ChemInform Abstract: DEFORMATION ELECTRON DENSITY OF α‐GLYCYLGLYCINE AT 82 K. II. THE X‐RAY DIFFRACTION STUDY

Abstract

A study of the charge density in α‐glycylglycine, C4H8N2O3, has been carried out, employing x‐ray and neutron diffraction data measured at a temperature of 82 K. The crystal is monoclinic, space group P21/c, with a cell of the following dimensions a=8.025(2), b=9.543(4), c=7.788(2) A, and β=106.50(2) ° at 82 K. X‐ray data were collected in the sinϑ/λ range 0.00–1.07 A−1, with Nb filtered MoKα radiation. The deformation densities were computed by an X–N method using positional parameters for all atoms from a neutron diffraction study and thermal parameters for nonhydrogen atoms from a high‐order x‐ray refinement. In the case of hydrogen atoms, the neutron thermal parameters were scaled up by 15% as suggested by a comparison of thermal parameters for nonhydrogen atoms from the neutron and the high‐order x‐ray refinements. Well‐defined peaks of deformation charge density are found for all covalent bonds, and polarization of the density towards the more electronegative atom in the bonds can be noticed. The deformation densities in the four N–H⋅⋅⋅O hydrogen bonds are very similar, with excess charge in the N–H bonds and electron deficient regions on the sides of the hydrogen atoms pointing towards the acceptor oxygen atoms. There is also excess electron density close to the oxygen atoms in the direction of each hydrogen bond, but there does not appear to be any systematic correlation between the H...O distance in the hydrogen bonds and the peak heights in the ’’lone‐pair’’ regions around the oxygen atoms.