). When it is placed in a magnetic field, NMR active nuclei (such as 1 H or 13 C) absorb electromagnetic energy. The resonant frequency, absorption energy and intensity of the emitted signal are proportional to the strength of the magnetic field. So, in a field of 21 Tesla, a proton resonates at a frequency of 900 MHz.

Chemical shift

Depending on the local electronic environment, different protons in a molecule resonate at slightly different frequencies. Since both this frequency shift and the fundamental resonant frequency are directly proportional to the magnitude of the magnetic field induction, this displacement is converted into a dimensionless quantity independent of the magnetic field, known as a chemical shift. Chemical shift is defined as a relative change relative to some reference samples. The frequency shift is extremely small compared to the main NMR frequency. The typical frequency shift is 100 Hz, whereas the base NMR frequency is on the order of 100 MHz. Thus, the chemical shift is often expressed in parts per million (ppm). In order to detect such a small frequency difference, the applied magnetic field must be constant inside the sample volume.

Since a chemical shift depends on the chemical structure of a substance, it is used to obtain structural information about the molecules in a sample. For example, the spectrum for ethanol (CH 3 CH 2 OH) gives 3 distinctive signals, that is, 3 chemical shifts: one for the CH 3 group, the second for the CH 2 group and the last for OH. The typical shift for a CH 3 group is approximately 1 ppm, for a CH 2 group attached to OH is 4 ppm, and for OH is approximately 2-3 ppm.

Due to molecular motion at room temperature, the signals of the 3 methyl protons are averaged out during the NMR process, which lasts only a few milliseconds. These protons degenerate and form peaks at the same chemical shift. The software allows you to analyze the size of the peaks in order to understand how many protons contribute to these peaks.

Spin-spin interaction

The most useful information for determining structure in a one-dimensional NMR spectrum is provided by the so-called spin-spin interaction between active NMR nuclei. This interaction results from transitions between different spin states of nuclei in chemical molecules, resulting in splitting of the NMR signals. This splitting can be simple or complex and, as a consequence, can be either easy to interpret or can be confusing to the experimenter.

This binding provides detailed information about the bonds of atoms in the molecule.

Second order interaction (strong)

Simple spin-spin coupling assumes that the coupling constant is small compared to the difference in chemical shifts between the signals. If the shift difference decreases (or the interaction constant increases), the intensity of the sample multiplets becomes distorted and becomes more difficult to analyze (especially if the system contains more than 2 spins). However, in high-power NMR spectrometers the distortion is usually moderate and this allows associated peaks to be easily interpreted.

Second-order effects decrease as the frequency difference between multiplets increases, so a high-frequency NMR spectrum shows less distortion than a low-frequency spectrum.

Application of NMR spectroscopy to the study of proteins

Most of the recent innovations in NMR spectroscopy are made in the so-called NMR spectroscopy of proteins, which is becoming a very important technique in modern biology and medicine. A common goal is to obtain high-resolution 3-dimensional protein structures, similar to images obtained in X-ray crystallography. Due to the presence of more atoms in a protein molecule compared to a simple organic compound, the basic 1H spectrum is crowded with overlapping signals, making direct analysis of the spectrum impossible. Therefore, multidimensional techniques have been developed to solve this problem.

To improve the results of these experiments, the tagged atom method is used using 13 C or 15 N. In this way, it becomes possible to obtain a 3D spectrum of a protein sample, which has become a breakthrough in modern pharmaceuticals. Recently, techniques (with both advantages and disadvantages) for obtaining 4D spectra and spectra of higher dimensions, based on nonlinear sampling methods with subsequent restoration of the free induction decay signal using special mathematical techniques, have become widespread.

Quantitative NMR Analysis

In the quantitative analysis of solutions, peak area can be used as a measure of concentration in the calibration plot method or the addition method. There are also known methods in which a graduated graph reflects the concentration dependence of the chemical shift. The use of the NMR method in inorganic analysis is based on the fact that in the presence of paramagnetic substances, the nuclear relaxation time accelerates. Measuring the relaxation rate can be performed by several methods. A reliable and universal one is, for example, the pulsed version of the NMR method, or, as it is usually called, the spin echo method. When measuring using this method, short-term radio frequency pulses are applied to the sample under study in a magnetic field at certain intervals in the region of resonant absorption. A spin echo signal appears in the receiving coil, the maximum amplitude of which is related to the relaxation time by a simple relationship. To carry out conventional analytical determinations there is no need to find the absolute values ​​of the relaxation rates. In these cases, we can limit ourselves to measuring some quantity proportional to them, for example, the amplitude of the resonant absorption signal. Amplitude measurements can be performed using simple, more accessible equipment. A significant advantage of the NMR method is the wide range of values ​​of the measured parameter. Using the spin echo setup, the relaxation time can be determined from 0.00001 to 100 s. with an error of 3...5%. This makes it possible to determine the concentration of a solution in a very wide range from 1...2 to 0.000001...0000001 mol/l. The most commonly used analytical technique is the calibration graph method. Heberlen U., Mehring M. High resolution NMR in solids. - M.: Mir. - 1980.

