1,572 research outputs found
Molecules and molecular crystals
No full textChapter 8: Molecules and molecular crystals (G. Gilli and P. Gilli);
INDEX:
8.1 Chemistry and X-ray crystallography 592;
8.1.1 Crystal and molecular structure 592;
8.1.2 The growth of structural information 594;
8.2 The nature of molecular crystals 595;
8.2.1 Intermolecular forces 595;
8.2.2 Thermodynamics of molecular crystals 616;
8.2.3 Free and lattice energy of a crystal from atom–atom potentials 619;
8.2.4 Polymorphism 622;
8.2.5 The prediction of crystal structures 623;
8.3 Elements of classical stereochemistry 627;
8.3.1 Structure: constitution, configuration, and conformation 627;
8.3.2 Isomerism 629;
8.3.3 Ring conformations 634;
8.4 Molecular structure and chemical bond 642;
8.4.1 Introduction 642;
8.4.2 Quantum-mechanical methods 643;
8.4.3 Qualitative bonding theories 645;
8.4.4 The VSEPR theory 647;
8.4.5 The VB theory 649;
8.4.6 Molecular mechanics (MM) 651;
8.4.7 Molecular mechanics, force fields, and molecular simulation (MS) 654 ;
8.5 Molecular hermeneutics: the interpretation of molecular structures 657;
8.5.1 Correlation methods in structural analysis 657;
8.5.2 Some three-centre–four-electron linear systems 659;
8.5.3 Nucleophilic addition to organometallic compound 661;
8.5.4 Nucleophilic addition to the carbonyl group 662;
8.5.5 Conformational rearrangements by structure-correlation methods 664;
8.5.6 Evidence for resonance-assisted H-bond (RAHB) by structure-correlation methods 669;
References 67
Conclusions. L'ancient et le nouveau: remarques sur l'historiographie des villes du Midi
No full textLa singularité urbaine des villes du Midi français a souvent été négligée. Coincées entre le modèle communal italien triomphant et le puissant mouvement urbain des villes flamandes ou rhénanes, elles apparaissent comme faiblement typées ; leur intégration dans l’espace capétien aurait également limité leur capacité d’autogouvernement et de discussions avec les autorités centrales. Pourtant, le dynamisme de ces cités méridionales est avéré, aussi bien dans leur élan démographique que dans la force des échanges économiques. Que dire également de leur place dans le développement des mouvements religieux contestaires et de leur rayonnement intellectuel facilité par la présence de grandes universités au recrutement international ! À la lumière des nombreux et récents travaux sur le sujet, le colloque offre la première approche comparative sur ces villes méridionales, en insistant sur les traits identitaires qui définissent leurs contours originaux, en particulier dans l’organisation politique
Introduction. L'autonomie et l'identité de la ville: une question sociale?
No full textLa singularité urbaine des villes du Midi français a souvent été négligée. Coincées entre le modèle communal italien triomphant et le puissant mouvement urbain des villes flamandes ou rhénanes, elles apparaissent comme faiblement typées ; leur intégration dans l’espace capétien aurait également limité leur capacité d’autogouvernement et de discussions avec les autorités centrales. Pourtant, le dynamisme de ces cités méridionales est avéré, aussi bien dans leur élan démographique que dans la force des échanges économiques. Que dire également de leur place dans le développement des mouvements religieux contestaires et de leur rayonnement intellectuel facilité par la présence de grandes universités au recrutement international ! À la lumière des nombreux et récents travaux sur le sujet, le colloque offre la première approche comparative sur ces villes méridionales, en insistant sur les traits identitaires qui définissent leurs contours originaux, en particulier dans l’organisation politique
Evidence for Intramolecular N-H...O Resonance-Asssisted Hydrogen Bonding in Beta-Enaminones and Related Heterodienes. A Combined Crystal-Structures, IR and NMR Spectroscopic, and Quantum-Mechanical Investigation
