Ammonium Bifluoride: A Study of Extinction and Electrostatic Properties. C.G. van Beek, S.J. van Reeuwijk* and B.M. Craven, Department of Crystallography, University of Pittsburgh, Pittsburgh, PA 15260. *Chemical Physics Laboratory, University of Twente, PO Box 217, 7500 AE Enschede, the Netherlands.

High resolution X-ray data for ammonium bifluoride, NH4F2H, collected at 102 K, indicate severe extinction (1.0 < Y < 0.63) and provide the opportunity to test extinction models. We have used the model by Becker & Coppens (1974) and the models by Sabine with correlated (1992) and uncorrelated scattering from mosaic blocks (unpublished). All three models fit the data well. However Sabine?s models avoid the high least squares correlation between block size and mosaic spread parameters which occurs with the Becker & Coppens model. Charge density parameters ( Stewart?s rigid pseudo atom model ) are nearly independent of the choice of extinction model.

Ammonium bifluoride, NH4F2H, contains two distinct F-H-F groups with extremely strong hydrogen bonds. Preliminary X-ray refinements indicate that neutron diffraction data will be necessary to obtain reliable deconvolution of thermal displacements from the charge density distribution, particularly for the hydrogen atoms.

This work was supported by NIH Grant GM35913.