Topological properties of hydrogen bonds: charge-density studies by the maximum entropy method

Abstract

The Maximum Entropy Method (MEM) is applied to lowtemperature single crystal x-ray diffraction data of amino acids and peptides. The resulting electron density maps are analyzed according to Bader’s Atoms in Molecules theory (Bader, 1994) towards the determination of the electron density and its Laplacian at bond critical points (BCPs). Energy densities at BCPs are calculated according to Abramov (1997), employing the local virial theorem. Geometrical, topological and energetic properties of 52 hydrogen bonds are studied. The electron densities, their principal curvatures and the energy densities at BCPs are found to depend systematically on the distance between hydrogen and acceptor atom. These relations appear different from the corresponding relations as described by Espinosa et al. (1999) for multipole densities. MEM electron densities at the BCPs of hydrogen bonds are higher than the multipole densities. The difference between MEM and multipole densities is more pronounced for stronger hydrogen bonds. The contribution of the prior density (overlapping spherical atomic electron densities) to the MEM density is studied. The prior density seems to contribute a large part to topological and energetic properties at BCPs of hydrogen bonds. However, the properties at BCPs of covalent bonds of MEM densities differ significantly from the prior. Thus, for research into chemical bonds an experimental electron density should not be substituted by a promolecule. References Abramov, Yu. A. (1997) Acta Cryst. A 53, 264. Bader, R. F. W. (1994): Atoms in Molecules. Oxford University Press. Espinosa, E., Souhassou, M., Lachekar, H. & Lecomte, C. (1999) Acta Cryst. B 55, 563.