Unstable intermediates. Part 174. Electron spin resonance spectrum of tetraoxodinitrate(1–) formed by radiolysis of dinitrogen tetraoxide

Abstract

Exposure of crystalline N₂O₄ to ⁶⁰Co γ-rays at 77 K gives three distinct paramagnetic centres, A, B, and C. The e.s.r. spectrum for A is characteristic of NO₂˙ in a variety of subtly different sites, and that for B, which shows the presence of two equivalent nitrogen atoms, is assigned to the species [N₂O₄]^–. The third centre, C, which is poorly defined, also contains two equivalent nitrogen atoms and is tentatively identified as [N₂O₄]⁺. Solutions of N₂O₄ in methanol (CD₃OD) give centres A and B′. The e.s.r. parameters for B are related to those for B′, and it is suggested that they are the same species, with centre B undergoing a minor libratory motion. Theunpairedelectron in centre B ([N₂O₄]^–) is in a σ* orbital having a smaller 2p : 2s ratio and a higher spin density on nitrogen than in the corresponding orbital of NO₂, from which it is derived. It is concluded that the electron is strongly confined to the two nitrogen atoms in [N₂O₄]^– and probably in [N₂O₄]⁺, which also appears to have a σ structure.