1. The essence of the phenomenon

   First of all, it should be noted that although the name of this phenomenon contains the word “nuclear,” NMR has nothing to do with nuclear physics and is in no way connected with radioactivity. If we talk about a strict description, then there is no way to do without the laws of quantum mechanics. According to these laws, the energy of interaction of the 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 imagined as an electrically charged ball rotating around its axis (although, strictly speaking, this is not so). 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 axis of the top precesses (rotates) around the vertical if it is not untwisted strictly vertically, but at a certain angle. In this case, the role of the magnetic field is played by the force of gravity.

   The precession frequency is determined both by the properties of the nucleus and the strength of the magnetic field: the stronger the field, the higher the frequency. Then, if, in addition to a constant external magnetic field, the core is affected by an alternating magnetic field, then the core begins to interact with this field - it seems to swing the core more strongly, the precession amplitude increases, and the core absorbs the energy of the alternating field. However, this will only happen under the condition of resonance, i.e., the coincidence of the precession frequency and the frequency of the external alternating field. This is similar to the classic example from school physics - soldiers marching across a bridge. If the frequency of the step coincides with the natural frequency of the bridge, then the bridge swings 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, absorption increases sharply, and the simplest magnetic resonance spectrum looks like this:

   

2. Fourier transform spectroscopy

   The first NMR spectrometers worked exactly as described above - the sample was placed in a constant magnetic field, and radio frequency radiation was continuously applied to it. Then either the frequency of the alternating field or the intensity of the constant magnetic field varied smoothly. The absorption of alternating field energy was recorded by a radio frequency bridge, the signal from which was output to a recorder or oscilloscope. But this method of signal recording 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 the signal induced in the RF coil by the freely precessing magnetic moments is recorded. 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 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 exciting pulse to record a spectrum is the simplest NMR experiment. However, there can be many such pulses of different durations, amplitudes, with different delays between them, etc., in an experiment, depending on what kind of manipulations the researcher needs to carry out 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.

3. Magnetic interactions in matter

   Magnetic resonance itself 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, the NMR method can provide a variety of 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 such interaction is the so-called chemical shift. Its essence is as follows: the electron shell of a 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 a 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, 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 - the protons of the methyl group CH 3 and the proton associated with the oxygen atom. As 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:

   

4. 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, but other isotopes of the same nucleus do not. In total, there are more than a hundred isotopes of various chemical elements that have magnetic nuclei, but in research usually no more than 1520 magnetic nuclei are used, everything else is exotic. Each nucleus has its own characteristic ratio of magnetic field and precession frequency, called the gyromagnetic ratio. For all nuclei these relations are known. Using them, you can select the frequency at which, under a given magnetic field, a signal from the nuclei the researcher needs will be observed.

   The most important nuclei for NMR are protons. They are the most abundant in nature, and they have a very high sensitivity. The nuclei of carbon, nitrogen and oxygen are very important for chemistry and biology, but scientists have not had much luck with them: the most common isotopes of carbon and oxygen, 12 C and 16 O, do not have a magnetic moment, the natural isotope of nitrogen 14 N has a moment, but it for a number of reasons it is very inconvenient for experiments. There are isotopes 13 C, 15 N and 17 O that are suitable for NMR experiments, but their natural abundance is very low and their sensitivity is very low compared to protons. Therefore, special isotope-enriched samples are often prepared for NMR studies, in which the natural isotope of a particular nucleus is replaced by the one needed for the experiments. In most cases, this procedure is very difficult and expensive, but sometimes it is the only opportunity to obtain the necessary information.

5. Electron paramagnetic and quadrupole resonance

   Speaking about 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 is that the resonance is observed at the magnetic moments not of atomic nuclei, but of the electron shell of the atom. EPR can only be observed in those molecules or chemical groups whose electron shell contains a 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 of use is significantly 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 a so-called paramagnetic probe, that is, a chemical group with an unpaired electron that binds to the molecule under study. But this approach has obvious disadvantages that limit the capabilities of this method. In addition, EPR does not have such a high spectral resolution (i.e., 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 fingers”. Some nuclei have what is called an electric quadrupole moment. This moment characterizes the deviation of the distribution of the electric charge of the nucleus from spherical symmetry. The interaction of this moment with the gradient of the electric field created by the crystalline structure of the substance leads to the splitting of the energy levels of the nucleus. In this case, one can observe a resonance 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 of application is even narrower than that of EPR.

