Modeling of the nuclear parameters for H atoms in X-ray charge-density studies.

Extensive and precise X-ray diffraction data for xylitol have been used to test different approaches to estimate nuclear parameters for H atoms in charge-density studies. The parameters from a neutron diffraction study of the same compound were taken as a reference. The resulting static charge densities obtained for the different approaches based on a multipole model were subjected to a topological analysis. The comparative analysis led to the following results. The procedure of extending the X-H bond to match bond lengths from neutron diffraction studies provides the best agreement with the neutron positional parameters. An isotropic model for the atomic displacements of H atoms is highly unsatisfactory and leads to significant deviations for the properties of the bond critical points including those that only involve non-H atoms. Anisotropic displacement parameters for H atoms can be derived from the X-ray data that are in agreement with the values from the neutron study, and the resulting charge-density models are in good agreement with the reference model. The anisotropic displacement parameters for H atoms are derived from the X-ray data as a sum of the external (rigid-body) and internal vibrations. The external vibrations are obtained from a TLS analysis of the ADPs of the non-H atoms and the internal vibrations from analysis of neutron diffraction studies of related compounds. The results from the analysis of positional and thermal parameters were combined to devise a 'best anisotropic' model, which was employed for three other systems where X-ray and neutron data were available. The results from the topological analysis of these systems confirm the success of the 'best anisotropic' model in providing parameters for the H atoms that give charge densities in agreement with the reference models based on H-atom parameters derived from neutron diffraction.

[1]  R. Mathiesen,et al.  The charge density of urea from synchrotron diffraction data. , 2004, Acta crystallographica. Section A, Foundations of crystallography.

[2]  P. Roversi,et al.  Approximate anisotropic displacement parameters for H atoms in molecular crystals , 2004 .

[3]  P Coppens,et al.  Density-optimized radial exponents for X-ray charge-density refinement from ab initio crystal calculations. , 2001, Acta crystallographica. Section A, Foundations of crystallography.

[4]  Burgi,et al.  Dynamics of molecules in crystals from multi-temperature anisotropic displacement parameters. II. Application To benzene (C6D6) and urea , 2000, Acta crystallographica. Section A, Foundations of crystallography.

[5]  Burgi,et al.  Dynamics of molecules in crystals from multi-temperature anisotropic displacement parameters. I. theory , 2000, Acta crystallographica. Section A, Foundations of crystallography.

[6]  Lenstra,et al.  Systematic intensity errors and model imperfection as the consequence of spectral truncation , 2000, Acta crystallographica. Section A, Foundations of crystallography.

[7]  M. Spackman.  Hydrogen bond energetics from topological analysis of experimental electron densities: Recognising the importance of the promolecule , 1999 .

[8]  S. Larsen,et al.  Properties of the Experimental Crystal Charge Density of Methylammonium Hydrogen Maleate. A Salt with a Very Short Intramolecular O−H−O Hydrogen Bond , 1998 .

[9]  P. Luger,et al.  TOPOLOGICAL ANALYSIS OF THE EXPERIMENTAL ELECTRON DENSITIES OF AMINO ACIDS. 1. D, L-ASPARTIC ACID AT 20 K , 1998 .

[10]  P. Roversi,et al.  Charge density in crystalline citrinin from X-ray diffraction at 19 K , 1996 .

[11]  R. Stewart,et al.  EXPERIMENTAL CHARGE DENSITY STUDY OF METHYLAMMONIUM HYDROGEN SUCCINATE MONOHYDRATE. A SALT WITH A VERY SHORT O-H-O HYDROGEN BOND , 1995 .

[12]  R. Stewart,et al.  Shell population and κ refinements with canonical and density-localized scattering factors in analytical form , 1984 .

[13]  H. Flack,et al.  On enantiomorph‐polarity estimation , 1983 .

[14]  F. Escudero,et al.  Atoms in molecules , 1982 .

[15]  R. Stewart.  Least-squares estimates of equivalent isotropic temperature factors: an application to H atoms in crystalline sucrose , 1976 .

[16]  F. L. Hirshfeld.  Can X‐ray data distinguish bonding effects from vibrational smearing? , 1976 .

[17]  J. Pople,et al.  Self‐Consistent Molecular‐Orbital Methods. I. Use of Gaussian Expansions of Slater‐Type Atomic Orbitals , 1969 .

[18]  William T. Simpson,et al.  Coherent X‐Ray Scattering for the Hydrogen Atom in the Hydrogen Molecule , 1965 .

[19]  Alfredsson,et al.  Influence of intermolecular interactions on multipole-refined electron densities. , 1999, Acta crystallographica. Section A, Foundations of crystallography.

[20]  G. A. Jeffrey,et al.  The Application of Charge Density Research to Chemistry and Drug Design , 1991 .

[21]  Y. W. Yang,et al.  Net atomic charges and molecular dipole moments from spherical‐atom X‐ray refinements, and the relation between atomic charge and shape , 1979 .