Magnetic systems, definition

The command molecule has the highest priority in the internal hierarchy of the spin system definition. Next comes the nucleus statement, each magnetically non-equivalent nucleus is defined, one nucleus per line followed by the coupling interaction between nuclei, one interaction per line. It is also possible to define a non-thermal equilibrium spin system state such as produced in multiple quantum coherence experiments. The required coherence may be selected using the spin state definition rather than by a pulse sequence, this not only simplifies the pulse sequence but also reduces calculation times. Fig. 4.3 illustrates the general layout of a spin system file. In the case of a single spin system, the molecule and endmol commands are redundant and may be omitted. 

In this review the definition of orientation and orientation functions or orientation averages will be considered in detail. This will be followed by a comprehensive account of the information which can be obtained by three spectroscopic techniques, infra-red and Raman spectroscopy and broad line nuclear magnetic resonance. The use of polarized fluorescence will not be discussed here, but is the subject of a contemporary review article by the author and J. H. Nobbs 1. The present review will be completed by consideration of the information which has been obtained on the development of molecular orientation in polyethylene terephthalate and poly(tetramethylene terephthalate) where there are also clearly defined changes in the conformation of the molecule. In this paper, particular attention will be given to the characterization of biaxially oriented films. Previous reviews of this subject have been given by the author and his colleagues, but have been concerned with discussion of results for uniaxially oriented systems only2,3). 

Any spin system that contains fluorine substituents that are chemically equivalent, but not magnetically equivalent is, by definition, second order. Such spectra can appear deceptively simple, or more commonly they can be amazingly complex. The fluorine and proton spectra of the simple, symmetrical compound, 1,1-difluoroethene exemplify the latter situation (Figures 2.5 and 2.6). 

The definitions of the first and second order magnetic perturbation operators are given helow. In the nonrelativistic formalism these operators are two-component operators, in the Kutzelnigg formalism all operators are to he multiplied hy the four-component matrix. All operators are given in the atomic unit system and we do not apply QED corrections so that the free electron g-factor is precisely equal to 2. 

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