Td. Pawlik et al., Electron nuclear double resonance study of pure and aquated rhodium complexes in silver chloride emulsions, J PHYS-COND, 10(50), 1998, pp. 11795-11810
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.