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

Abstract

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 θ/λ)max = 1.20 Å−1. A multipolar pseudo-atom density model was fitted against the 6805 observed data with I > 3σ(I), [R(F) = 0.016,Rw (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 ρ(r), ∇2 ρ(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.

Transferability of Multipole Charge-Density Parameters: Application to Very High Resolution Oligopeptide and Protein Structures

“…At present, the database includes results from ultra high resolution (d`0.45 A Ê ) studies of the amino-acid and peptide crystals: Leu-enkephaline trihydrate Tyr-GlyGly-Phe-Leu (Wiest et al, 1994;Pichon-Pesme et al, 1992), N-acetyl l-tryptophane methylamide (Souhassou et al, 1991), N-acetyl-,-dehydro-phenylalanine methylamide , l-arginine phosphate (Espinosa et al, 1996), Tyr-Gly-Gly (Lachekar, 1997), Gly-Asp (Lachekar, 1997), Gly-Gly-Gly (PichonPesme & Lecomte, 1998) and l-histidine phosphate (Mata et al, 1997).…”

Topological analysis of the electron density in hydrogen bonds

“…Therefore, Table 3. Topological properties at the CP of some pure covalent (H 2 , B 2 , N 2 , O 2 ) and some pure ionic (LiCl, NaCl, NaF, KF) interatomic interactions (Bader, 1990), as well as the XÐH (X = C, N, O) covalent bonds observed in LAP (Espinosa et al, 1996;Espinosa, 1994), two symmetrical OÐHÐO interactions (Boukhris, 1995;Flensburg et al, 1995) and the HÁ Á ÁO HBs used in this study…”

“…To demonstrate a tentative classi®cation, Table 3 shows the topological properties at the CP of some pure covalent (H 2 , B 2 , N 2 , O 2 ) and ionic (LiCl, NaCl, NaF, KF) interactions (Bader, 1990), as well as those of the XÐH (X = C, N, O) covalent bonds in LAP (Espinosa et al, 1996). In the topological analysis of &(r), Bader (1990) classi®es covalent and ionic interactions as shared and closed- ) are 1.9 and 2.6, 0.5 and 1.1, and 1.6 and 1.5 for the ®rst and second equations in (a), (b) and (c), respectively.…”

“…Often, the Hirshfeld rigid-bond test (Hirshfeld, 1976) applied to neutron results fails where high-order X-ray and multipolar re®nements succeed. However, in many cases, a satisfactory description can be derived (Espinosa et al, 1996), especially when it involves positional and displacement parameters describing H atoms only. Thus, it is not surprising to ®nd many more X À (X + N) than X À N studies in the literature.…”

“…However, if we want to obtain the most accurate model of &(r), X À N and X À (X + N) methods are, in principle, methodologically better. Both include neutron information before the re®nement of the electron density parameters: in the X À N method all atomic positions and displacement parameters are obtained from neutron data and are kept ®xed in the multipolar re®nement (Coppens, 1967); in the X À (X + N) method only part of the information derived from neutron data (mainly for the H atoms) is included and not re®ned (Espinosa et al, 1996;Espinosa, 1994). Because thermal diffuse scattering (TDS) and extinction effects are different in X-ray and neutron experiments, the usual problem of the X À N method is the description of thermal smearing, which does not match very well with high-order X-ray results (Blessing, 1995).…”