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The Origin of the Difference Between the 13 C and 17 O Shift Behaviour of Carbonyl Compounds RCOX: Ab initio Calculation of the Shielding Tensors
Author(s) -
Dahn Hans,
Carrupt PierreAlain
Publication year - 1997
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/(sici)1097-458x(199709)35:9<577::aid-omr129>3.0.co;2-e
Subject(s) - chemistry , substituent , ab initio , atomic orbital , chemical shift , crystallography , excitation , acceptor , computational chemistry , ab initio quantum chemistry methods , molecular orbital , stereochemistry , molecule , electron , organic chemistry , physics , condensed matter physics , quantum mechanics , electrical engineering , engineering
In carbonyl compounds RCOX (X=H, Me, SiR 3 , SR, Cl, F, OMe, OH, NH 2 , O ‐ ; R=H, Me), the 17 O shift values of the carbonyl group depend on the electron donor–acceptor properties of X, whereas the 13 C shift values are determined by other factors too. By IGLO ab initio calculations, the difference between 13 C and 17 O has been traced to differences in the relative importance of the shielding tensor component in the direction of the C—O bond (σ zz ). The deshielding contribution of this component is mainly determined by the energy of the n–π* excitation; donor–acceptor‐type interactions influence the level of both orbitals. As the n orbital of this transition is essentially localized on the O atom, the excitation acts less on 13 C and is thus mainly responsible for the difference in substituent sensitivity between 13 C and 17 O shifts. An analogous difference of substituent sensitivities between 13 C and 17 O shifts exists in aroyl compounds p ‐YC 6 H 4 COX towards changes in the para substituent Y; it is explained on the same basis as the effect of X upon RCOX, without the necessity of referring to a ‘reverse’ substituent effect. Finally, the surprising absence of substituent effects upon the 17 O shift in aroyl cations p ‐YC 6 H 4 CO + can be explained by the fact that, for symmetry reasons, the n–π*‐type excitation is absent in linear compounds. © 1997 by John Wiley & Sons, Ltd.