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The origin of shielding constants

المؤلف:  Peter Atkins، Julio de Paula

المصدر:  ATKINS PHYSICAL CHEMISTRY

الجزء والصفحة:  521

2025-12-10

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The origin of shielding constants

The calculation of shielding constants is difficult, even for small molecules, for it re quires detailed information about the distribution of electron density in the ground and excited states and the excitation energies of the molecule. Nevertheless, consider able success has been achieved with the calculation for diatomic molecules and small molecules such as H₂O and CH₄ and even large molecules, such as proteins, are within the scope of some types of calculation. Nevertheless, it is easier to understand the different contributions to chemical shifts by studying the large body of empirical information now available for large molecules. The empirical approach supposes that the observed shielding constant is the sum of three contributions:

σ=σ(local) + σ(neighbour) + σ(solvent)

The local contribution, σ(local), is essentially the contribution of the electrons of the atom that contains the nucleus in question. The neighbouring group contribution, σ(neighbour), is the contribution from the groups of atoms that form the rest of the molecule. The solvent contribution, σ(solvent), is the contribution from the solvent molecules.