Mohamed Souhassou

Topological analysis of the electron density in hydrogen bonds

Topological analysis of the experimental electron density rho(r) in hydrogen-bonding regions has been carried out for a large number of organic compounds using different multipole models and techniques. Relevant systematic relationships between topological properties at the critical points and the usual geometric parameters are pointed out. Results involving X-ray data only and joint X-ray and neutron data, as well as special hydrogen bonding cases (symmetric, bifurcated, peptide bonds, etc.) are included and analysed in the same framework. A new classification of hydrogen bonds using the positive curvature of the electron density at the critical point [lambda(3)(r(CP))] is proposed.

Numerical computation of critical properties and atomic basins from three-dimensional grid electron densities

InteGriTy is a software package that performs topological analysis following the AIM (atoms in molecules) approach on electron densities given on three-dimensional grids. Tricubic interpolation is used to obtain the density, its gradient and the Hessian matrix at any required position. Critical points and integrated atomic properties have been derived from theoretical densities calculated for the compounds NaCl and TTF-(2,5)Cl(2)BQ (tetrathiafulvalene-2,5-dichlorobenzoquinone), thus covering the different kinds of chemical bonds: ionic, covalent, hydrogen bonds and other intermolecular contacts.

Topological analysis of experimental electron densities

Practical computing algorithms are described for analysing the topology of experimental electron density distributions represented as either three-dimensional grid densities or multipolar pseudoatom superpositions. The algorithms are implemented in the program NEWPROP, results from which are illustrated with applications to two N-acetyl, C-methylamide blocked amino acid crystal structures.

Electron density in ammonium dihydrogen phosphate: non-uniqueness of the multipolar model in simple inorganic structures

X-ray and neutron diffraction data of a single crystal of ammonium dihydrogen phosphate have been used for the determination of the electron density using multipolar expansion of the density around each nucleus. As the ammonium group was found to be nearly neutral from unconstrained multipole refinement, constrained refinements have been performed with the charge of the ammonium group ranging from zero to one. On the other hand, the expansion of the radial functions of the phosphorus atom was varied. All refinements led to almost the same agreement factors and residual densities. The consequences of such uncertainties on the topology of the electron density are discussed, namely the topology of the P-O bond critical point.

On the accurate estimation of intermolecular interactions and charge transfer: the case of TTF-CA

High-resolution X-ray diffraction experiments and state-of-the-art density functional theory calculations have been performed. The validity of the atoms-in-molecules approach is tested for the neutral-ionic transition of TTF-CA which involves a transfer of less than one electron between the donor and acceptor molecules. Foremost, crystallographical data have been reassessed along the temperature-induced neutral-ionic phase transition undergone by this charge transfer complex. Based on accurate X-ray structures at 105 and 15 K, topological analysis of both DFT and the experimental multipolar electron densities allowed detailed characterization of intra- and interstack intermolecular interactions. Direct quantification of the intermolecular charge transfer and the dipole moment are discussed.

Recovering experimental and theoretical electron densities in corundum using the multipolar model: IUCr Multipole Refinement Project

This electron-density study on corundum (alpha-Al2O3) is part of the Multipole Refinement Project supported by the IUCr Commission on Charge, Spin and Momentum Densities. For this purpose, eight different data sets (two experimental and six theoretical) were chosen from which the electron density was derived by multipolar refinement (using the MOLLY program). The two experimental data sets were collected on a conventional CAD4 and at ESRF, ID11 with a CCD detector, respectively. The theoretical data sets consist of static, dynamic, static noisy and dynamic noisy moduli of structure factors calculated at the Hartree-Fock (HF) and density functional theory (DFT) levels. Comparisons of deformation and residual densities show that the multipolar analysis works satisfactorily but also indicate some drawbacks in the refinement. Some solutions and improvements during the refinements are proposed like contraction or expansion of the inner atomic shells or increasing the order of the spherical harmonic expansion.

On the precision and accuracy of structural analysis of light-induced metastable states

Bragg diffraction data were collected on single crystals of the spin-crossover complex [Fe(phen)2(NCS)2] in its low-spin and light-induced metastable high-spin states. Experimental variables included the temperature (32 and 15 K), the X-ray source (sealed tube and synchrotron), and the time interval between laser light excitation of the sample (λ = 647 nm). From a comparison of the structural parameters refined, it is shown that photo-crystallographic measurements suffer significantly and systematically from bias if the probed sample contains residual ground-state species, resulting from an incomplete photo-conversion or a significant metastable- to ground-state relaxation. It follows that a 4% population of species in a different spin state affects the Fe—N bond lengths by more than three standard deviations, and the FeN6 polyhedron volume by as much as seven standard deviations, while the mean atomic position misfit exceeds 0.005 A.

Out-of-equilibrium charge density distribution of spin crossover complexes from steady-state photocrystallographic measurements : experimental methodology and results

Abstract The electron density distribution of a light-induced molecular excited state, i.e. the high spin metastable state of [Fe(phen)2(NCS)2], was determined from steady-state photocrystallographic measurements. We defined the experimental conditions under which the accuracy of the measured diffraction data is compatible with an electron density analysis. These include: (i) a large structural and electronic contrast between high spin (HS) and low spin (LS) states, (ii) an efficient photoconversion under light irradiation and (iii) slow relaxation of the HS metastable state. Multipolar modeling of the electron density yielded a deformation density and 3d-orbital populations for Fe(II) characteristic of a high spin (t2g4eg2) electron configuration and support the assumption of significant σ-donation and π-backbonding of the Fe—N interactions. The electron density distribution in the intermolecular regions confirms anisotropic intermolecular interactions with possibly a layer topology parallel to the orthorhombic (ab) plane, related to the system cooperativity.

Experimental electron density in crystalline H3PO4

X-ray diffraction data for H3PO4 crystals have been measured to dmin = 0.46 A resolution, and used to model the electron-density distribution with the hydrogen structure of the crystals adopted from an earlier neutron diffraction analysis. The molecule is asymmetric in the crystal with site symmetry 1 (C1), but the local symmetries of the pseudoatomic densities are, within experimental error, equivalent as they would be under idealized 3m (C3v) molecular symmetry. Although the experimental analysis entailed substantial problems with absorption and extinction corrections, the static deformation density from the experiment agrees very well with that from a polarized split-valence molecular orbital wavefunction for an isolated molecule with the crystallographic molecular geometry. Hydrogen bonding in the crystal polarizes the molecules P==O acceptor group towards P(+)--O-, and appears to relocalize the lone-pair density of the P--OH donor groups. Crystal data: anhydrous orthophosphoric acid, H3PO4, M(r) = 98.00, room temperature, P2(1)/c, a = 5.7572 (13), b = 4.8310 (17), c = 11.5743 (21) A, beta = 95.274 (12) degrees, V = 320.55 (25) A3, Z = 4, dx = 2.030 mg mm-3, mu = 0.660 mm-1 for lambda(Mo K alpha) = 0.7107 A, F(000) = 200 e-, R(parallel F) = 0.026 for 3512 unique reflections.

First spin-resolved electron distributions in crystals from combined polarized neutron and X-ray diffraction experiments

A method to map spin-resolved electron distribution from combined polarized neutron and X-ray diffraction is described and applied for the first time to a molecular magnet and it is shown that spin up density is 5% more contracted than spin down density.