From a spherical crystal of L-alanine an extensive set (20Mo < 100°) of X-ray diffraction data has been measured at 23 (1) K. No phase transition has been observed on cooling, and the cell parameters of the orthorhomt ic crystals (space group />2,2121) are, at the temperature of data collection, a = 5.928 (1) k, b = 12.260 (2) A, c = 5.794 (1) A. The contraction of the cell volume, with respect to the room temperature value, amounts to 2.2%. Intensity data, carefully corrected for scan-truncation losses, have been analyzed with various models. The best fit was reached by interpreting the electron density function in terms of pseudoatoms (multipoles). The analysis of the correlation coefficients has shown that the quality and quantity of the data set allowed a reliable deconvolution of thermal motion from the charge density. Precise and accurate geometry, as well as experimental deformation maps of both charge density and electrostatic potential, has been obtained. Ab initio calculations of atomic populations and group charges for the zwitterionic amino acid have been performed with various basis sets, up to the 6-31G** level. Derived results are compared with those of the multipole analysis of the X-ray diffraction data.

The electrostatic properties of crystals of α-glycine have been obtained from extensive X-ray diffraction data collected at approximately 23 K and carefully processed, including corrections for scan truncation losses, anisotropic extinction, and multiple reflection. From a multipole parameterization of the X-ray intensities we have obtained an unusually preciseand we are confident, accuratemodel of the total electron distribution in the crystal including the topological features, atom and group charges, the dipole moment for the glycine zwitterion, electrostatic potentials, electric field gradients at the nucleii of the three hydrogen atoms of the ammonium group, and intermolecular electrostatic energies within the crystal. We have also calculated the total interaction energies involving the six distinct types of intermolecular pairings and examined these energies in terms of the molecular arrangement.

The CH··O contacts in the 3,4-bis(dimethylamino)-3-cyclobutene-1,2-dione (DMACB) crystal have been characterized through a topological analysis of its experimental and theoretical densities, derived from a multipole refinement of X-ray diffraction data and from periodic Hartree−Fock calculations, respectively. The existence or the lack of an H··O bond critical pointthat is a point through the two nuclei where the gradient of the electron density vanishesallows us to distinguish between bonded and non bonded CH··O contacts, regardless of the value of their H··O separation. The 23 unique bonded contacts in DMACB are characterized by a large and nearly constant (∼140°) C−H−O angle, denoting the importance of the electrostatic energy contribution to such interactions. Instead, the nonbonded ones (four unique for H··O separations up to 3.0 Å) are more bent and may even be folded down to 90°, since their dominant van der Waals contribution to the interaction energy is independent of the C−H−O angle. The CH··O angular distribution observed for H··O separations greater than 2.7 Å is only apparently isotropic, since such isotropy clearly disappears when the bonded and nonbonded contacts are identified and their angular distributions separately analyzed. The Koch and Popelier criteria (J. Phys. Chem. 1995, 99, 9747) to establish H-bonds are, for the first time, applied in their entirety to a large set of CH··O contacts in a crystalline phase. The criteria are always satisfied by all of the bonded intermolecular CH··O contacts, with a single exception concerning one long bond and one of the six criteria only. The expressions proposed by Espinosa et al. (Chem. Phys. Lett. 1998, 285, 170), relating the potential energy densities at the critical point to the H-bond strengths, fail when applied to the weak CH··O interactions present in the DMACB crystal. The reasons for such a failure are outlined and new relationships are proposed. The importance of the promolecular charge distributions in defining topological properties of interest to the CH··O bonds is investigated. The criticism raised by Spackman (Chem. Phys. Lett. 1999, 301, 425) as to the lack of additional information provided by the experimental results to the description of such weak interactions is discussed. It is shown that the promolecular model yields significantly different electron density values at the critical point and in some instances even different topologies, compared to the corresponding multipole or theoretical densities. On the other hand, when the electron density topologies are the same, the values obtained from either electron density for the potential or kinetic energy density at the critical point, are very much alike.

Anisotropic displacement parameters (ADPs) are compared for H atoms estimated using three recently described procedures, both among themselves and with neutron diffraction results. The results convincingly demonstrate that all methods are capable of giving excellent results for several benchmark systems and identify systematic discrepancies for several atom types. A revised and extended library of internal H-atom mean-square displacements is presented for use with Madsen's SHADE web server [J. Appl. Cryst. (2006), 39, 757-758; http://shade.ki.ku.dk], and the improvement over the original SHADE results is substantial, suggesting that this is now the most readily and widely applicable of the three approximate procedures. Using this new library--SHADE2--it is shown that, in line with expectations, a segmented rigid-body description of the heavy atoms yields only a small improvement in the agreement with neutron results. The SHADE2 library, now incorporated in the SHADE web server, is recommended as a routine procedure for deriving estimates of H-atom ADPs suitable for use in charge-density studies on molecular crystals, and its widespread use should reveal remaining deficiencies and perhaps overcome the inherent bias in the majority of such studies.

