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.

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...

The total experimental electron density distributions rho(r) of zwitterionic L- and DL-alanine crystals, as derived from extensive sets of X-ray diffracted intensities collected at 23 and 19 K, are compared to gain an insight into the different physical properties of the two related chiral compounds in the solid state and to explore the extent of the rho(r) transferability. Relevant parameters that characterize the two crystal forms are obtained, showing differences and similarities in terms of (i) geometric descriptors, (ii) topological indexes, (iii) molecular electrostatic potential Phi(r) distributions, (iv) atomic volumes and charges, (v) molecular electric moments, and (vi) electrostatic interaction energies. To assess the relative stability of the racemate with respect to the pure enantiomer, the crystal lattice energies, as obtained through DFT fully periodic calculations, are also discussed and compared with the experimental sublimation enthalpies after correction for the proton-transfer energies. In-crystal group charges, evaluated with the quantum theory of atoms in molecules, are found to be transferable between the racemic and the pure enantiomer, at variance with group volumes. Similarly, molecular first and third moments are not strictly transferable and indicate that for the zwitterionic alanine molecule the molecular charge distribution in the DL-crystal is more polarized in the c direction by about 10%. By contrast, quantitative agreement is observed for second and fourth moments. Significant differences arise from (1) the crystal packing of the dipole vectors, which are aligned in an antiparallel fashion in the L-crystal, to be compared with a parallel alignment in the racemate, due the polar space group Pna21 of the latter, (2) the strongly attractive electrostatic energy of a homochiral pair in the L-crystal, which is opposed to the corresponding heterochiral pair in the DL-crystal form. The difference between these Ees values amounts to 135-150 kJ mol(-1). Despite this, the two crystal forms are predicted as equally thermodynamically favored by the theoretical P-B3LYP estimates of the crystal lattice energies. Finally, the necessity of an upgrading of the dispersion and exchange-repulsion terms currently adopted within the experimental charge density approach to intermolecular interactions is recognized and discussed.

An experimental study of the electron-density distribution rho(r) in an angiotensin II receptor antagonist 1 has been made on the basis of single-crystal X-ray diffraction data collected at a low temperature. The crystal structure of 1 consists of infinite ribbons in which molecules are connected by an N-H...N hydrogen bond and several interactions of the C-H...O, C-H...N, and C-H...S type. The molecular conformation, characterized by the syn orientation of a tetrazole and a pyrimidinone ring with respect to a phenyl spacer group, is stabilized by two short SO and SN intramolecular contacts between a substituted thiophene fragment and the other two heterocycles of 1. The electrostatic nature of these interactions is documented. Furthermore, the Laplacian of rho(r) in the plane defined by the sulfur, oxygen, and nitrogen atoms involved in these interactions shows their strongly directional character as the regions of charge concentration on the valence shell of the nitrogen and oxygen atoms directly face the regions of charge depletion on the valence shell of the sulfur atom. All the chemical bonds and the relevant intra- and intermolecular interactions of 1 have been quantitatively described by the topological analysis of rho(r). Simple relationships between the bond path lengths (R(b)) and the values of rho at the bond critical points (rho(bcp)) have been obtained for the 28 C-C bonds, the seven N-C bonds, and the four O-C bonds. For the first two classes of bonds the relationship is in the form of a straight line, whose parameters, for the C-C bonds, agree, within experimental uncertainty, with those previously derived in our laboratory from a 19 K X-ray diffraction study of crystals of a different compound. Maps of the molecular electrostatic potential phi(r) derived from the experimental charge density display features that are important for the drug-receptor recognition of 1.

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