6. Advantages and disadvantages of NMR

   

   NMR is the most powerful and informative method for studying molecules. Strictly speaking, this is not one method, it is a large number of different types of experiments, i.e., pulse sequences. Although they are all based on the phenomenon of NMR, each of these experiments is designed to obtain some specific specific information. The number of these experiments is measured in 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 feasible, of course, not always. One of the main advantages of NMR is that, on the one hand, its natural probes, i.e. magnetic nuclei, are distributed throughout the molecule, and on the other hand, it allows one 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 part of it.

   NMR has two main disadvantages. Firstly, it is 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, an NMR experiment can be carried out for even several weeks. Secondly, it is expensive. NMR spectrometers are among the most expensive scientific instruments, costing at least hundreds of thousands of dollars, and the most expensive spectrometers cost several million. Not all laboratories, especially in Russia, can afford to have such scientific equipment.

7. Magnets for NMR spectrometers

   One of the most important and expensive parts of the 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 fields as high as possible. The magnetic field is created by the electric current in the solenoid - the stronger the current, the larger 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, for a very long time, high-field NMR spectrometers have used superconducting magnets, i.e., magnets in which the solenoid wire is in a superconducting state. In this case, the electrical resistance of the wire is zero, and no energy is released at any current value. The superconducting state can only be achieved at very low temperatures, just a few degrees Kelvin, the temperature of liquid helium. (High-temperature superconductivity is still the domain of purely 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 make them expensive. A superconducting magnet is built on the principle of a thermos-matryoshka. The solenoid is located 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 to ensure that the helium evaporates as slowly as possible. Finally, the nitrogen shell is isolated from room temperature by an external vacuum layer. Such a system is capable of maintaining the desired temperature of a superconducting magnet for a very long time, although this requires regularly adding liquid nitrogen and helium to 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 starting the magnet, the current runs through superconducting wires with virtually no losses for many years.

8. 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 the use of NMR. The inhomogeneity of the field is created by so-called gradient coils, which work in tandem 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 relationship between magnetic field and frequency. By changing the magnitude of the magnetic field (or, which is essentially the same thing, the frequency of signal observation), you can change the layer that will produce the signal. In this way, it is possible to “scan” the sample throughout its entire 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 the sample. The most interesting and important, from a practical point of view, application of NMR tomography was found in medicine. In this case, the “specimen” being studied 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 interesting 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.

9. History of discovery

   

   The year of discovery of NMR is considered to be 1945, when the Americans Felix Bloch from Stanford and, independently of him, Edward Purcell 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 had been theoretically predicted, and several attempts had been 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 Evgeniy Zavoisky. It is now well known that Zavoisky also observed the NMR signal, this was before the war, in 1941. However, he had at his disposal a low-quality magnet with poor field uniformity; the results were poorly reproducible and therefore remained unpublished. To be fair, it should be noted that Zavoisky was not the only one who observed NMR before its “official” discovery. In particular, the American physicist Isidor Rabi (Nobel Prize winner in 1944 for his study of the magnetic properties of nuclei in atomic and molecular beams) also observed NMR in the late 30s, but considered it an instrumental artifact. One way or another, our country retains priority in the experimental detection of magnetic resonance. Although Zavoisky himself began to deal with other problems soon after the war, his discovery played a huge role in the development of science in Kazan. Kazan still remains one of the world's leading scientific centers for EPR spectroscopy.

10. 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 Zeeman, Otto Stern, Isidor Rabi, Wolfgang Pauli. But there were four Nobel Prizes directly related to NMR. In 1952, the prize was awarded to Felix Bloch and Edward Purcell for the discovery of nuclear magnetic resonance. This is the only “NMR” Nobel Prize in physics. In 1991, the Swiss Richard Ernst, who worked at the famous ETH in Zurich, received the prize in chemistry. 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 winner of the prize, also in chemistry, was Kurt Wüthrich, who worked with Ernst in neighboring buildings at the same Technical School. He received the prize for developing methods for determining the three-dimensional structure of proteins in solution. Previously, the only method to determine the spatial conformation of large biomacromolecules was 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 tomography. The Soviet discoverer of EPR, E.K. Zavoisky, alas, did not receive the Nobel Prize.

