Peter Péchy

17O NMR Spectra of Carbonyl Compounds ArCOX: Influence of Groups X on the Polarity of the Carbonyl Group

The 17O shift values of para‐substituted benzoyl compounds p‐YC6H4COX (X=H, CF3, COOEt, Br, Cl, F, SEt, OCOAr, OH, O‐Na+, NH2) were measured. The 17O shielding values δ(17O) of these and other aryl‐unsubstituted benzoyl compounds (Y=H) follow the electron‐donating power of X (resonance), whereas inductive electron attractors (CF3, COOEt) show only small effects. The sensitivity of the shift values to donor/attractor para substituents Y, measured as the Hammett–Taft ρ+=δ(17O)/σ+, follows both the resonance and the inductive effects; combined with the δ(17O) values, it is proposed as an empirical measure of the polarity (electrophilicity) of the carbonyl group (35 types of COX in Table 1).

Concerning the chemical shift data (17O, 13C, 1H) of formaldehyde

The 17O NMR signal of formaldehyde was measured and the literature values of the 13C and 1H signals are discussed. They are compared with values obtained by other spectroscopic methods and by ab initio calculation.

NMR of Terminal Oxygen—17—Ab initio IGLO Study of the High Shielding of O (and C, N) in Linear Heteronuclear π‐Systems

Linear heteronuclear π‐systems of sp character, when compared with corresponding sp2 systems, show increased shift to higher field for all atoms concerned (O, C, N); acetylene compared with ethylene is a model case. In 17O NMR, the molecules NO+, PhCO+, RCNO, NξCξO−, RNξCξO and NO2+ were examined (in addition to known CO, CO2, COS and N2O). For these molecules, 13C and 15N shieldings were also analysed. As had been shown before for 13C and 15N, the increase in shielding of 17O 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 Δσ). Ab initio IGLO calculations were executed to evaluate Δσ ( = σ‖‐σ⊥). For each of the atoms concerned, the resulting calculated average values agreed well with the experimental values of isotropic shielding, σiso. As in the case of C and N, the values of σ‖ of 17O in linear compounds are close to that of the diamagnetic shielding of the isolated O atom, σd (atom). The particular case of organic isocyanates, RNCO, which, although not completely linear, show related behaviour, is discussed.