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Review Chapter in Nuclear Magnetic Resonance (NMR)

Studienarbeit 2018 45 Seiten

Chemie - Sonstiges

Leseprobe

Introduction:

Nuclear magnetic resonance (NMR) is a physical phenomenon that depends on the quantitative mechanical magnetic properties of the nucleus of the atom. NMR is used to denote a set of scientific methodologies and techniques. This phenomenon is used to study molecules in terms of structure and spatial composition. The phenomenon is mainly based on the fact that all atomic nuclei that have an odd number of protons or neutrons have intrinsic magnetic torque and angular momentum. The nuclei used in these techniques are the nucleus of the hydrogen atom H1, the most abundant isotope of hydrogen, Carbon-13. Other isotopes can be used but their uses are less.The spining motion of these elements revolves around a magnetic moment (M) axis. When these nuclei are placed between poles of an external magnetic field, they influence the energy levels of the spin energy level of these nuclei, resulting in dissociation of energy splitting The spherical motion to two different energy levels is based on the direction of the magnetic momentum resulting from the spindle motion:

- Low energy level Here is the magnetic torque in the direction of the external magnetic field.
- High energy level Here is the magnetic torque in the direction of an anti-magnetic field outside.

The difference in energy between these two levels can be increased by increasing the external magnetic field strength - as shown in Fig. 6-1 - so that these nuclei are placed in an external magnetic field (between the poles of a large magnet) and radiated by Radiowave radiation. And move to the higher energy level, resulting in a change in the direction of the movement of the nucleus of the nucleus, and then return the nuclei from the high level of energy to the low level again and so, and launches On this phenomenon is the phenomenon of magnetic nuclear resonance. The absorption of energy can be detected and enlarged as a linear spectrum and is called the magnetic resonance signal resonance signal and each molecule shows several AMTSASAT express the electronic conditions surrounding each nucleus, which determines the type of the bond and other atoms associated with this nucleus, and therefore uses the analysis of nuclear resonance Magnetic in the identification of the structural structure of the molecules.

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Form (6.1): The power of the electromagnetic motion and expresses the spectrum.

of electromagnetism in the area of radio radiometers frequency in Hz, hertz MHz (Hz), MHz, (1MHz = 106 Hz) There are a limited number of elements containing nuclei with strong magnetic properties allowing the practical application of the possibility of Its by the NMR spectrometer-as we mentioned – like: The hydrogen 1h, the 13C carbon plus some other elements, such as: Boron 11B, fluorine 19F, phosphorus 31P. These elements are also characterized by the fact that their atoms contain an odd number of odd numbers of protons or neutrons, with a number of Mallow (Spin Magnetic (Magnetic Quantum number) has ±½ and the number of possible directions of the magnetic moment = 2 The energy levels resulting from the different directions of the magnetic moment can be calculated by the following

equation: E = - m µ Bo / I

Where: E is the power of the level Bo magnetic field strength Outer m Magnetic quantum number I number quantum magnetically Μ magnetic torque.

Thus, the power of the levels in the case of nuclei with an inducte is equal to ½: e1 =-½ Μ Bo/½ Where E1= -½ µ Bo/ ½ where: m= + ½ • E = - µBo

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The following table (6.1) illustrates the condition of protons and neutrons, as well as the spinning of some nuclei. As can be seen from the table, the spindle of Hirogen-1, phosphorus-31, fluorine-19 and carbon-13 equals ½.

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Table (6.1): The spinning of some nuclei.

in the case of nuclei where the number of protons and neutrons is my husband, Its movement is in one direction, so that the number of the cobalt you have is equal to zero.

In the case of nuclei in which the number of protons or neutrons is individual, their thrust is in two directions, and the number of their cobalt is equal to ½.

In the absence of the external magnetic field, the magnetic torque of these nuclei can be found in any direction, the energy of these trends is equal, and the number of nuclei (protons) present at these levels is also equal.

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Figure (6-2): direction of the nuclei when placed in a magnetic field.

In the presence of the outer magnetic field, the energy of the movement is separated into two levels: one, high and the other, low in energy-as we have already explained-and so we find that these nuclei under these conditions directs itself so that the direction of magnetic torque toward the outer magnetic field, to be At a low level, some nuclei continue to reverse the external magnetic field and rotate the nuclei so that they change direction to each one in the direction of the field and once the field is reversed – as evident in-(Fig. 6.2).

The low level of energy (M = ½) is always occupied by a larger number of protons than the high level of energy (M =-½) Because each system tends to exist at the low energy level. Form (6.2): the tendency of the nuclei when placed in a magnetic field and the value of magnetic torque express a constant value for the one type of nuclei, and it was found that when those nuclei with magnetic properties in an external magnetic field intensity of 14.000 Gauss at room temperature 300 K)) be 1.000010 kernel In the low energy level, while we find only 1.000000 cores in the high energy level, the difference in the number of nuclei at both levels is ten nuclei and is responsible for the absorption of energy in the magnetic nuclear resonance.

