NMR of Terminal Oxygen—17—Ab initio IGLO Study of the High Shielding of O (and C, N) in Linear Heteronuclear π-Systems
✍ Scribed by Hans Dahn; Peter Péchy; Pierre-Alain Carrupt
- Publisher
- John Wiley and Sons
- Year
- 1996
- Tongue
- English
- Weight
- 578 KB
- Volume
- 34
- Category
- Article
- ISSN
- 0749-1581
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✦ Synopsis
Linear heteronuclear Ir-systems of sp character, when compared with corresponding sp2 systems, show increased shift to higher field for all atoms concerned (0, C, N); acetylene compared with ethylene is a model case. In I7O NMR, the molecules NO+, PhCO+, RCGNO, N=C=O-, RN=C=O and NO2+ were examined (in addition to known CO, CO, , COS and N,O). For these molecules, "C and 15N shieldings were also analysed. As had been shown before for 13C and "N, the increase in shielding of "0 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 Au). Ab iniiio IGLO calculations were executed to evaluate Au (=allul). For each of the atoms concerned, the resulting calculated average values agreed well with the experimental values of isotropic shielding, u,,,,. As in the case of C and N, the values of u,, of I7O in linear compounds are close to that of the diamagnetic shielding of the isolated 0 atom, ud (atom). The particular case of organic isocyanates, RNCO, which, although not completely linear, show related behaviour, is discussed. KEY WORDS ' ' 0 NMR; ab initio calculations; shielding anisotropy; linear x-systems; NO' ;NO,+ ; acyl cations; isocyanates 'Relative to Me,CN=O, 6("O) =1538 ppm, Ref. 10; 6("N) = +578 ppm. 'Relative to MeC(0)OMe. 6("O) =361 ppm, 6(13C) =171 ppm. hRelative to MeC(0)SMe. 6(170) =511 pprn," a('%) =195 PPm. I Ref. 12. 'Relative to PhCH=N(O)Ph, 6(170) -377 pprn', a('%) =134 ppm;13 6(16N) --95. 'Relative to PhN=N(O)Ph, 6("O) -456 pprn', 6("N)(NO) = -57 ppm, 6("N)(NPh) --47 ppm. ' This work: 0.5 M aqueous solution of KCNO. " Relative to MeC(O)NMe,, 6(170) -348 ppm, 6("C) =170 ppm, 6(16N) = -281 ppm. " Relative to MeNO,, 6(170) = 590 ppm, 6(16N) = 0 ppm. Relative to Me,CO, 6("O) =571 ppm, a('%) -207 ppm. Ref. 14. Relative to CH,=CH,, -123 ppm. Relative to PhNNPh, 6(16N) -+129 ppm. ' Relative to RMeC=NMe, a('%) = 168 ppm ( R -Me) 6(16N) = -74 ppm (R = Et).