Molecular Orbital Contributions to the Chemical Shift

The IGLO method permits the identification of localized molecular orbital contributions to the chemical shift that can be interpreted on a physicochemical basis. Contributions, e.g. for carbon Is core electrons, for C-H and C-C bonds, etc. can be identified. Even double and triple bonds can be described. In this section, the MO contributions to the methylene carbon chemical shift will be investigated (Fig. 24). For simplicity, in the following, we will refer to the theoretical scale a rather than to the experimental scale S which is measured with respect to a reference. 

The following essential features are found with little variation for all the systems presented in this study. 

These findings show that NMR is indeed a strictly local method the chemical shift of a nucleus is determined by the electrons in its immediate vicinity. The influence of remote residues is fairly small and acts mainly by changing the neighboring MOs. The direct contribution of distant molecular orbitals is negligible. 

3 Method of ab initio Simulation of Solid State NMR Spectra 

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