No full textThe resonance-assisted hydrogen bond (RAHB) is a model of synergistic interplay between pi -delocalization and hydrogen-bond (H-bond) strengthening originally introduced (Gilli, G.; Bellucci, F.; Ferretti, V.; Bertolasi, V. J. Am. Chem. Sec. 1989, 111, 1023; Bertolasi, V.; Gilli, P.; Ferretti, V.1 Gilli, G. J. Aln. Chem. Sec. 1991, 113, 4917) for explaining the abnormally strong intramolecular O-H...O bonds formed by the ...O=C-C=C-OH... beta -enolone fragment I which are typical of B-diketone enols. The applicability of this model to the intramolecular N-H...O hydrogen bonds formed by a number of heteroconjugated systems (...O=C-C=C-NH..., beta -enaminones II; O=C-C=N-NH..., ketohydrazones III; and ...O=N-C=C-NH..., nitrosoenamines IV) is investigated. The X-ray crystal structures of five molecules which close a six-membered ring by an intramolecular N-H...O bond through the resonant ...O=X-C=X-NH... (X = C, N) fragments II-IV are compared to those of two other molecules closing the same ring through the nonresonant ...O=C-C-C-NH... beta -aminone moiety V. Experimental findings are complemented by a CSD (Cambridge Structural Database) search of all compounds forming intramolecular N-H...O bonds through the molecular fragments II-V and by a comprehensive analysis of the IR v(NH) stretching frequencies and H-1 NMR delta (NH) chemical shifts available for compounds of these classes of known crystal structure. It is shown that all the descriptors of H-bond strength [d(N...O) shorthening, decrease of v(NH), increase of delta (NH), and increase of pi -delocalization within the heteroconjugated fragment] are mutually intercorrelated according to RAHB rules, which can then account for the strength of heteronuclear N-H...O bonds in II-IV as well as for that of the homonuclear O-H...O bonds in I. Heteronuclear N-H...O bonds appear, however, to have distinctive features. In particular, their strength turns out to be partially hampered by the proton affinity difference (BPA) between the N and O atoms, so that very strong H-bonds (2.65 greater than or equal to d(N...O) greater than or equal to 2.48 Angstrom, 3200 greater than or equal to v(NH) greater than or equal to 2340 cm(-1), 13 less than or equal to delta (NH) less than or equal to 18 ppm) can occur only when the pi -delocalization of the heterodienic moiety is associated with proper electron-attracting substituents which are able to decrease this Delta PA by increasing the NH acidity. Moreover, at variance with strong O-H...O RAHBs, whose protons are mostly found in nearly symmetrical positions, even the strongest N-H...O RAHBs are highly dissymmetric, despite the very similar changes undergone by both IR and H-1 NMR spectra in O-H...O and N-H...O H-bonded systems. Specificities of heteronuclear H-bonds are shown to be interpretable by the electrostatic-covalent H-bond model (ECHBM) which was previously developed for the homonuclear case (Gilli, P.; Bertolasi, V.; Ferretti, V.; Gilli, C. J. Am. Chem. Sec. 1994, 116, 909). The conclusions drawn are corroborated by extended DFT quantum-mechanical calculations at the B3LYP/6-31+G(d,p)B3LYP/6-31+G(d,p) level of theory and by full geometry optimization carried out on 27 variously substituted heterodienes II-IV and nonresonant beta -aminones v. calculations allow the estimation of H-bond energies that are found to be approximately 2.75 kcal mol(-1) for nonresonant V and 5.22, 6.12, and 7.03 kcal mol(-1) for unsubstituted resonant II, III, and IV, respectively
Noncovalent Interactions in Crystals
No full textNoncovalent Interactions in Crystals (P. Gilli and G. Gilli); in: Supramolecular Chemistry: from Molecules to Nanomaterials (J.W. Steed and P.A. Gale eds.) - Volume 6: Supramolecular Materials Chemistry
ABSTRACT:
The over 500 000 structures collected in current crystallographic databases represent the greatest archive of noncovalent molecular interactions ever conceived by man. Their analysis provides an invaluable basis for understanding these interactions in the crystalline state and for transferring this knowledge to gas phase and condensed phases, such as pure liquids or solutions in polar and nonpolar solvents. This chapter is intended to review the different classes of noncovalent interactions and to supply the mathematical background for their description.
