NMR spectroscopy. Applications of NMR spectroscopy Magnets for NMR spectrometers
Allyl cleavage- addiction spin-spin interaction constants between protons in allylic systems ( 4 J ) which largely depends on the torsion angle between the planes formed by the atoms HC 2 C 3 and C 1 C 2 C 3.
Annulens- cyclic conjugate systems.
Atropic molecules- molecules of compounds that do not produce a ring current.
Bond angle (θ) - the angle between two bonds on one carbon atom.
Vicinal interaction - interaction between nuclei that are separated by three bonds.
Off-resonance decoupling(off resonance decoupling) - allows you to distinguish between the signals of CH 3, CH 2, CH groups and the quaternary carbon atom. To observe off-resonance decoupling, a frequency is used that is close to the chemical shift, but does not correspond to the resonant frequency of the signal. This suppression leads to a reduction in the number of interactions, to the point that only direct ones are recorded. J(C,H) interactions.
Geminal interaction - interaction between nuclei that are separated by two bonds.
Heteronuclear correlation spectroscopy (HETCOR)- in these experiments, the chemical shifts of the 1 H spectra are placed on one axis, while the 13 C chemical shifts are placed on the other axis. HETCOR - heteronuclear variant of COSY, which uses indirect heteronuclear spin-spin interactions between 1 H and 13 C.
HMQC - HETeronuclearMultyQuantumCorrelation- registration 1 N with decoupling from 13 C.
HSQC - HETeronuclear MultiQuantum Correlation- HMQC option
COLOC - CORrelation Long (very long)
HMBC (HETeronuclear MultiplBond Correlation)- a variant of the HMQC experiment for detecting long-range heteronuclear spin-spin interactions. HMBC produces a higher signal-to-noise ratio than the HMQC experiment.
Gyromagnetic ratio (γ ) - one of the characteristics of the magnetic properties of the nucleus.
Homoallylic interaction- interaction through 5 bonds in the allylic system.
Further interaction - interaction between nuclei that are separated by more than 3 links (usually through 4-5 links).
Sensor- a device that provides transmission of pulses to the sample and registration of resonance signals. Sensors are broadband and selectively tuned. They are installed in the active region of the magnet.
Dihedral (torsion) angle- the angle formed by two planes between the connections under consideration.
Two-dimensionalJ-spectra. Two-dimensional J-spectroscopy is characterized by the presence of one frequency coordinate associated with the SSV and a second coordinate associated with chemical shifts. The most widespread is the contour representation of two-dimensional J-spectra in two mutually perpendicular coordinates.
Two-dimensional NMR spectroscopy - experiments using pulse sequences, which makes it possible to obtain the NMR spectrum in a representation in which the information is distributed over two frequency coordinates and is enriched with information about the interdependence of NMR parameters. The result is a square spectrum with two orthogonal axes and a signal that has a maximum in the frequency representation at the point with coordinates (, ), i.e., on the diagonal.
Delta scale (δ -scale) - a scale in which the chemical shift of TMS protons is taken as zero.
Diamagnetic shift- shift of the resonant signal to the weak field region (large values δ ).
Diatropic molecules- canceled from 4 n+2 π electrons, which, according to Hückel’s rule, are aromatic.
Doublet - a signal of two interacting nuclei, which is represented in the 1H NMR spectrum by two lines of the same intensity.
Isochronous nuclei- nuclei having the same chemical shift value. Often they are chemically equivalent, that is, they have the same chemical environment.
Integral signal intensity(area under the curve) - measured by an integrator and shown in the form of steps, the height of which is proportional to the area and shows relative number protons.
Pulsed spectroscopy - a method of excitation of magnetic nuclei - using short and powerful (hundreds of kilowatts) high-frequency pulses. A pulse with a carrier frequency ν o and duration t p creates an excitation band in the frequency range +1/t p. If the pulse length is several microseconds, and ν o approximately corresponds to the center of the resonance frequency region for a given type of nuclei, then the band will cover the entire frequency range, ensuring simultaneous excitation of all nuclei. As a result, an exponentially decaying sine wave (ESW) is recorded. It contains information about both the frequency, i.e., in fact, the chemical shift, and the shape of the line. The more familiar form for us - the spectrum in frequency representation - is obtained from the SIS using a mathematical procedure called the Fourier transform.
