Use of Quantum Theory of Atoms in Molecules in the Search for Appropriate Hydrogen Atom Locations in X-ray Diffraction Based Studies

A novel approach for the determination of hydrogen positions in studies based on X-ray diffraction is presented. The comparison of proton positions with their corresponding electron density maxima has been performed by means of the Quantum Theory of Atoms in Molecules. The observed differences mainly depend on the type and electronegativity of the atom covalently bonded to the hydrogen, changing in the range of Delta r = 0.004 divided by 0.032 angstrom, while being simultaneously independent from the involvement of atom in the hydrogen bonding. It is postulated that the Delta r correction should be taken into account in electron density based studies. In theoretical single point calculations (sp) based on X-ray diffraction analysis, an elongation of hydrogen covalent bonds with Delta r corrections instead of neutronographic values is proposed. This procedure utilized in this work for the analysis of intramolecular O-H center dot center dot center dot O hydrogen bonding properties leads to good agreement between quantum theoretical results for the optimized structure and X-ray based single point calculations.