Origin of the Chemical Shift

This shows that the TB MO calculation correctly predicts the origin of the chemical shift and electronic structure associated with the structure of the polymers. Furthermore, TB INDO/S calculations have been carried out on the seven polyacetylene chains which take an orthorhombic form(9). From these results it has been demonstrated that the chemical shift is very sensitive to intermolecular interactions and the TB MO calculation provides useful information about the band structure. 

In previous chapters we have described the origin of the chemical shift and of indirect spin coupling, and we have seen a number of illustrations of high resolution NMR spectra. We now need to look more carefully at the way in which the effects of chemical shifts and spin coupling can be added to the basic treatment of spin physics that we studied in Chapter 2. In this chapter we explore the ways in which nuclei interact not only with the applied magnetic field but also with each other. The steady-state quantum mechanical approach of Chapter 2 can easily be expanded by using a Hamiltonian that includes chemical shifts and couplings. 

Why is the displacement of the an different for [Ala ] and [Leu ] The reason for this is not clear at present, but this is a very important question in reaching and understanding of the correlation between structures and the l5N shielding tensor. The 15N shielding tensor may be useful for conformational analysis of solid polypeptides, if the origin of the chemical shift displacements can be elucidated. 

The origins of the chemical shifts are probably not sufficiently well understood (as yet), to allow a quantitative discussion of aromatic character in the annulenes. If such a concept is considered meaningful it would probably best be defined in terms of the degree of bond alternation therein, which is of pivotal importance to the jr-electron properties (see Sections B and C). Apart from theoretical calculations, a number of physical methods have demonstrated their ability to estimate the extent of bond alternation in annulenes (crystallographic analysis, electronic/vibronic spectral analysis, diamagnetic anisotropy/susceptibility exaltation measurements and of course n.m.r.), see ref. > for a full discussion. (Furthermore the known correlation between n.m.r. vicinal coupling constants and carbon-carbon bond orders is of potential utility in any determination of bond alternation 65>). 

So far, we have discussed the nucleus in terms of a specific isotope in our case, P. NMR spectroscopy is only useful to us because there is a difference between phosphoms atoms in different local enviromnents. Each phosphorus atom in its own local enviromnent is assigned its own chemical shift value, measured in ppm. We know from C- and H-NMR that the origin of the chemical shift lies in the influence (shielding and deshielding) that the neighbouring atoms assert on the phosphoms atom. Mathematically, we can describe this phenomenon by the following equation ... 

We will not discuss the physical origin of the chemical shift phenomenon in detail, and merely note that it is based on a rather complicated quantum-mechanical interaction between the Bo field and the paired bonding electrons and lone pairs, leading to a local magnetic field at the position of the nudeus that is in most cases reduced as compared to the hypothetical bare nuclear spin. We would like to note, however, that a... 

---