Properties of the Crystal Electron Density of Methylammonium Deuterium Maleate. Dennis Madsen*+, Claus Flensburg* and Sine Larsen*. *Centre for Crystallographic Studies, Dept. of Chem., University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark. +ESRF, BP220, 38043 Grenoble Cedex, France.

The charge density of methylammonium deuterium maleate (MADMA) has been determined from a set of low temperature X-ray and neutron diffraction data with maximum resolutions of 1.08AA-1 and 0.72AA-1. The program VALRAY was used in the refinements of both data sets. The analysis of the neutron data showed that MADMA crystallizes in Pnam with a crystallographic mirror plane perpendicular to the C-C double bond in the anion. The two very short intramolecular O-H-O hydrogen bonds were found to be symmetric. A topological analysis has been performed and all critical points in the experimental electron density were located. The integrated properties of the atomic basins in the experimental electron density were furthermore determined. The topological analysis showed that the two very short intramolecular hydrogen bonds are covalent in character. The conjugated system in the hydrogen maleate ion was found not to include the region with the very short intramolecular hydrogen bond.