The Origin of the Difference Between the 13C and 17O Shift Behaviour of Carbonyl Compounds RCOX: Ab initio Calculation of the Shielding Tensors
✍ Scribed by Hans Dahn; Pierre-Alain Carrupt
- Publisher
- John Wiley and Sons
- Year
- 1997
- Tongue
- English
- Weight
- 378 KB
- Volume
- 35
- Category
- Article
- ISSN
- 0749-1581
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✦ Synopsis
In carbonyl compounds RCOX (X = H, Me, SR, Cl, F, OMe, OH, O-; R = H, Me), the 17O shift SiR 3
, N H 2 , values of the carbonyl group depend on the electron donor-acceptor properties of X, whereas the 13C shift values are determined by other factors too. By IGLO ab initio calculations, the di †erence between 13C and 17O has been traced to di †erences in the relative importance of the shielding tensor component in the direction of the CÈO bond
The deshielding contribution of this component is mainly determined by the energy of the n-p* excitation ; (r zz ). donor-acceptor-type interactions inÑuence the level of both orbitals. As the n orbital of this transition is essentially localized on the O atom, the excitation acts less on 13C and is thus mainly responsible for the di †erence in substituent sensitivity between 13C and 17O shifts. An analogous di †erence of substituent sensitivities between 13C and 17O shifts exists in aroyl compounds towards changes in the para substituent Y ; it is explained p-YC 6 H 4 COX on the same basis as the e †ect of X upon RCOX, without the necessity of referring to a "reverseÏ substituent e †ect. Finally, the surprising absence of substituent e †ects upon the 17O shift in aroyl cations can be p-YC 6 H 4 CO' explained by the fact that, for symmetry reasons, the n-p*-type excitation is absent in linear compounds. 1997