Study of phase determination by anomalous neutron scattering on Cd-histidine dihydrate

A study of experimental phase determination by anomalous neutron scattering techniques has been undertaken on a small structure, bis(L-histidine) cadmium dihydrate, in order to investigate the precision in the anomalous phases and its dependence on various parameters. Bijvoet differences have been measured for a limited number of reflections at four wavelengths including one (0.59A) at the low-wavelength side of the 113Cd resonance. Anomalous phases have been derived from these differences by a two-wavelength method. The experimental phases are compared to phases calculated from the structural model of Cd-histidine. The structure has been redetermined in order to complete an X-ray structural model by localization of the hydrogen atoms. The figure-of-merit for the differences between anomalous phases and phase angles calculated from the refined model is 98.4%. The internal consistency in experimental phases of symmetry-related reflections indicates an even higher precision of the phase determination with a figure-of-merit of 99.6%