Radiation mechanisms. Part 5.—Nitrite and nitrate ions in aqueous glasses

Abstract

Acidic, neutral and alkaline aqueous glasses containing various metal nitrites or nitrates have been exposed to 60Co γ-rays at 77 K and their e.s.r. spectra recorded over a range of temperatures. In alkaline glasses, nitrite gave NO2 and NO2–2 at 77 K, but NO2–3 was clearly distinguished when trapped O– was lost (NO–2+ O–→ NO2–3). Nitrate gave only NO2–3 under these conditions. In neutral glasses, nitrite again gave NO2 and NO2–2, but the latter was readily converted to HNO–2. No NO2–3 was formed on annealing. Nitrate in neutral glasses gave NO2–3 and NO2 in relative concentrations that varied with the cation. The NO2 radical signal was lost reversibly on slight warming above 77 K, which suggests that these radicals are only weakly associated with solvent. Two species with small hyperfine coupling to 14N (≈3 G) were detected in amounts that were again cation dependent. One was clearly NO3(which did not rotate on slight annealing) but the other, species A, having g1= 2.008, g2= 2.006, g3= 2.000, A1= 3.3 G, A2= 3.15 G and A3= 1.6 G, was not identifiable with any known nitrogen radical. Species A was never formed in acidic glasses. We tentatively suggest NO2–4 as a possible formulation (OH + NO–3→ NO2–4+(H+)).Acidic glasses containing nitrate gave only NO2 and NO3. The effect of added methanol was studied and it is concluded that NO2 is formed from NO2–3 and not via NO3 as has been previously supposed.