Pulsed NMR- a method of exciting magnetic nuclei using short and powerful (hundreds of kilowatts) high-frequency pulses. During the pulse, all nuclei simultaneously are excited, and then, after the pulse stops, the nuclei return (relax) to their original ground state. The loss of energy by relaxing nuclei leads to the appearance of a signal, which is the sum of signals from all nuclei and is described by a large number of damped sinusoidal curves on a time scale, each of which corresponds to a certain resonant frequency.
Spin-spin interaction constant (SSIC)- quantitative characteristics of the interaction of different nuclei.
Correlation spectroscopy (COSY) - experiment with two 90 o pulses. In this type of two-dimensional spectroscopy, the chemical shifts of spin-coupled magnetic nuclei are correlated. Two-dimensional COZY spectroscopy, under certain conditions, helps to reveal the presence of very small constants that are usually invisible in one-dimensional spectra.
COSY- experiments in which the pulse duration is varied. This makes it possible to reduce the size of diagonal peaks that make it difficult to identify nearby cross-peaks (COSY45, COSY60).
DQF-COSY - double quantized filter - suppresses singlets on the diagonal and interference corresponding to them.
COSYLR (long rank)- COZY experiment, which allows you to determine long-range interactions.
TOCSY - TotalCorrelationSpectroscopy- shooting mode, which allows you to obtain cross-peaks between all spins of the system in a spectrum saturated with signals by transferring magnetization through bonds in the structural fragment under study. Most often used to study biomolecules.
Larmor frequency- precession frequency in NMR.
Magnetically equivalent are those nuclei that have the same resonant frequency and a common characteristic value of the spin-spin interaction constant with the nuclei of any neighboring group.
Multiquantum coherences- superposition states, when two or more interacting spin ½ are reoriented simultaneously.
Multidimensional NMR- registration of NMR spectra with more than one frequency scale.
Multiplet - a signal of one group that appears as several lines.
Indirect spin interaction - interaction between nuclei, which is transmitted within the molecule through a system of bonds and is not averaged during rapid molecular motion.
Paramagnetic particles - particles containing an unpaired electron, which has a very large magnetic moment.
Paramagnetic shift- shift of the resonant signal to the region of strong field (large values δ ).
Paratropic molecules - canceled with the number of π electrons equal to 4 n.
The direct spin-spin interaction constant is a constant characterizing the interaction between nuclei that are separated by one bond.
Direct spin-spin interaction- interaction between nuclei, which is transmitted through space.
Resonant signal - spectral line corresponding to energy absorption during the transition between eigenstates caused by a high-frequency oscillator.
Relaxation processes - loss of energy at the upper level and return to the lower energy level due to non-radiative processes.
WITH viping- a gradual change in the magnetic field, as a result of which resonance conditions are achieved.
First order spectra- spectra in which the difference in chemical shifts of individual groups of magnetically equivalent nuclei ν o significantly greater than the spin-spin interaction constant J .
Spin-lattice relaxation - the process of relaxation (energy loss), the mechanism of which is associated with interaction with local electromagnetic fields of the environment.
Spin-spin relaxation - the relaxation process is carried out as a result of the transfer of energy from one excited nucleus to another.
Spin-spin interaction of electrons- interaction resulting from the magnetic interaction of different nuclei, which can be transmitted through electrons of chemical bonds of directly unbound nuclei.
Spin system- this is a group of nuclei that interact with each other, but do not interact with nuclei that are not part of the spin system.
Chemical shift - displacement of the signal of the nucleus under study relative to the signal of the nuclei of the standard substance.
Chemically equivalent nuclei- nuclei that have the same resonant frequency and the same chemical environment.
Shimmy - in NMR spectroscopy, this is the name for electromagnetic coils that create magnetic fields of low intensity, which correct inhomogeneities in a strong magnetic field.
Broadband interchange(1 N broadband decoupling) - the use of strong irradiation, which covers the entire range of proton chemical shifts, in order to completely remove all 13 C 1 H interactions.
Shielding - change in the position of the resonant signal under the influence of induced magnetic fields of other nuclei.
Van der Waals effect- an effect that occurs during a strong spatial interaction between a proton and a neighboring group and causes a decrease in the spherical symmetry of the electronic distribution and an increase in the paramagnetic contribution to the screening effect, which, in turn, leads to a shift of the signal to a weaker field.
Zeeman effect- splitting of energy levels in a magnetic field.
Roof effect- increase in the intensity of central lines and decrease in the intensity of distant lines in the multiplet.
Magnetic anisotropy effect(the so-called cone of anisotropy) is the result of exposure to secondary induced magnetic fields.