For the sake of clarity, the treatment distinguishes between physical and chemical interactions. Physical interactions are considered essentially independent of molecular constitution and deriving from (i) van der Waals forces (atomic repulsion/exchange and attraction/dispersion terms); (ii) electrostatic multipolar forces (mostly monopolar and dipolar terms); and (iii) hydrophobic forces, a kind of interaction that develops in crystal clathrates and water solutions. Conversely, interactions of chemical nature are strictly related to the physicochemical properties of molecules with particular concern for (iv) groups that are either Bronsted acids (proton donors, D–H) or Bronsted bases (proton acceptors, :A) and may interact by forming D–H· · ·:A hydrogen bonds and (v) groups that are either Lewis bases (electron donors, D:) or Lewis acids (electron acceptors, :A) and may interact by forming D:-->A electron donor–acceptor (EDA) or charge-transfer (CT) interactions.
Special emphasis is given to interactions that play a determinant structure-directing role in molecular interaction and recognition phenomena, such as hydrogen and halogen bonding.
INDEX:
1 Introduction;
1.1 The birth of structural chemistry;
2 A Chemical Classification of Crystals;
2.1 Chemical forces in crystals;
2.2 Metallic crystals;
2.3 Covalent crystals;
2.4 Ionic crystals;
2.5 Molecular crystals;
3 Nonbonded Forces in Molecular Crystals. A Classification;
4 Mostly Physical Intermolecular Forces in Crystals;
4.1 vdW nonbonded forces;
4.2 Electrostatic multipolar forces;
4.3 Hydrophobic forces;
5 Mostly Chemical Intermolecular Forces in Crystals;
5.1 Charge-transfer (CT) or electron donor–acceptor (EDA) interactions;
5.2 Hydrogen bond (H-bond);
6 Conclusions;
Reference
Equilibrium analysis of cellular neural networks
No full textCellular neural networks are dynamical systems, described by a large set of coupled nonlinear differential equations. The equilibrium point analysis is an important step for understanding the global dynamics and for providing design rules. We yield a set of sufficient conditions (and a simple algorithm for checking them) ensuring the existence of at least one stable equilibrium point. Such conditions give rise to simple constraints, that extend the class of CNN, for which the existence of a stable equilibrium point is rigorously proved. In addition, they are suitable for design and easy to check, because they are directly expressed in term of the template elements
Out-of-plane deformation pathways of the R(X=)C-NR2 fragment present in amides, thioamides, amidines, enamines, and anilines. A concerted study making use of structural data, molecular mechanics, and ab initio calculations
No full textThe R(X=)C-NR(1)R(2) (R(1), R(2) = alkyl groups) fragment is present in many classes of molecules and assumes, usually, a planar conformation owing to the C-N partial double-bond character. It can undergo,however, a cis-trans isomerization process by rotation around the C-N bond and concomitant nitrogen pyramidalization. In a previous paper (Gilli, G.; Bertolasi, V.; Bellucci, F.; Ferretti, V. J. Am. Chem. Sec. 1986, 108, 2420) the isomerization pathway was mapped by the use of some 90 crystal structures containing the fragment of interest, and a semiempirical potential giving the total energy of the fragment during its deformation was proposed. In the present work the previous sample of crystal structures is updated to the current state of the crystallographic databases; the observed geometries are compared with the out-of-plane deformation energy maps obtained by ab initio SCF calculations at the 4-31G level for sample molecules (thioformamide, formamide, formamidine, vinylamine, and aniline) representative of the five chemical classes investigated. It is shown that the originally proposed potential is validated by this analysis and that the values of the energetic barriers involved in the reaction, evaluated from the ab initio energy maps, are in good agreement with the available experimental data. It is found, moreover, that there is a specific class of compounds (o- and p-nitroanilines and polyconjugated enamines and amidines) which deviate from the general behavior. The crystal structures of two of these compounds are reported, and possible reasons for the discrepancies discussed
Towards an unified hydrogen-bond theory
