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Title

NMR of terminal oxygen.17. Ab initio IGLO study of the high shielding of O (and C, N) in linear heteronuclear pi-systems

Authors
Dahn, H.
Pechy, P.
Published in Magnetic Resonance in Chemistry. 1996, vol. 34, no. 4, p. 283-288
Abstract Linear heteronuclear pi-systems of sp character, when compared with corresponding sp(2) systems, show increased shift to higher field for all atoms concerned (O, C, N); acetylene compared with ethylene is a model case. In O-17 NMR, the molecules NO+, PhCO(+), RC=NO, N=C=O-, RN=C=O and NO,+ were examined (in addition to known CO, CO2, COS and N2O). For these molecules, C-13 and N-15 shieldings were also analysed. As had been shown before for C-13 and N-15, the increase in shielding of O-17 in linear molecules is mainly attributable to the vanishing of the deshielding component along the linear axis of the molecule (manifest also in the increase in the anisotropy of shielding Delta sigma). Ab initio IGLO calculations were executed to evaluate Delta sigma (=sigma(parallel to)-sigma(perpendicular to)). For each of the atoms concerned, the resulting calculated average values agreed well with the experimental values of isotropic shielding, sigma(iso). As in the case of C and N, the values of sigma(parallel to) Of O-17 in linear compounds are close to that of the diamagnetic shielding of the isolated O atom, sigma(d) (atom). The particular case of organic isocyanates, RNCO, which, although not completely linear, show related behaviour, is discussed
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