Nuclear quadrupole resonance (NQR) - observed for nuclei with spin quantum number I > 1/2 due to the nonspherical distribution of nuclear charge. Such nuclei can interact with gradients of external electric fields, especially with gradients of the fields of the electron shells of the molecule in which the nucleus is located and have spin states characterized by different energies even in the absence of an applied external magnetic field.
Nuclear magneton The nuclear magneton value is calculated using the formula:
Nuclear magnetic resonance(NMR) is a physical phenomenon used to study the properties of molecules when atomic nuclei are irradiated with radio waves in a magnetic field.
Nuclear factor - the ratio of the charge of a nucleus to its mass.
NMR spectroscopy is a non-destructive analysis method. Modern pulsed NMR Fourier spectroscopy allows analysis at 80 mag. cores. NMR spectroscopy is one of the main. Phys.-Chem. methods of analysis, its data is used for unambiguous identification as intervals. chemical products r-tions, and target in-in. In addition to structural assignments and quantities. analysis, NMR spectroscopy brings information about conformational equilibria, diffusion of atoms and molecules in solids, internal. movements, hydrogen bonds and association in liquids, keto-enol tautomerism, metallo- and prototropy, order and distribution of units in polymer chains, adsorption of substances, electronic structure of ionic crystals, liquid crystals, etc. NMR spectroscopy is a source of information on the structure of biopolymers , including protein molecules in solutions, comparable in reliability to the data of X-ray diffraction analysis. In the 80s The rapid introduction of NMR spectroscopy and tomography methods into medicine began for the diagnosis of complex diseases and for medical examination of the population.
The number and position of lines in the NMR spectra unambiguously characterize all fractions of crude oil, synthetic. rubbers, plastics, shale, coal, medicines, drugs, chemical products. and pharmaceutical prom-sti, etc.
The intensity and width of the NMR line of water or oil make it possible to accurately measure the moisture and oil content of seeds and the safety of grain. When detuning from water signals, it is possible to record the gluten content in each grain, which, like oil content analysis, allows for accelerated agricultural selection. crops
The use of increasingly stronger magnets. fields (up to 14 T in serial devices and up to 19 T in experimental installations) provides the ability to completely determine the structure of protein molecules in solutions, express analysis of biol. fluids (concentrations of endogenous metabolites in blood, urine, lymph, cerebrospinal fluid), quality control of new polymer materials. In this case, numerous variants of multiquantum and multidimensional Fourier spectroscopic spectroscopy are used. techniques.
The NMR phenomenon was discovered by F. Bloch and E. Purcell (1946), for which they were awarded the Nobel Prize (1952).
The phenomenon of nuclear magnetic resonance can be used not only in physics and chemistry, but also in medicine: the human body is a collection of the same organic and inorganic molecules.
To observe this phenomenon, an object is placed in a constant magnetic field and exposed to radio frequency and gradient magnetic fields. In the inductor coil surrounding the object under study, an alternating electromotive force (EMF) arises, the amplitude-frequency spectrum of which and time-transient characteristics carry information about the spatial density of resonating atomic nuclei, as well as other parameters specific only to nuclear magnetic resonance. Computer processing of this information generates a three-dimensional image that characterizes the density of chemically equivalent nuclei, nuclear magnetic resonance relaxation times, distribution of fluid flow rates, diffusion of molecules and biochemical metabolic processes in living tissues.
The essence of NMR introscopy (or magnetic resonance imaging) is, in fact, the implementation of a special kind of quantitative analysis of the amplitude of the nuclear magnetic resonance signal. In conventional NMR spectroscopy, one strives to achieve the best possible resolution of spectral lines. To achieve this, the magnetic systems are adjusted in such a way as to create the best possible field uniformity within the sample. In NMR introscopy methods, on the contrary, the magnetic field created is obviously non-uniform. 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 setting any code for gradations of the amplitude of NMR signals (brightness or color on the monitor screen), you can obtain a conventional image (tomogram) of sections of the internal structure of the object.
NMR introscopy and NMR tomography were first invented in the world in 1960 by V. A. Ivanov. An incompetent expert rejected the application for an invention (method and device) “... due to the obvious uselessness of the proposed solution,” so the copyright certificate for this was issued only more than 10 years later. Thus, it is officially recognized that the author of NMR tomography is not the team of the Nobel laureates listed below, but a Russian scientist. Despite this legal fact, the Nobel Prize was awarded for NMR tomography not to V. A. Ivanov. Spectral devices
For accurate study of spectra, such simple devices as a narrow slit limiting the light beam and a prism are no longer sufficient. Instruments are needed that provide a clear spectrum, i.e., instruments that can well separate waves of different lengths and do not allow individual parts of the spectrum to overlap. Such devices are called spectral devices. Most often, the main part of the spectral apparatus is a prism or diffraction grating.
