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Edwin D. Becker stresses that New conceptual approaches to NMR, together with remarkable advances in materials, computers, electronics, and magnet and probe technology, have brought NMR to its current level, with no indication of an end in sight (Becker xv). Hence, it is impossible to imagine the development of organic chemistry without physical methods of study. This issue represents special interest in recent years because the ability to apply different physical methods can be useful in solving the posed tasks.
Thus, in order to accomplish this target, it is necessary to be efficient in main principles and limitations of used methods. The applied physical methods in organic chemistry are variable. Nevertheless, the most frequently used types of spectroscopy are ultraviolet, infrared and nucleic magnetic resonance (NMR). NMR spectroscopy represents special interest because of its specific use of theoretical knowledge, equipment and the way of conducting the experiment.
The issue of NMR spectroscopy will be interesting for a wide audience. The professionals and students will find it useful in daily life. Everything that a person uses can be exposed to the analysis with the application of NMR spectroscopy. Physics, chemistry, medicine, food industry, and so on use NMR spectroscopy in order to improve the quality through the careful analysis of substances' structure.
The main purpose of the present paper is to highlight the issue of NMR spectroscopy and introduce the spectral data for the correct use of the study method, to have an idea of the limits of applied method. Moreover, it is important to be efficient in what areas NMR spectroscopy can be applied and how it can be used. Understanding of the core of the process plays an integral role for human life. It can help people better understanding the surrounding nature and through this analysis distinguishing what people need and in what ways it can be improved.
In order to accomplish the fixed target of the paper, it is necessary to elaborate a set of sources such as scientific articles, primary and secondary books. The theme of NMR spectroscopy requires the analysis of scientifically proved data conducted in recent times. Thus, the qualitative and quantitative methods of analysis can help in accomplishing the purpose of the present paper.
Thesis Statement: NMR spectroscopy is characterized by a wide application at present time in the era of technological progress. Nevertheless, NMR spectroscopy has a set of both disadvantages and advantages. The notion of NMR spectroscopy qualities are necessary for having an idea about its strengths and weaknesses in order to decide what method of analysis can serve best in the chosen area.
2. NMR Spectroscopy.
Edwin Becker states that "The first nuclear magnetic resonance (NMR) was detected early in 1938 in a molecular beam, and the first studies of NMR in bulk materials were carried out about 8 years later. Over the following decades, NMR has grown from an interesting and important study of a physical phenomenon to an indispensable technique in a very wide variety of fields. In organic chemistry NMR is arguably one of the two most important tools for the elucidation of molecular structure. In structural biology NMR rivals x-rays crystallography in providing precise three-dimensional structure for proteins and other marcomolecules, but NMR goes beyond x-rays crystallography in furnishing information on internal mobility and overall molecular motion in both large and small molecules" (Becker 1).
It is worth adding that the phenomenon of magnetic resonance was discovered in 1945-1946 by two independent groups of scientists, which were headed and inspired by F. Bloch and E. Purcell. In 1952 Felix Bloch and Edward Mills Purcell were awarded by the Nobel Prize in Physics for the development of new methods for conducting the precise nuclear magnetic measurements.
NMR spectroscopy is a method in which the investigated sample is placed in a magnetic field and is illuminated by radio waves. These radio waves cause the nucleus of atoms in the molecules of the sample produce signals that can be decrypted using the method of conversion into the accessible form. This method is a Fourier transform algorithm. It is a complex equation (Picture 1).
Then the signals of nuclei are analyzed in order to determine different things related to the molecule and its surroundings; for example, the structure of the molecule, chemical shift, which is determined by the chemical structure of the studied substance, the chemical structure of substances, conformation of molecules, the effects of mutual influence, and transformations inside the molecular. In order to completely understand the method of NMR spectroscopy, it is necessary to look at the nucleus as it rotates. Scientists have defined this process as resonance. When the positively charged nucleus rotates, then its moving charge creates a magnetic moment. In the absence of a magnetic field they are directed at random, but if to put them in a uniform magnetic field, all the magnetic moments will be built in the field.
