Evaluation of Charges from the NMR Chemical Shift

The study of chemical shifts in NMR provides an example of the sort of physical quantities to which one may refer. As is well known, the total magnetic field to which a nucleus of a molecule is subject is the sum of the external field ext and of the induced field created by the electrons surrounding the nucleus under study. The latter field is proportional to the external field and of opposite sign. The effective fidd thus acting on the nucleus is  

On the other hand, several attempts have been made to connect chemical shifts with charge distributions given by semiempirical methods whose parameters have been adjusted to other molecular properties. A linear condition of the type a = — AQc — BQn+C has been found to be quite satisfactory for the chemical shift of the proton in aminoacids . For 0 the results of similar attempts have not been equally encouraging ), the reason possibly being the charge definition 

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