ELECTRONIC PARAMAGNETIC RESONANCE
The essence of the method
The essence of the phenomenon of electron paramagnetic resonance is the resonant absorption of electromagnetic radiation by unpaired electrons. An electron has a spin and an associated magnetic moment.
If we place a free radical with a resulting angular momentum J in a magnetic field with a strength B 0 , then for J nonzero, the degeneracy in the magnetic field is removed, and as a result of interaction with the magnetic field, 2J+1 levels arise, the position of which is described by the expression: W =gβB 0 M, (where M = +J, +J-1, …-J) and is determined by the Zeeman interaction of the magnetic field with the magnetic moment J. The splitting of electron energy levels is shown in the figure.
Energy levels and allowed transitions for an atom with nuclear spin 1 in a constant (A) and alternating (B) field.
If we now apply an electromagnetic field with frequency ν, polarized in a plane perpendicular to the magnetic field vector B 0 , to the paramagnetic center, then it will cause magnetic dipole transitions that obey the selection rule ΔM = 1. When the energy of the electronic transition coincides with the energy of the photoelectromagnetic wave, a resonant reaction will occur absorption of microwave radiation. Thus, the resonance condition is determined by the fundamental magnetic resonance relation
Absorption of microwave field energy is observed if there is a population difference between the levels.
At thermal equilibrium, there is a small difference in the populations of the Zeeman levels, determined by the Boltzmann distribution = exp(gβB 0 /kT). In such a system, when transitions are excited, equality of populations of energy sublevels should very quickly occur and absorption of the microwave field should disappear. However, in reality there are many different interaction mechanisms, as a result of which the electron non-radiatively passes into its original state. The effect of constant absorption intensity with increasing power occurs due to electrons that do not have time to relax, and is called saturation. Saturation appears at high microwave radiation power and can significantly distort the results of measuring the concentration of centers by the EPR method.
Method value
The EPR method provides unique information about paramagnetic centers. It clearly distinguishes impurity ions isomorphically included in the lattice from microinclusions. In this case, complete information is obtained about a given ion in the crystal: valence, coordination, local symmetry, hybridization of electrons, how many and in what structural positions of electrons it is included, the orientation of the axes of the crystal field at the location of this ion, a complete characteristic of the crystal field and detailed information about the chemical bond . And, what is very important, the method allows you to determine the concentration of paramagnetic centers in regions of the crystal with different structures.
But the EPR spectrum is not only a characteristic of an ion in a crystal, but also of the crystal itself, features of the distribution of electron density, crystal field, ionicity-covalence in a crystal, and finally, simply a diagnostic characteristic of a mineral, since each ion in each mineral has its own unique parameters. In this case, the paramagnetic center is a kind of probe, providing spectroscopic and structural characteristics of its microenvironment.
This property is used in the so-called. the method of spin labels and probes, based on the introduction of a stable paramagnetic center into the system under study. As such a paramagnetic center, as a rule, a nitroxyl radical is used, characterized by anisotropic g And A tensors.
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 (NMR) spectroscopy is the most powerful tool for elucidating the structure of organic substances. In this type of spectroscopy, the sample under study is placed in a magnetic field and irradiated with radio frequency electromagnetic radiation.
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Rice. 11-13. Protons in a magnetic field: a - in the absence of a magnetic field; b - in an external magnetic field; c - in an external magnetic field after absorption of radio frequency radiation (spins occupy a higher energy level)
radiation. Hydrogen atoms in different parts of the molecule absorb radiation of different wavelengths (frequencies). Under certain conditions, other atoms can also absorb radio frequency radiation, but we will limit ourselves to considering spectroscopy on hydrogen atoms as the most important and common type of NMR spectroscopy.
The nucleus of a hydrogen atom consists of one proton. This proton rotates around its axis and, like any rotating charged object, is a magnet. In the absence of an external magnetic field, proton spins are randomly oriented, but in a magnetic field only two spin orientations are possible (Fig. 11-13), which are called spin states. Spin states in which the magnetic moment (shown by the arrow) is oriented along the field have slightly lower energy than spin states in which the magnetic moment is oriented against the field. The energy difference between the two spin states corresponds to the energy of a photon of radio frequency radiation. When this radiation affects the sample under study, protons move from a lower energy level to a higher one, and energy is absorbed.
