Correlation between Accurate Electron Density and Linear Optical Properties in Amino Acid Derivatives: l‑Histidinium Hydrogen Oxalate

The accurate electron
density and linear optical properties of l-histidinium hydrogen
oxalate are discussed. Two high-resolution single crystal X-ray diffraction
experiments were performed and compared with density functional calculations
in the solid state as well as in the gas phase. The crystal packing
and the hydrogen bond network are accurately investigated using topological
analysis based on quantum theory of atoms in molecules, Hirshfeld
surface analysis, and electrostatic potential mapping. The refractive
indices are computed from couple perturbed Kohn–Sham calculations
and measured experimentally. Moreover, distributed atomic polarizabilities
are used to analyze the origin of the linear susceptibility in the
crystal, in order to separate molecular and intermolecular causes.
The optical properties are also correlated with the electron density
distribution. This compound also offers the possibility to test the electron density building block approach for material science
and different refinement schemes for accurate positions and displacement
parameters of hydrogen atoms, in the absence of neutron diffraction
data.