TRANSFERABILITY AS A PRINCIPLE IN CHEMISTRY - THE CHARGE DENSITY OF CH₃NH+₃ IN TWO SALTS. Dennis Madsen, Claus Flensburg, Sine Larsen, Centre for Crystallographic Studies, University of Copenhagen

Transferability in chemistry has for some years been demonstrated on theoretical charge densities¹. We have compared topological features of the experimental charge densities of the CH₃NH+₃ ion in two different salts, methyl ammonium hydrogen succinate monohydrate (MAHS)² and methyl ammonium deuterium maleate (MADMA). For the latter neutron and X-ray diffraction data were measured at 122 K to resolutions of 0.732 Å^-1, and 1.08 Å^-1 respectively. Parameters for the hydrogen atoms obtained from the refinement of the neutron data were used in the multipole refinement of the X-ray data with the program VALRAY³. The bond critical points (bcp's) in the experimental charge density were found, and the bond paths determined. The MADMA crystallizes in space group Pnam with the cation in a general position and two different anions on crystallographic mirror planes perpendicular to the double bonds. The bcp's in the two very short (2.40Å) symmetric intramolecular hydrogen bonds both have negative Laplacians (equal within the uncertainty) indicating the covalent character of these interactions. The chemically equivalent bcp's for the CH₃NH+₃ ion in MADMA and MAHS have been compared. This showed that equivalent bcp's have identical [[rho]], and ¡² [[rho]] within the uncertainty, the only exception being the C-N bcp, which is in a crystallographic mirror plane in MAHS and in a general position in MADMA. These results show that transferability may also be applicable to experimental charge densities. Further studies to determine the charges and volumes of the atoms will be attempted to make definitive conclusions.

¹Chang, C. and Bader, R. F. W. (1992), J. Phys. Chem. 96, p. 1654-1662. ²Flensburg, C. et al. (1995), J. Phys. Chem. 99, p. 10130-10141. 3Stewart, R. F. and Flensburg, C. (1996), Personal Communication.