Magnetic resonances

Magnetic resonance techniques, EPR (ESR) and NMR, can be used [341,342] to obtain information about atomic, ionic, molecular and crystallographic states before, during and after solid state reactions. Only a very restricted use has been made of the NMR of solids [342—345]. 

The intensity of the EPR resonance absorption is a measure of the number of paramagnetic centres present [346], while the type of line observed and the measured g factor are indications of the interactions of the paramagnetic particles and of their distribution within the matrix. Such spectra are much more sensitive to changes in crystal field and atomic orientations than X-ray diffraction and are not dependent upon crystallinity [347]. The nature of the paramagnetic particles may be discerned from the superfine structure of the spectrum. 

The high sensitivity and selectivity of the EPR response enables diamagnetic systems to be doped with very low concentrations of paramagnetic ions, the fate of which can be followed during the progress of a reaction. The criteria [347] for the use of such tracer ions are that they should give a distinct EPR spectrum, occupy a single coordination site and have the same valency as, and a similar diffusion coefficient to, the host matrix ion. Kinetic data are usually obtained by comparison with standard materials. 

There are two kinds of transitions that may be responsible for magnetic resonance  

ESR occurs at a much higher frequency than the NMR in the same magnetic field, because the magnetic moment of an electron is about 1800 times that of a proton. ESR is observed in the microwave region (9-38 GHz), nuclear spin resonance at radio frequencies (10-950 MHz). 

In ordinary absorption spectroscopy one observes the interaction between an oscillating electric field and matter, resulting in transition between naturally present energy levels of a system of electrically charged dipoles. So it would be appropriate to call this electric resonance spectroscopy . 

Magnetic resonance spectroscopy deals with the observation of the interaction between an oscillating magnetic field and matter, which results in transition between energy levels of the magnetic dipoles, the degeneracy of which is usually removed by an externally applied steady magnetic field. 

The important practical difference between electric and magnetic resonance spectroscopy is that the former technique usually permits observation of transitions in the absence of externally applied fields. In magnetic resonance spectroscopy this is hardly ever possible. 

The annelide (18) forms micelles of a tractable size for n.m.r. spectroscopy only in 33% aqueous tetrabutylammonium hydroxide. Under these conditions Ba exchange with free ligand is considerably faster than for the analogous compound in which methyl replaces octadecyl.  

Radio- and microwaves also have several other fields of application in spectroscopy. Molecular rotational transitions correspond to this wavelength region. Radiometers can be used for passive remote sensing, of e.g. temperature and air humidity, and radar systems can be utilized for active measurements of e.g. oil slicks at sea. Finally, radio astronomy is a fascinating field, yielding information on the most remote parts of the universe. 

We will now study the influence of a rotating magnetic field on a magnetic moment which precesses with an angular frequency Wq ternal magnetic field Bq as illustrated in Fig. 7.1 (Sect.2.3.1) 

As we have seen, a resonance will be induced by a rotating field. However, it is easier to produce a linearly oscillating field and such a field can always be resolved into two counter-rotating components, out of which one can be made resonant. The other component is then very far from resonance and will normally have a negligible effect. 

As we have seen, a resonance will be induced by a rotating field. However, it is easier to produce a linearly oscillating field and such a field can always 

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J A measure of the coupling constant in nuclear magnetic resonance. 

One has seen that the number of individual components in a hydrocarbon cut increases rapidly with its boiling point. It is thereby out of the question to resolve such a cut to its individual components instead of the analysis by family given by mass spectrometry, one may prefer a distribution by type of carbon. This can be done by infrared absorption spectrometry which also has other applications in the petroleum industry. Another distribution is possible which describes a cut in tei ns of a set of structural patterns using nuclear magnetic resonance of hydrogen (or carbon) this can thus describe the average molecule in the fraction under study. 

Starting from these methods, as we will see further on, nuclear magnetic resonance (NMR) of carbon has provided an absolute percentage of aromatic, paraffinic, and naphthenic carbons. 

Determining the Parameters of a Petroleum Fraction by Nuclear Magnetic Resonance... 

Schematic view showing the principle of nuclear magnetic resonance.

Brown, J.K. and W.R. Ladner Jr (1960), Distribution in coallike materials by high-resolution nuclear magnetic resonance spectroscopy . Fuel, Vol. 39, p. 87. 

NMR Nuclear magnetic resonance [223, 224] Chemical shift of splitting of nuclear spin states in a magnetic field H [225], C [226, 227], N [228], F [229], 2 Xe [230] Other Techniques Chemical state diffusion of adsorbed species... 

Equation (A1.6.64) describes the relaxation to equilibrium of a two-level system in tenns of a vector equation. It is the analogue of tire Bloch equation, originally developed for magnetic resonance, in the optical regime and hence is called the optical Bloch equation. 

Ernst R R, Bodenhausen G and Wokaun A 1987 Principles of Nuclear Magnetic Resonance in One and Two Dimensions (Oxford Clarendon)... 

Lynden-Bell R M and Harris R K 1969 Nuclear Magnetic Resonance Spectroscopy (London Nelson) pp 81-3... 

Venanzi T J 1982 Nuclear magnetic resonance coupling constants and electronic structure in molecules J. Chem. Educ. 59 144-8... 

Morris G A and Freeman R 1979 Enhancement of nuclear magnetic resonance signals by polarization transfer J. Am. Chem. See. 101 760-2... 

Bax A 1982 2-Dimensional Nuclear Magnetic Resonance in Liquids (Delft Delft University Press)... 

Siichter C P 1990 Principles of Magnetic Resonance (Beriin Springer)... 

Sliohter C P 1989 Principies of Magnetic Resonance 3rd edn (Berlin Springer)... 

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