Spectroscopic studies of silver(O) centres formed radiolytically in water–ethanol solvents at 4 and 77 K

Abstract

Exposure of dilute solutions of silver perchlorate in water–ethanol systems to ⁶⁰Co γ-rays or X-rays at 4 and 77 K gave Ag(II) and Ag(O) centres, identified by e.s.r. spectroscopy. As shown by Li and Kevan, the latter centres at 4 K remain almost identical through the complete solvent range except for an abrupt increase in line width at ca. 0.14 mole fraction (EtOH). The isotropic hyperfine coupling is ca. 90 % of the atomic value in these media. This sudden change is attributed to a phase transition. The narrow features are thought to be characteristic of a well defined salt-hydrate phase, whilst the broad features are characteristic of a glass phase. This suggestion is strongly supported by an i.r. study of systems in the 0.1–0.2 mole fraction (EtOH) range in which —OH oscillators in an —OD environment were studied. The ice phase at 77 K is characterised by a narrow O—H stretching feature, whilst the glass phase shows a much broader feature in the same region. In the 0.13 mole fraction system both phases are detected, but the ice-phase was lost on reaching 0.14 mole fraction (EtOH). We suggest that at 4 K, the dominant Ag(O) centre is the tetrasolvate, and that there is a steady change from water to ethanol in the primary solvent shell.

On annealing the solutions to 77 K, or on irradiating at 77 K, a range of different species were formed. Most noticeable is that the major species up to a mole fraction (EtOH) of ca. 0.3 have reduced values of A_iso(Ag), whereas for more ethanolic systems there is little change on annealing to 77 K. However, on further annealing, species with enhanced values of A_iso(Ag) grew in, and over most of the mole-fraction range, an atomic species with narrow lines and, presumably, no coordinated solvent, was detected. Formation of the atomic species competes with formation of Ag⁺₂ and Ag³⁺₄ centres, which tend to dominate in ethanol-rich systems.

Concurrent optical studies of the systems irradiated at 77 K show that there is a marked shift in the first absorption band to low frequencies for the major primary centres relative to the atomic value. This is greatest for the aqueous centres, and there was a clear shift towards the pure atom value as the ethanol concentration was increased. Similarly, on annealing the solutions above 77 K the band narrowed and shifted towards the gas-phase frequency.

These results show that even when the values for A_iso(Ag) decrease, the centres nevertheless become more atom-like in their optical spectra. This apparent contradiction is discussed in terms of loss of solvation of the parent Ag⁺_solv ions. For strongly aqueous systems it is suggested that on annealing, only one water molecule is partially or completely lost, leaving an asymmetric centre. The SOMO for this unit is an s–p hybrid, the p-orbital admixture accounting for the reduction in A_iso(Ag), the gain in anisotropy and the negative shift in g values. For more ethanolic, glassy systems, there is a general loosening of bonding as the solvent molecules move away from silver, the process of solvent loss being catastrophic once it is thermally initiated. The resulting atoms are either trapped or move through the glass and react with Ag⁺ ions to give Ag⁺₂ units.