Martha Dolphin Bruch stresses that "to simplify the study of the motions of the nuclear moments during an NMR experiment, a set of axes may be imagined to be rotating in the same direction and at the same rate that the moments are processing. This frame is called the rotating frame, and while it seems to be only a fictitious rotation, it will be shown later that the rotating frame may be considered as physically existing within the spectrometer" (Bruch 12).
Thermal motion of a molecule creates a torque that forces the magnetic moment to wiggle. When radio waves are placed on a rotating nucleus, then it bends more, and sometimes may even turn round. It is possible to detect a magnetic moment in the direction perpendicular (90o) to the attached magnetic field, when the magnetic moment is tilted away from the applied magnetic field.
Different nuclei resonate at different frequencies. It means that it is necessary to get into the carbon atom by radio wave with a frequency different from the one that is required to the nucleus of hydrogen in order to make it turn round. It also means that the same atoms in different environments, such as the hydrogen atom attached to an oxygen atom and the hydrogen atom attached to carbon atom, will turn round at different frequencies. It is possible to determine how a molecule is constructed, observing at what frequencies different nuclei are turned round. Besides, it can give answers to other questions concerning the properties of molecules.
The traditional method of NMR spectroscopy has many disadvantages. First of all, it requires a lot of time to construct each spectrum. Secondly, it is characterized as very sensible to the outside interference, and generally obtained spectra have significant noise. That is why, it is necessary to follow silence while using this method. Thirdly, it is not suitable for creating high-frequency spectrometers (300, 400, 500 and more MHz). Therefore, pulsed spectroscopy (PW) is based on Fourier, which transforms the received signal that is currently used in modern NMR devices. At present time, all NMR spectrometers are based on a powerful superconducting magnet with a constant magnetic field. NMR spectroscopy is characterized by a set of advantages. They are regarded to be the following ones: high resolution; the ability to conduct a quantitative account of resonant nuclei; the parameters characterizing the phenomenon in a comfortable form for researchers and consumers.
The main components of NMR spectrometer are considered to be the following ones: magnet, which creates polarized nuclear spin system of magnetic filed H0; transmitter that provides a sounding field H1; the sensor, in which under the influence of H0 and H1 in the sample, a signal of the NMR arises; receiver, which strengths the signal; registration system; the device of information processing; the stabilization system of resonance conditions; programming system of NMR registration; correction system of the magnetic field. The core of the NMR spectrometer is considered to be a powerful magnet.
In other words, the major part of NMR spectrometer is a refrigerator, which is filled with two cold liquids, that is, liquid nitrogen and liquid helium. Liquid nitrogen has a temperature of -195o C, and the liquid helium boils at -269o C. Liquid helium is placed in the central part of the refrigerator, and it cools the superconducting coils, which creates a magnetic field at -269o C. This is surrounded by liquid nitrogen, which prevents too rapid evaporation of liquid helium. Hole at the top of the device is used to load samples in the spectrometer. When the sample is loaded into the machine, a jet of air rotates the tube in order to make the sample more homogeneous for the scanning process.
When the sample is prepared for NMR spectroscopy in the solution condition, then the solvent or a part of it should be saturated with deuterium. It means that a portion of hydrogen atoms in a molecule of solvent should be replaced by deuterium atoms. The nucleus of a hydrogen atom consists of a single proton, while the nucleus of a deuterium consists of a proton and a neutron. That is why it is necessary to "tie" NMR to the fixed frequency, in order to avoid systematic error during the withdrawal of the spectrum.
Thus, when the sample is in the magnetic field and rotates, the device is tied to a particular frequency; it is possibly to start withdrawing the spectrum. First of all, the radio-frequency generator sends to the sample impulses, that is, short bursts of radio waves. These waves are absorbed and transmitted through a sample to the receiver, which picks up the signal from the sample. Then, the information is transmitted to a computer connected to the NMR spectrometer, in which data is transferred and analyzed.