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The degree of precision and accuracy in molecular geometry attainable with modem X-ray diffraction and cryogenic techniques is documented by presenting the results of a low-temperature study of syn-l,6:8,13 biscarbonyl[ 14]annulene, tricycio[8.4.1.13,8 ]hexadeca-2,4,6,8,10,12,14-heptaene-15,16-dione. Intensity data up to sin0/2 = 1.14,~, -I have been measured from a spherical crystal at T --19 K. Their multipolar analysis, up to the octopole level for C and O atoms, and to quadrupoles for anisotropically described H atoms, has led to precisions in the coordinates of the heavy atoms which are better than 0.0004,~,. Root mean square (r.m.s.) amplitudes of vibration for C and O atoms have been determined to a precision of ca 0.0006 ,~. Based on the least-squares or's, uncertainties for the C--C and C--O bond lengths are 0.0005 ,~, and those of the C--H bond distances 0.006--0.007 ,A,. The topological properties of the experimentally derived charge density of the annulene molecule in the crystal have been determined by evaluating the location and nature of its critical points (points where Vp = 0). Contour maps of ,Oex p and its negative Laplacian (--VZ,Oexp) are presented. The occurrence of a bond critical point midway between the two C atoms of the carbonyl bridges seems to indicate the existence of a chemically unexpected weak bond between the two C atoms.

For the fungal metabolite citrinin, CI3Hl4O5, the total experimental electron distribution p(r) and its Laplacian V2p(r) have been obtained from an extensive set (36 564 measurements) of single-crystal X-ray diffracted intensities at a temperature of 19 f 2 K. Relevant steps in data collection and processing are reported. The resulting 7698 independent intensity data have been analysed with a multipole (pseudoatoms) formalism. The topological properties of p(r) have been determined according to the quantum theory of atoms in molecules. CC and CO bond path lengths have been obtained by numerical integration; their values are found to be well correlated with those of the electron density at the bond critical points. Topological features have been used to characterize the extension of the conjugated system of the molecule, and to confirm the stability of its rings, particularly the two formed by intramolecular H bonds. Maps of V2p(r) are presented, showing details in the valence charge distribution and providing a very sensitive tool for analysing dependence of the density on the model adopted to interpret X-ray data. The known chemical reactivity of the molecule towards nucleophiles at a csp2 atom is confirmed by the shape of the molecular reactive surface (the zero envelope of V2p(r)).Key words: experimental electron density, low-temperature X-ray diffraction, topological analysis, Laplacian of p.Resume : En se basant sur un ensemble extensif (36 564 mesures) d'intensitks de diffraction des rayons X par un cristal unique obtenues i une tempkrature de 19 + 2 K, on a obtenu la distribution expkrimentale totale, p(r), ainsi que son laplacien, v2p(r), pour le mCtabolite de champignon citrinine, CI,H,,O5. On a ensuite analyst les 7698 donnCes indkpendantes d'intensitt k I'aide du formalisme multipble (pseudoatomes). On a determink les propriCtCs topologiques de p(r) suivant la thCorie quantique des atomes dans les molCcules. On a dCterminC les longueurs des liaisons CC et CO par leur inttgration numCrique; on a trouvt que leurs valeurs donnent une bonne corrilation avec celles obtenues i partir de la densite Clectronique des points critiques de liaison. On a utilisC des caractkristiques topologiques pour caractkriser l'extension du systtme conjuguC de la molCcule et pour confirmer la stabilitC de ses noyaux, particulibrement les deux formCs par les liaisons hydrogbnes intramoliculaires. On prisente des cartes de VZp(r) qui montrent les details de la distribution de charge de valence et qui fournissent un outil trbs sensible pour analyser la dtpendance de la densitC sur le modble adopt6 pour interpriter les donnCes de diffraction des rayons X. En se basant sur la forme de la surface molCculaire rCactive (l'enveloppe zCro du V2p(r)), on a confirm6 la rCactivitC connue de la molCcule vis-i-vis des nuclCophiles au niveau d'un atome de carbone sp2.Mots clis : densit6 klectronique expkrimentale, diffraction des rayons X ? I basse temptrature, diffraction, analyse topologique, laplacien de p.[Traduit par la rtdacti...

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