The NMR spectroscopy method is based on the magnetic properties of nuclei. The nuclei of atoms carry a positive charge and rotate around their axis. The rotation of the charge leads to the appearance of a magnetic dipole.

The angular momentum of rotation, which can be described by the spin quantum number (I). The numerical value of the spin quantum number is equal to the sum of the spin quantum numbers of protons and neutrons included in the nucleus.

The spin quantum number can take the value

If the number of nucleons is even, then the value I = 0, or an integer. These are the nuclei C 12, H 2, N 14; such nuclei do not absorb radio frequency radiation and do not produce signals in NMR spectroscopy.

I = ± 1 / 2 H 1 , P 31 , F 19 - absorb radio frequency radiation and produce an NMR spectrum signal.

I = ± 1 1/2 CL 35, Br 79 - non-symmetrical charge distribution over the surface of the nucleus. Which leads to the emergence of a quadropole moment. Such nuclei are not studied by NMR spectroscopy.

PMR - spectroscopy

The numerical value of I (I = ±1/2) determines the number of possible orientations of the nucleus in an external magnetic field in accordance with the formula:

From this formula it is clear that the number of orientations is 2.

In order to make the transition of a proton located at a lower level to a higher one, it needs to be given an energy equal to the difference in the energy of these levels, that is, irradiated with radiation of a strictly defined purity. The difference in energy levels (ΔΕ) depends on the magnitude of the imposed magnetic field (H 0) and the magnetic nature of the nuclei, described by the magnetic moment (μ). This value is determined by rotation:

, Where

h – Planck’s constant

Magnitude of external magnetic field

γ – proportionality coefficient, called the gyromagnetic ratio, determines the relationship between the spin quantum number I and the magnetic moment μ.

– basic NMR equation, it connects the magnitude of the external magnetic field, the magnetic nature of the nuclei and the purity of radiation at which the absorption of radiation energy occurs and the nuclei move between levels.

From the above record it is clear that for the same nuclei, protons, there is a strict relationship between the value of H 0 and μ.

So, for example, in order for proton nuclei in an external magnetic field of 14000 Gauss to move to a higher magnetic level, they need to be irradiated with a frequency of 60 MHz; if up to 23000 Gauss, then radiation with a frequency of 100 MHz will be required.

Thus, from the above it follows that the main parts of an NMR spectrometer should be a powerful magnet and a source of radio frequency radiation.

The analyzing substance is placed in an ampoule made of special types of glass 5 mm thick. We place the ampoule in the gap of a magnet, for a more uniform distribution of the magnetic field inside the ampoule, it rotates around its axis, with the help of a coil the radiation is generated continuously by radio frequency radiation. The frequency of this radiation varies over a small range. At some point in time, when the frequency exactly corresponds to the NMR spectroscopy equation, absorption of radiation energy is observed and the protons reorient their spin - this absorption of energy is recorded by the receiving coil in the form of a narrow peak.

In some spectrometer models μ=const, and in small aisles the value of H 0 changes. To register the spectrum, 0.4 ml of a substance is needed; if a solid substance is dissolved in a suitable solution, it is necessary to take 10-50 ml/g of the substance.

To obtain a high-quality spectrum, it is necessary to use solutions with a concentration of 10–20%. The NMR sensitivity limit corresponds to 5%.

To increase sensitivity using a computer, many hours of signal accumulation are used, while the useful signal increases in intensity.

In the further improvement of the NMR spectrodistribution technique, the use of Fourier - signal conversion began. In this case, the sample is not irradiated with radiation with a slowly varying frequency, but with radiation connecting all frequencies in one packet. In this case, radiation of one frequency is absorbed, and the protons move to the upper energy level, then the short pulse is turned off and after that the excited protons begin to lose the absorbed energy and move to the lower level. This energy phenomenon is recorded by the system as a series of millisecond pulses that decay over time.

The ideal solvent is a substance that does not contain protons, that is, carbon tetrachloride and carbon sulfur, but some substances do not dissolve in these solutions, so any solvents in the molecules of which the atoms of the light isotope H1 are replaced by atoms of the heavy isotope deuterium are used. The isotope frequency must correspond to 99%.

СDCl 3 – deuterium

Deuterium does not produce a signal in NMR spectra. A further development of the method was the use of a high-speed computer and further signal conversion. In this case, instead of the last scan of the radiation frequency, instantaneous radiation containing all possible frequencies is superimposed on the sample. In this case, instantaneous excitation of all nuclei and reorientation of their spins occurs. After the radiation is turned off, the nuclei begin to release energy and move to a lower energy level. This burst of energy lasts several seconds and consists of a series of microsecond pulses, which are recorded by the recording system in the form of a fork.