Isotope and bonding effects in ammonium oxalate monohydrate, determined by the combined use of neutron and X-ray diffraction analyses

Three-dimensional neutron and X-ray diffraction structural analyses have been carried out on protonated, deuterated and null-matrix (63 % H, 37 % D) crystals of ammonium oxalate monohydrate. The null-matrix composition is such that the neutron hydrogen and deuterium scattering lengths cancel (bH=--0.378x 10 t 2 cm; bD=065x l0 -12 cm). Absorption and extinction corrections were applied and refinement was by full-matrix least-squares calculations with anisotropic temperature factors. It was shown statistically that different sets of coordinates and temperature factors resulted if extinction corrections were not applied. In the null-matrix crystal the deuterium atoms were found to show a preference for the ammonium ion sites and the protons for the water sites, with a separation factor (H/D)w.,ter/(H/D) . . . . . ium of 107 (2). There was no significant expansion of the hydrogen bonds on deuteration. X-N maps for the deuterated and protonated forms (difference syntheses derived from neutron parameters and X-ray data) revealed deviations from spherical symmetry of the atoms, consistent with bonding and lone-pair electron distributions. These features were not observed with the X-ray data for the null-matrix crystals because of excessive extinction and would not have been seen for the deuterated and protonated crystals had extinction corrections not been applied.