By increasing the intensity of the magnetic field, the energy difference between the two levels increases, thus increasing the number of nuclei in the low energy level relative to the number of nuclei in the high energy level.

NMR devices differ from each other in the intensity of the magnetic field used, and by increasing the intensity of the magnetic field we get a good separation of the absorption resulting from the different nuclei in the molecules.

Relaxation process relaxation when the energy absorption of the radio rays occurs, the nuclei move from the low energy level to the higher energy level, resulting in system deviation from the heat balance and if the high-level nuclei of energy are not returned to the low-level time The absorption process cannot continue and this is called saturation saturation and absorption in this case is very small and may not be detected in practice, but what happens in the chemical systems that the absorbed energy is usually lost quickly so the absorption process continues and can be detected, the process The energy acquired in this case is called the process of relaxation relaxation procedure the time that it takes to lose this energy is called relaxation time and the process of relaxation operation is done in two ways:

First: longitudinal relaxation longitudinal or spin-lattice relaxation relax By the power lost from the nucleus to the rest of the carrot. The efficiency of this method is expressed by the time it takes in the process of transferring energy from the nucleus at the high energy level to the low energy level, and the smaller the time indicates the efficiency of energy transmission, resulting in a widening of the absorption curve broadening, this process occurs in the case of liquids, solvents and gases.

Second: relax transverse transverse or spin-spin relaxation is relaxed by the effect of the movements of the neighboring nuclei, and this process occurs with the transmission of energy from the nucleus at the high energy level to another nucleus adjacent to the low energy level, and this method with Importance in the case of solids.

NMR Spectrum Magnetic (nuclear resonance spectrum) the magnetic resonance absorption spectrum is recorded for the nuclei of one type of element that has magnetic properties within the same carrot. This is because each type of nuclei of the atoms of the elements absorbs the energy of the rays on a different frequency, and the NMR device is characterized by its ability to distinguish one type of the intention of the elements relative to the conditions surrounding these nuclei in the carrot.

Nucleus of hydrogen atom (proton): When a single absorption of the nuclei of hydrogen atoms occurs, we will not get any useful information for the synthesis of particles but the presence of the nuclei of hydrogen atoms in the molecule leads to the presence of these nuclei in different electronic conditions for the distribution of electrons in The bond between the nucleus of hydrogen and the other atom. This variation in the electronic distribution around the hydrogen nuclei in the molecule leads to the absorption of these nuclei at different frequencies Thus, the number of absorption reflects the different types of hydrogen atoms in the molecule.

We find that the hydrogen in both the-OH &-CH2 &-CH3 differs in terms of the surrounding electronic conditions, thereby absorbing each type of protons on a different frequency, and the intensity of absorption in each group, proportional to the number of protons in this group and thus we get useful information for the installation Molecular.

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Nuclear magnatic resonance devices (Fig. 6.3)

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Figure (6-4) Schematic sketch of magnetic nuclear resonance

differ from other spectral analysis devices where the presence of magnetic energy levels in which the transition process occurs depends on the presence of a strong external magnetic field, while in other spectral analysis methods its energy levels are considered (levels Electronic and vibratory energy) is an intrinsic characteristic of molecules. EMR electromagnetic rays used in NMR devices with very large wavelength radiowave. Therefore, the units used in the production and detection of these rays are different from other spectral analysis devices.

In the above-mentioned spectral analysis devices-UV-VL & IR can induce absorption by altering the radiation power (wavelength or frequency) and absorption occurs at the wavelength in which the radiation energy is equal to the difference in energy between energy levels, but it is found difficult to control the change of wavelength in the The Radiowave is used in NMR devices with sufficient precision and therefore NMR devices use a fixed beam of radio rays, while changing the intensity of the magnetic field so that the absorption of the beam occurs when the energy of the radiation is equal.Since each proton (the nucleus of the hydrogen atom) in the molecule has its own energy, the absorption of the different protons in the molecule occurs by altering the intensity of the magnetic field in the presence of a fixed packet with an appropriate frequency of radio rays.

Magnetic Nuclear Resonance spectrometer: Magnetic nuclear resonance devices consist of five main parts as described later (6.4).

(1) Magnet uses magnets to separate the magnetic energy levels of different nuclei, can use permanent magnet permanent mangnet or electromangnet electric magnet, and the specimen is placed in the machine between the polar A magnet that is required to give a homogeneous magnetic field homogeneous field and be consistent to an appropriate degree.