In the experiment, first carried out in practice by Purcell, a sample is placed in a glass bulb about 5 mm in diameter, is put between the poles of a strong electromagnet. Then, in order to improve the homogeneity of the magnetic field, the bulb is rotated, and the magnetic field acting on it, is gradually increasing. Radio frequency generator of high quality is used as the source of radiation. Nuclei begin to resonate under the influence of increasing the magnetic fields, on which the spectrometer is configured. Besides, the shielded nuclei resonate at a frequency slightly higher than the nucleus are deprived of the electronic shells.
Energy absorption is fixed by the radio bridge and then it is recorded by the recorder. Frequency is increased until it reaches a certain limit, above which the resonance is impossible. The spectrometer records several spectrums because the currents that pass from the bridge are very small. That is why the spectrometer records several dozen of passes. All received signals are summed at the final schedule, the quality of which depends on the signal/noise of the device. The sample in this method is exposed to radio-frequent radiation of the constant frequency, while the strength of the magnetic field varies, so it is also called the method of continuous irradiation, that is, continuous wave (CW).
In fact, "The accuracy of the integration procedure depends on the signal-to-noise-ratio (S/N). For a precise integration a maximum S/N is required. For a precision better than 1%, S/N of 250:1 (1H), 300:1 (19F) and 600:1 (31P) have been proposed in the literature. It should be noted that there is no reason for these different numbers; one and the same S/N should yield the same precision, irrespective of the NMR frequency (nucleus)" (Diehl et al., 16).
Impulsive version is characterized by nuclei excitation with the help of a short pulse, the length of which is few microseconds, instead of continuous wave that is used in CW-method. The amplitudes of frequency components of the impulse are decreased with increasing distance from Î½0. Nevertheless, it is desired that all nuclei are irradiated equally, it is necessary to use the hard pulses, that is, short pulses of high power. The pulse duration depends on the different aspects. Thus, it is important to choose the pulse duration so that the width of the spectrum should be more than the width of the spectrum on one or two orders of magnitude. The power reaches to several thousand of watts.
As a result of impulse spectroscopy it is possible to obtain not only the usual spectrum with visible peaks of resonance, but the image of damped resonant oscillations, in which the signals are mixed from all the resonant nuclei. It is called the free induction decay (FID). In order to convert this spectrum, it is necessary to use mathematical methods, the so-called Fourier transformation, by which any function can be represented as the sum of the set of harmonic oscillations.
In other words, "the required acquisition time depends on the smallest line width in the spectrum, and truncation of the NMR signal in the time domain [free induction decay (FID)] must be avoided. If truncation occurs, signal forms with (substantial) "wiggles" appear in the spectra, and, in combination with FID baseline correction modes, wrong intensities will result" (Diehl et al., 16).
3. NMR Spectroscopy Application.
The phenomenon of nuclear magnetic resonance can be used not only in physics and chemistry, but also in medicine. A human body is the totality of organic and inorganic molecules. Thus, NMR spectrometry can be applied in medicine, for structural analysis (determining the chemical shifts, the analysis of molecules structure); and other applications (identification by metal-transponder reader, the manipulation of quantum information, that is, quantum computer). Webb writes that "Chemical shift, fine structure, and peak intensities constitute a "fingerprint" of every molecule, allowing for its identification and for quantification in multicomponent samples, including living organisms. Multiplicity patterns and integrated peak areas yield information on molecular structure. Chemical-shift values provide structural information on molecular over time indicate reaction kinetics. For small molecules in solution, analysis of chemical shift and fine structure of every atomic nucleus is often no problem in a modern high-resolution spectrometer. Even in large molecules such as proteins the full spectroscopic information is accessible" (Webb 2).