No full textThough the hydrogen bond (H-bond) is known since 1920, all attempts to predict its geometry and energetics from the structure of the interacting molecules have been so far unsuccessful. This problem is addressed here through the Electrostatic-Covalent H-Bond Model (ECHBM) derived from the systematic analysis of structural and spectroscopic data of a large number of O-H ... O H-bonds, according to which: (i) weak II-bonds are electrostatic in nature but become increasingly covalent with increasing strength, very strong bonds being essentially three-centre-four-electron covalent bonds; (ii) strong H-bonds belong to a limited number of classes (three for X-H ... X homonuclear and four for X-H ... Y heteronuclear II-bond); and (iii) within each class, H-bonds are the stronger the smaller is Delta PA, the difference between the proton affinities of the I-I-bond donor and acceptor atoms. It is shown that this model leads to a thorough classification of II-bonds in chemical classes which, in turn, can be used to predict the H-bond strength from the simple knowledge of the chemical formula. A number of applications to homonuclear O-H ... O and heteronuclear N-H ... O/O-H ... N H-bonds is illustrated by both systematic analysis of crystal structure data and DFT theoretical calculations at the B3LYP/6-31 + G(d,p)//B3LYP/6-31 + G(d,p) level of theory
The Nature of Solid-State N-H...O/O-H...N tautomeric Competition in Resonant Systems. Intramolecular Proton Transfer in Low-Barrier Hydrogen Bonds Formed by the ...O=C-C=N-NH...<-/->...HO-C=C-N=N... Ketohydrazone-Azoenol System. A Variable Temperature X-ray Crystallographic and DFT Computational Study
No full textThe tautomeric ...O=C-C=N-NH... ...HO-C=C-N=N... ketohydrazone-azoenol system may form strong N-H...O/O-H...N intramolecular resonance-assisted H-bonds (RAHBs) which are sometimes of the low-barrier H-bond type (LBHB) with dynamic exchange of the proton in the solid state. The problem of the N-H...O/O-H...N competition in these compounds is studied here through variable temperature (100, 150, 200, and 295 K) crystal-structure determination of pF = 1-(4-F-phenylazo)2-naphthol and oF = 1-(2-F-phenylazo)2-naphthol, two molecules that, on the ground of previous studies (Gilli, P; Bertolasi, V.; Ferretti, V.; Gilli, G. J. Am. Chem. Soc. 2000, 122, 10405), were expected to represent an almost perfect balance of the two tautomers. According to predictions, the two molecules form remarkably strong bonds (d(N...O) = 2.53-2.55 Angstrom) of double-minimum or LBHB type with dynamic N-H...O/O-H...N exchange in the solid state. The enthalpy differences between the two minima, as measured by van't Hoff methods from the X-ray-determined proton populations, are very small and amount to DeltaH° = -0.120 and DeltaH° = -0.156 kcal mol(-1) in favor of the N-H...O form for pF and oF, respectively. Successive emulation of pF by DFT methods at the B3LYP/6-31+G(d,p)// B3LYP/6-31+G(d,p) level has shown that both energetic and geometric experimental aspects can be almost perfectly reproduced. Generalization of these results was sought by performing DFT calculations at the same level of theory along the complete proton-transfer (PT) pathway for five test molecules designed in such a way that the RAHB formed changes smoothly from weak N-H...O to strong O-H... through very strong N-H...O/O-H...N bond of LBHB type. A systematic correlation analysis of H-bond energies, H-bond and pi-conjugated fragment geometries, and H-bond Bader's AIM topological properties performed along the PT-pathways leads to the following conclusions: (a) any X-H...Y H-bonded system is fully characterized by its intrinsic PT-barrier, that is, the symmetric barrier occurring when the proton affinities of X and Y are identical; (b) the intrinsic X-H...Y bond associated with the symmetric barrier is the strongest possible bond in that system and will be single-minimum (single-well, no-barrier) or double-minimum (double-well, low-barrier) according to whether the intrinsic PT-barrier is lower or slightly higher than the zero-point vibrational level of the proton; (c) with reference to the intrinsic H-bond, the effect of chemical substitution can only be that of making more and more dissymmetric the PT-barrier, while the two H-bonds split in a higher-energy bond which is stronger because closer to the transition-state structure and in a lower-energy one (the stable form) which is weaker because farther from it; (d) complete dissymmetrization of the PT-barrier will increasingly weaken the more stable H-bond until the formation of an extreme dissymmetric single-minimum or dissymmetric single-well H-bond
Former, enseigner, eduquer dans l'Occident medieval (1100-1450) Parijs:SEDES ,2000 2-7181-9215-1
No full textItem does not contain fulltextP. Gilli Former, enseigner, eduquer dans l'Occident medieval (1100-1450) Parijs:SEDES ,2000 2-7181-9215-
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