Protonation Nitrogen Shielding and NOE in Aqueous Nitrite and Solid-State ¹⁵N NMR of NOBF₄ and NO₂BF₄

Nitrogen-15 chemical shift measurements carried out under NO at 1 atm and pH ≥ 2.2 yield the value Δ(δ¹⁵N) = −30.4 ppm for the total protonation shielding of NO₂⁻(aq). Measurements at higher acidity levels reflect the increasing significance of NO⁺ and confirm the presence of two ¹⁵N resonances, one sharp and one broadened, in the region around 45% D₂SO₄. Assignment of the sharp peak to NO⁺ is confirmed by solid-state NMR of NOBF₄. Broadening in the other suggests that exchange between HNO₂ and some form of acidium ion (e.g. H₂NO₂⁺) precedes NO⁺ release, but no NOE is detected in 42.4% D₂SO₄ and it is inferred that no proton-to-nitrogen connnectivity occurs in this species. Nitrogen-15 chemical shifts and chemical shift anisotropics have been obtained in the solid state at natural isotopic abundance for the nitrosonium and nitronium ions in their respective tetrafluoroboride salts.