Electron nuclear double resonance study of pure and aquated rhodium complexes in silver chloride emulsions

Detailed information has been obtained on the structure of pure and aquated Rh2+ complexes in AgCl using multifrequency EPR and powder ENDOR spectrosc opy. Because of the anisotropy of the Rh2+ centre g matrix it was possible to selectively excite Rh2+ centres with a specific angle between the z axis of the g matrix and the magnetic field axis. As a result, almost single-cr ystalline-like ENDOR spectra were obtained and the angular dependence of th e ENDOR spectra could be obtained by scanning the magnetic field over the r ange of the Rh2+ EPR spectra. Three different Rh2+ centres were detected th at are distinguished by different thermal stabilities, different parameters of the g matrix and the hyperfine matrices of the central Rh nucleus and l igand H, Cl and Ag nuclei. A pure Rh2+ centre without H2O ligands could onl y be generated in an emulsion where great care was taken to prevent the aqu ation of the [RhCl6](3-) dopant ion in the dopant solution and during the p recipitation. This centre had the highest thermal stability. In an emulsion prepared conventionally two Rh2+ centres were found that are associated wi th one and two H2O ligands bound to the Rh2+ ion in the plane perpendicular to the g matrix z axis. Using the results of the EPR and ENDOR analysis in combination with the results of a total energy calculation two models are proposed that contain the positions of the H2O ligands and the charge compe nsating vacancies.