Nuclear Magnetic Resonance

The nuclear states produced by a magnetic field are studied in nuclear magnetic resonance spectroscopy.

The frequency of the radiation that corresponds to the nuclear magnetic energy level spacings and the weakness of the radiation absorption that must be expected lead to a spectrometer of a radically different kind from those prism instruments used for electronic and vibrational spectral analysis. The arrangements most frequently used. The principle magnetic acts on the nuclei of the sample to produce energy levels like those indicated as resistors. Transitions between these levels are stimulated by radiation from the radio frequency transmitter, which sends out electromagnetic radiation from the frequency of the radiation is such that the quanta of radiation have an energy matching the nuclear energy level spacing. The receiver coil, which is oriented at right angles to the transmitter, receives no signal unless the sample provides this coupling with the transmitter. The signal obtained as a function of magnetic field to 60 MHZ is applied on a and b for several simple compounds. The identification fo the hydrogen atom or groups of hydrogen atoms that produce a given signal can be made by a simple comparison of these spectra or can be more definitely established by the use of deuterium subtitled derivates. 

This indication of the operation of an nmr spectrometer implies that a magnetic field is imposed on the sample and that the frequency of the radiation is varied.

Since it is here possible to control the energy level spacing by manipulating B, the equality can be brought about either by adjusting v after some time fixed value of B is chosen or by adjusting B after some fixed value of v has been selected. The latter procedure turns out to be experimentally more satisfactory. A fixed frequency, usually about 60 MHz is supplied by the transmitter, and the magnetic field is varied through a small range until is satisfied. At this point the sample absorbs and emits radiation, the transmitter and receiver are coupled, the circuit can be said to be in resonance, and a signal is produced from the receiver circuit.

The signal obtained as a function of magnetic field for a fixed frequency of 60 MHz a and b for several simple compounds. The identification of the hydrogen atom or groups of hydrogen atoms that produce a given signal can be made by a simple comparison of these spectra or can be more definitely established by the use of deuterium substituted derivates. If spectra are obtained at higher resolution, a considerable complexity appears, as by the solid curves and the lower curves.

The nuclear energy level splitting depends on the nuclear magnetic moment and the magnetic field strength. The experimental results indicate that even if the absorption of only hydrogen atoms is studied, a number of closely spaced absorption are observed.

Chemical shifts: The environments of the nuclei of a molecule affect the overall pattern and the detailed structure of nmr spectra. The factors that lead to the different resonances can often be treated separately from the factors that lead to the finer splitting indicated there. The separation in the positions of the spectral lines associated with hydrogen atoms in different chemical environments is called the chemical shift. These shifts can be conviently reported as the difference in magnetic field necessary for absorption by some reference. This difference is usually reported as the chemical shift. These shifts can be conveniently reported as the difference in magnetic field necessary for absorption compared with that the chemical shift δ, defined as;

δ = Bref - Bsample/Bref × 106

As a result of chemical shifts, the nmr spectrum is a portrayal of the chemical environment fo the various atoms, hydrogen for example, of the material environment. So an analysis of a spectrum of an hydrogen atoms and often to the molecular structure of the sample. In this respect nmr complements infrared and ultraviolet spectroscopy in the elucidation of the structures of large molecules.

Fine structure: the hydrogen atoms of the methyl group experience a magnetic field that depends on the applied field, on the chemical shift effect of the shielding electrons, and on the influence fo the magnetic field fo the nucleus of the hydrogen atom adjacent to the carbonyl group. This nucleus indicates, can line up with or against the principle magnetic field. The methyl hydrogen atoms therefore experience a slightly greater or lesser magnetic field, depending on the orientation of the lone hydrogen atom. 

A number of important features of nmr spectroscopy have not been dealt with in this brief introduction. It is frequently of interest, for example, to examine the mechanism by which the radiation is able to interact with the magnetic nuclei to turn them to a different orientation. This has not been treated here. Likewise, no mention has been made of the fact that if atoms, like hydrogen atoms of a water sodium hydroxide solution, move their position from one molecule to another so that they occupy a given position for less than the nmr spectrum shows a single absorption at a position characteristic of the one environment and the other characteristic of the other. In fact, it is one of the most interesting aspects of nmr spectroscopy.

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