Reactions of AgI ions in alcohols after radiolysis at 77 K

Abstract

Solutions of silver perchlorate in methanol, ethanol, ethane-1,2-diol, isopropyl alcohol, allyl and propargyl alcohols have been exposed to ⁶⁰Co γ-rays at 77 K and the resulting radicals studied using ESR spectroscopy. In addition to ĊH₂OH solvent radicals, methanolic solutions gave Ag^II centres and a number of discrete Ag° species. The latter exhibited a range of hyperfine couplings, reflecting various degrees of solvation. Their relative yields were shown to vary according to initial Ag^I concentration and with added sodium perchlorate. After annealing to ca. 120 K, Ag° gave way to the molecular cluster cations Ag⁺₂, Ag²⁺₃, Ag³⁺₄ and Ag⁴⁺₅, previously identified by ESR and optical spectroscopy in a range of systems and shown to be formed by the sequential addition of Ag^I ions. Ag⁴⁺₅ formation was favoured either by high initial Ag^I or high counterion (nitrate or perchlorate) concentration. During the silver aggregation process the solvent radicals (CH₂OH) decayed and the adduct [Ag . CH₂OH]⁺ was formed [A_iso(Ag)= 128 G]. On annealing to the softening point of the matrix these species were lost. No higher-nuclearity molecular centres were detected. However, a weak singlet in the free-spin region grew in. This is tentatively assigned to conduction electron spin resonance (CESR) in pseudo-metallic, Ag⁰_n particles. On further melting the Ag⁰_n signal disappeared and the sample became black, indicating the eventual formation of colloidal silver. Similar reactions were observed in ethanol, ethane-1,2-diol and isopropyl alcohol.

The allyl alcohol system did not give any clearly defined Ag° centres, only solvent radicals being observed at 77K. Two possible silver–solvent adducts were detected after annealing, one with a larger A_iso(Ag) value than expected (ca. 360 G). Results for propargyl alcohol were similar, except that only one silver–solvent species was formed. Possible structures for these centres are discussed. When both systems were annealed to the matrix softening point, the adduct signals decayed and an isotropic singlet in the free-spin region was revealed. This signal was particularly strong in the propargyl system. The g value and linewidth are similar to values recently reported for CESR signals from Ag⁰_n latent-image particles formed in silver halide matrices by photolysis.