Pharmacological sphere is another place of application NMR spectroscopy. Diehl and coauthors support this information by providing the data that a set of pharmacological companies expressed the desire to use NMR spectroscopy method in their production. It considerably helped because "Due to the broad range of possibilities for application" (Diehl et al., 17). Among the international pharmacopoeias are the following: "the European Pharmacopoeia, the British Pharmacopoeia (BP) and the United States Pharmacopeia USP29, cite use of NMR spectroscopy for identification purposes and quantitative NMR spectroscopy for evaluation of the composition of polymers, mainly excipients and impurities in drugs" (Diehl et al., 17).
Bernd W.K. Diehl, Frank Malz and Ulrike Holgrabe, the authors of Federal Institute for Materials Research and Testing and Institute of Pharmacy and Food Chemistry in Germany, have introduced their vision of NMR spectroscopy in pharmaceutical sphere of application. They consider that NMR spectroscopy method represents huge possibilities because it has been successfully applied according to the statistical data and the results of the conducted studies (Holzgrabe et al., 1999).
NMR spectrometry has allowed determining different substances in a mixture. "NMR spectroscopy can be used in different fields of the quality evaluation of drugs: to identify a drug, to determine the level of impurities and elucidate their structure, and to observe the course of decomposition; to evaluate the content of residual solvents; to determine the isomeric composition: the ratio of diastereomers and the enantiomeric excess (ee) by means of chiral additives; to determine molar ratios of (protonated) basic drugs and (deprotonated organic acids in respective salts" (Diehl et al., 15).
In summing up, it is important to point out that nuclear magnetic resonance (NMR) has obtained much emphasis in recent decades and currently it is successfully applied in different spheres of human life. Besides, it is necessary to note that it is almost the most powerful, flexible and sensitive spectroscopic tool. Using this method, scientists are exploring the core of the structure of any substance: gases, fluids, solids, etc. NMR spectroscopy is so versatile that it provides the possibility to explore the connections that contain the major part of the periodic table of elements.
"NMR has become one of the best methods for obtaining anatomical images of human subjects and animals (under the common name magnetic resonance imaging, (MRI) and for exploring physiological processes. Materials science uses NMR spectroscopy and imaging to describe the structure, motion, and electronic properties of heterogeneous and technologically important substances. NMR is widely used in the food industry to measure moisture content and to access the quality of certain foodstuffs. NMR is used to measure the flow of liquids in pipes in industrial processes and to observe the flow of blood in human beings" (Becker 1).
Taking into account NMR spectroscopy, it is necessary to stress that the obvious advantage of this method is in a relatively low background, high selectivity measurements, the small spectral interference, which allows detecting weak signals and analysis of very small absolute number of items. To the disadvantages of NMR spectroscopy it is necessary to relate time costs, sensibility of the mechanism to outside noises, and it is unsuitable for creating high-frequency spectrometers (Freeman 1997).
Today the studies by NMR spectroscopy are maintained in almost all developed countries, because it possesses the great advantages and a high rate of application. It is directly related to the nanostructure of chemistry, and to the most important applications of NMR field today, that is, nanobiology nanomedicine. NMR spectrum provides with the absolute reliability in determining the structure and composition of a substance since there are no two connections similar to each other in nature.
Medicine is the sphere where the NMR spectroscopy method is successfully applied (Chary and Govil 2008). With the help of this method, physicians have received the possibility to determine the pathology and successfully treat it at the very beginning.
In order to vividly introduce the issue of NMR spectrometry and its application in daily life, it is necessary to provide an example that will completely reveal the theme. The capabilities of spectroscopy are considered to be extremely high. That is why the use of NMR spectroscopy is used in different spheres, one of which is the surrounding environment. Scientists use this method in order to understand the human factor in globalization process through the analysis of heavy metals in different layers of ice and snow of Greenland and Antarctica. Melting of ice causes a negative effect on the surrounding nature, including the climate change.