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The electric field contribution to the screening constant in 13C NMR and its dependence on the longitudinal bond polarizabilities. the syn/anti anisochrony problem in amides

par Piccinni Leopardi, Carla ;Reisse, Jacques
Référence Journal of Magnetic Resonance (1969), 42, 1, page (60-72)
Publication Publié, 1981

Article révisé par les pairs

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Résumé :

The electric field contribution to the screening constant in 13C NMR depends on the charge borne by the observed carbon atom. This charge can be estimated either by quantum mechanics or by classical methods. It appears that each of these approaches is difficult to handle for complex molecules. The classical approach has the merit of explicitly showing the influence of the various bonds connected to the carbon under study. Nevertheless, the great dependence of the calculated value on the longitudinal bond polarizabilities appears to be a limiting factor because these polarizabilities are not known with sufficient precision. This problem is discussed in detail, and the influence of the choice of a set of polarizability values is illustrated for the itsyn/anti anisochrony of the α (to nitrogen) carbon atoms in N,N-dialkylamides. The influence of other parameters, such as field gradient and dipole position, is also examined. The accuracy of the electric field contribution to the screening constant is then critically discussed. The origin of the syn/anti anisochrony does not appear to be due to the electric field effect. Other possible contributions are discussed but no definitive conclusion can be drawn. © 1981.