Unstable intermediates. Part CL. Nitrogen dioxide, nitrogen trioxide, and nitrate (2–) in irradiated lead(II) nitrate: electron spin resonance evidence for electron transfer to and from lead(II) ion

Abstract

All three of the title radicals have e.s.r. spectra which display satellite lines assignable to ²⁰⁷Pb hyperfine components. The radical NO₃^2– interacts strongly with three magnetically equivalent Pb²⁺ ions and weakly with a fourth. The hyperfine coupling is anisotropic, suggesting the acquisition of spin density by the 6p manifold. This is supported by the large shift to high field for the NO₃^2– features, compared with results for this species in alkali-metal salts. Single-crystal studies show that the principal directions of the ²⁰⁷Pb hyperfine-tensor components are strongly displaced from those of the ¹⁴N hyperfine tensor and hence various anomalous features of the powder spectrum are interpreted. A radical, thought to be NO₃, is also formed by ⁶⁰Co γ rays at 77 K. This also displays a large shift to high field (low g values) relative to ‘normal’ NO₃, and strong, almost isotropic, hyperfine coupling to three neighbouring Pb²⁺ ions. This species is stationary at 4·2 K, but exhibits in-plane rotation at 77 K and free rotation at ca. 200 K. On storage at room temperature it is slowly replaced by a species with normal magnetic properties for NO₃ and no detectable coupling to ²⁰⁷Pb nuclei. At room temperature, features for NO₃^2– also decay, being replaced by those characteristic of NO₂ undergoing in-plane rotation. This exhibits isotropic coupling to three equivalent Pb²⁺ ions. On cooling at 77 K, the radicals become stationary on the e.s.r. time scale and exhibit strong, almost isotropic, coupling to a single Pb²⁺ ion and weak coupling to two other ions. These results are discussed in the light of the ability of Pb²⁺ to act both as an electron donor and acceptor.