Hydrogen atoms in a molecule are in different chemical environments. Some are part of methyl groups, others are connected to oxygen atoms or a benzene ring, others are located next to double bonds, etc. This small difference in the electronic environment is enough to change the energy difference between spin states and, therefore, the frequency of absorbed radiation.
The NMR spectrum arises as a result of the absorption of radio frequency radiation by a substance located in a magnetic field. NMR spectroscopy allows one to distinguish between hydrogen atoms in a molecule that are in different chemical environments.
NMR spectra
When scanning the radiation frequency at certain frequency values, absorption of radiation by hydrogen atoms in the molecule is observed; the specific value of the absorption frequency depends on the environment of the atoms
Rice. 11-14. Typical NMR spectrum: a - spectrum; b - integral curve giving the peak area
hydrogen. Knowing in which region of the spectrum the absorption peaks of certain types of hydrogen atoms are located, it is possible to draw certain conclusions about the structure of the molecule. In Fig. Figures 11-14 show a typical NMR spectrum of a substance in which there are three types of hydrogen atoms. The position of signals on the chemical shift scale 5 is measured in parts per million (ppm) of the radio frequency. Usually all signals are located in the area in Fig. 11-14, the chemical shifts of the signals are 1.0, 3.5 and The right part of the spectrum is called the high-field region, and the left is called the low-field region. In NMR spectra, the peaks are traditionally shown pointing upward rather than downward, as in IR spectra.
To interpret the spectrum and obtain structural information from it, three types of spectral parameters are important:
1) position of the signal on the -scale (characterizes the type of hydrogen atom);
2) signal area (characterizes the number of hydrogen atoms of a given type);
3) multiplicity (shape) of the signal (characterizes the number of closely located hydrogen atoms of other types).
Let's take a closer look at these parameters using the example of the spectrum of chloroethane (Fig. 11-15). First of all, let's pay attention to the position of the signals in the spectrum, or, in other words, to the values of the chemical shifts. Signal a (protons of the group is at 1.0 ppm, which
Rice. 11-15. NMR spectrum of chloroethane
(see scan)
indicates that these hydrogen atoms are not located next to an electronegative atom, while the shift of the signal b (protons of group ) is The values of the chemical shifts of frequently occurring groups must be remembered in the same way as the frequencies of absorption bands in IR spectra. The most important chemical shifts are given in table. 11-2.
Then we analyze the area of the peaks, which is proportional to the number of hydrogen atoms of a given type. In Fig. 11-15 relative areas are indicated by numbers in parentheses. They are defined using the integral curve located above the spectrum. The signal area is proportional to the height of the “step” of the integral curve. In the spectrum under discussion, the ratio of signal areas is 2:3, which corresponds to the ratio of the number of methylene protons to the number of methyl protons
Finally, consider the shape or structure of signals, which is usually called multiplicity. The methyl group signal is a triplet (three peaks), while the methylene group signal is four peaks (quartet). Multiplicity provides information about how many hydrogen atoms are bonded to an adjacent carbon atom. The number of peaks in a multiplet is always one greater than the number of hydrogen atoms of the neighboring carbon atom (Table 11-3).
Thus, if there is a singlet signal in the spectrum, this means that the molecule of the substance includes a group of hydrogen atoms, in the vicinity of which there are no other hydrogen atoms. In the spectrum in Fig. 11-15 the signal of the megyl group is a triplet. This means that there are two hydrogen atoms adjacent to the carbon atom.
Likewise, the methylene group signal is a quartet because there are three hydrogen atoms in the neighborhood.
It is useful to learn how to predict the expected NMR spectrum based on the structural formula of a substance. Having mastered this procedure, it is easy to move on to solving the inverse problem - establishing the structure of a substance from its NMR spectrum. Below you will see examples of predicting spectra based on structure. You will then be asked to interpret the spectra to determine the structure of the unknown substance.
Prediction of NMR spectra based on structural formula
To predict NMR spectra, follow these procedures.
1. Draw the complete structural formula of the substance.
2. Circle the equivalent hydrogen atoms. Determine the number of hydrogen atoms of each type.
3. Using the table. 11-2 (or your memory), determine the approximate values of the chemical shifts of the signals of each type of hydrogen atom.
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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.
