Oxygen-l7 nuclear magnetic resonance. Part I. Oxygen–nitrogen groupings

Abstract

Oxygen-l7 shifts are reported for N₂O₃, N₂O₄, MeONO, Me₂NNO, NO₂⁺, and NOCl, with ¹⁷O in natural abundance. These are compared with literature values of ¹⁷O shifts in other oxygen–nitrogen groupings, and with the corresponding nitrogen shifts. The shifts of adjacent oxygen and nitrogen in the same chromophore are roughly in the proportion of their respective values of 〈r^–3〉, which gives the dependence of the paramagnetic shielding term (σ_p) on the radius of the 2p orbitals. The oxygen resonances tend to move downfield as the lowest-energy electronic absorption moves to longer wavelengths, but the scatter is large. The oxygen shielding in XNO and XNO₂ compounds (as in RCOX) reflects the conjugative effect of the substituent X. This contrasts with the nitrogen shielding, in which inductive effects are relatively more important. Inductive effects may however be among those responsible for oxygen shift relationships, in certain sets of compound, that differ from the overall trends. The relative importance of the orbital terms that contribute to σ_p is compared for oxygen and nitrogen in these groupings.