(2) Field Intensity Change Unit Magnetic field Sweep Generator magnetic field strength is changed by a coil file in the face of magnet poles and this file is connected to a variable Sweep generator Generator when the DC constant power change in the coil changes the intensity of the magnetic field in the area The sample is in slight limits and this change is within 1000 Hz of the Magnetic Nuclear Resonance spectrometer which uses a 60 MHz frequency, which is called 60 MHz NMR

(3) The source of the production of radio waves radiofrequency transmitter produces radio rays from the radio-ray oscillator radiofrequency oscil Lator where the coil is fed into a double wire wrapped around the specimen which is called a transmitter coil transmission coil and is vertically centered on the direction of the magnetic field. The radio frequency production unit is selected on the basis of the required radiometer, which therefore depends on the intensity of the magnetic field used in the device, for example, in the case of magnetic use of 14 kg, the required frequency is 60 MHz .

(4) sample mode Holder and Probe used to Aviv glass diameter of 5mm interior for the placement of the samples this tube is connected to turbine-air-operated turpenes, by which the tube can rotate around its vertical axis several hundred cycles per minute x Cycle/min, and this rotation reduces the susceptibility resulting from heterogeneity in the magnetic field of the tar Lg.

Radiofrequency Receiver or Detector detector unit can detect the absorption of radio rays by another file of the wire surround the sample also and be perpendicular to both the transmission file and magnetic field and is called the receiver file Receiver coil and generates an electric overflow that moves to the future Receiver where it is maximized and recorded.

Electronic Integrator Integration Unit all magnetic nuclear resonance devices contain a unit to measure the area under each absorption curve called Electronic Integrator Integration unit and this area is directly proportional to the number of protons responsible for this absorption.

As mentioned earlier, NMR devices differ from each other in the intensity of the field used and thus in the radio frequency used, the different devices are differentiated based on the frequency of the radiation used in the device.

NMR 60 MHz Device: A device that uses a 60 MHz frequency and to obtain this frequency uses a magnetic field strength of about 14 kilograms and this magnetic field is working to separate the energy levels so that they are in the range of radio rays used in the device. Other devices used are: 90, 100, 220, 300, 360 and 500 MHz and by increasing the intensity of the field we get these high frequencies for radio rays.

In some devices, it proves the intensity of the field Fixed Magnetic field for example at 14 kpa and then changes the frequency Vary the Frequency until the Location of the Renen Resonance and these are the most common, since each nucleus-e.g. hydrogen 1h, fluorine 19F, phosphorus 31P or carbon 13C-Has a Resonance swing at a different frequency.In the top 300 MHz devices, which require a very strong field, a strong magnet is submerged in a liquid helium, called superconducting magnet because it has a strong field of high field in the sense that the magnet file here delivers the entire voltage so that the resistance equals zero.

In order to connect the magnet coil magnet file with high efficiency it must be kept at a very low temperature up to the temperature of the liquefied helium, but if the temperature of the magnet file rises the resistance increases and heat starts and the helium begins to boil (the boiling point of Helium 4.3 degree absolute e) quenching obstruction of the magnetic field occurs.

These devices are called Fourier transform nuclear magnetic resonance (FT NMR spectrometer).

Magnetic resonance imaging (MRI) machine.

Sample preparation handling can be done 1h-NR for liquid or solid samples after a solution of which AVI is a suitable solvent where the weight of the sample is dissolved in the range of 30 mg in the solvent and requires that the solvent does not contain hydrogen in its composition.

In the case of polar compounds requiring a polar solvent such as water or ethanol, a solvent containing a hydrogen isotope should be used, which is a diotrium since it has no absorption at the 1H-NR and is called such solvent deuterated solvents and is expensive.

Examples of commonly used solvents in this area are:

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and to prepare the sample for analysis by the RNI device n Magnetic nuclear we need about 20-30 milligrams of solids or 50 micoculter of the liquid sample and dissolve the solid sample or loosen the liquid sample by about 0.5 ml of the appropriate solvent, then put the sample in the analysis tube (5mm I.D. glass tube), and if there is a filter should be To be transparent, the solution in the tube must be about 3-7 cm in height, and the sample is added to the standard reference substance which is often the fourth methyl Ceylon Tetra methyl Silan and is called (TMS) then covered the tube with plastic cover and then placed the tube inside the turbine and then In the place where it is between the magnet poles and pushes a stream of air through the pump pump Spin the tube at high speed and then we work location for TMS material at zero and then we scan the sample on the chart of the absorption spectrum of samples (Fig. 6.5).

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Form (6.5): NMR chart for drawing spectrum of chemical transition absorption

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Details

Seiten
45
Jahr
2018
ISBN (eBook)
9783668814769
ISBN (Buch)
9783668814776
Sprache
Englisch
Katalognummer
v442841
Note
A
Schlagworte
NMR Review Carbon NMR Proton NMR Phosphrous NMR

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Titel: Review Chapter in Nuclear Magnetic Resonance (NMR)