Chapter 1 NMR chemical shift and electronic structure

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The nuclear magnetic resonance (NMR) chemical shift is affected by a change of the electronic structure arising from structural changes. NMR chemical shifts in the solid state provide useful information about the electronic structure of a polymer or polymers with a fixed structure. Furthermore, in the solid state, the components of the full chemical shift tensor can often be determined. The complete chemical shift tensor provides information on the local symmetry of the electron cloud around the nucleus and much more detailed knowledge of the electronic structure of the polymer compared with the average chemical shift associated with the structure. Theoretical calculations of NMR chemical shifts for polymer systems have been achieved using two main approaches. One approach is that model compounds, such as the dimer and trimer, as a local structure of polymer chains, are used in the calculation by combining quantum chemistry and statistical mechanics. Another approach is to employ the tight-binding molecular orbital (TB MO) theory to describe the electronic structure of linear polymers with periodic structure within the framework of the linear combination of atomic orbitals (LCAO) approximation for the electronic eigenfunctions. These approaches lead to the determination of the spatial structure and/or electronic structure of polymer systems including polypeptides in the solution and solid state.