Electron density study of a new non-linear optical material: L-arginine phosphate monohydrate (LAP). Comparison between X-X and X-(X+N) refinements

The crystal structure, thermal vibrations and electron density of L-arginine phosphate monohydrate (formally C6H15N4O2+.H2PO4-.H2O) have been analysed using 130 K single-crystal X-ray diffraction data to a resolution of (sin theta/lambda)(max) = 1.20 Angstrom(-1). A multipolar pseudo-atom density model was fitted against the 6805 observed data with I > 3 sigma(I), [R(F) = 0.016, R(w)(F) = 0.014, S = 1.39] in order to map the static valence-electron density distribution. Positional and thermal vibration parameters for H atoms were taken from neutron diffraction results. A comparison between the electron density rho(r), del(2) rho(r) and the electrostatic potential calculated from X-X and X-(X + N) refinements shows that reliable results may be obtained from X-X data only.