General principles chemical shifts

In principle all the X-ray emission methods can give chemical state information from small shifts and line shape changes (cf, XPS and AES in Chapter 5). Though done for molecular studies to derive electronic structure information, this type of work is rarely done for materials analysis. The reasons are the instrumental resolution of commercial systems is not adequate and the emission lines routinely used for elemental analysis are often not those most useftil for chemical shift meas-ure-ments. The latter generally involve shallower levels (narrower natural line widths), meaning longer wavelength (softer) X-ray emission. 

The following descriptions will be couched primarily in terms of chemical shift changes, which are the most widely used for the measurement of dissociation constants the general principles apply equally to measurements based on other NMR parameters, and these will be touched on later. 

Our treatment is considerably simplified by restricting ourselves to nuclei with / = /2, as we can cover most of the general principles without tedious algebraic manipulations. In addition, nuclei with / = /2 are studied far more extensively than others. It will be helpful to use the widely employed system of notation in which each nucleus of spin /2 is denoted by some letter of the alphabet A, B, X, etc. We choose letters of the alphabet representative of relative chemical shifts that is, for two nuclei that have a small chemical shift relative to each other, we choose two letters of the alphabet that are close to each other, and for nuclei that have large relative chemical shifts, we use letters from opposite ends of the alphabet. (We define small and large chemical shifts later.) Different nuclear species (e.g., H and F) are also represented by letters from opposite ends of the alphabet, because their chemical shifts differ by many megahertz. 

Figure 15 presents the pulse sequence for a general separated local field (SLF) experiment.133-136 The basic principle of the SLF technique is that a spinning-sideband pattern, from which the heteronuclear dipolar coupling can be extracted, is obtained in the indirect dimension for each resolved resonance in the direct dimension, i.e., the dipolar interaction is separated from the chemical shift interaction. In the original SLF papers, a homonuclear decoupling method is applied in t, but recently McElheny et al. 

Detailed analysis of spectra of this type leads in principle to the evaluation of. /axI. 1- ax I I- aa I. /xx 1- General equations for line positions and intensities of the X part of an X AA X spin system have been presented by Harris for situations where the remote XX coupling is zero. The appearance of the X part of such a spectrum consists of a doublet of separation. /ax+- ax centred at vX, (the chemical shift of the X nucleus), which has relative intensity 2 " , ... 

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