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It is well known [1] that carbon-bound halogen atoms (frequently iodine and bromine, but also chlorine and even fluorine) can act as electron acceptors and form short contacts with different species (both neutral and negatively charged) that possess an ability to act as an electron donor. [2] These noncovalent interactions can be strong enough to control the aggregation of organic molecules in solid, [3] liquid, [4] and gas [5, 6] phases and the term ªhalogen bondingº [2, 6] has been suggested in order to emphasize the similarity with hydrogen bonding. While metal-ion coordination [7] is frequently the key feature in inorganic supramolecular systems, hydrogen bonding [8] is by far the most frequently used tool to assemble organic molecules in solid, liquid, or gas phases, and it plays an important role in stabilizing supramolecular aggregates even in water.Here we show that when the recognition pattern controlling the self-assembly process can be based on either hydrogen or halogen bonding, the latter can dominate over the former and, under appropriate conditions, will single out the molecules that will be involved in the construction of supramolecular architectures. We have already described specific I´´´N intermolecular interactions occurring between a,w-diiodoperfluoroalkanes and dinitrogen hydrocarbons. [9±11] These interactions are strong enough to overcome the low affinity [12] that exists between perfluorocarbons and hydrocarbons to assemble them into oligomeric structures in the liquid phase [4] and into crystalline 1D networks in the solid phase. [9±11] methacrylate as a comonomer. The copolymer 1 was shown to have good film-forming properties on a solid substrate. The fluorescence of the polymer was effectively quenched by photochemically generated acid and afforded finely resolved fluorescent images on a solid substrate. Experimental Section4: Potassium carbonate (82.9 g, 0.60 mol) and benzyl bromide (136.8 g, 0.80 mol) were added to a solution of 5-fluoro-2-nitrophenol (3) (94.3 g, 0.60 mol) in DMF (200 mL). The resulting mixture was stirred at 100 8C for 3 h, filtered, and the filtrate diluted with diethyl ether. The organic layer was washed with water and dried with anhydrous magnesium sulfate. Concentration, followed by vacuum distillation (142 ± 146 8C, 0.

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The crystal structure of the 7 form of isotactic polypropylene (7-iPP) is refined with the Rietveld method on X-ray diffraction data collected at low (-120 °C) temperature. The analysis, leading to the proposal of the totally novel crystal architecture with nonparallel chain axes, is discussed in detail, and the reliability of the proposed structure is assessed, also with reference to alternative models. While the overall structure is best represented in terms of the statistical copresence of anticlined isochiral helices at each crystallographic position, as implied by space group Fddd, local packing modes which cannot retain this feature are satisfactorily described in terms of space groups F2dd or Fd2d. Some relevant implications of the 7isotactic polypropylene crystal structure on the crystalline morphology of this polymer are presented, while issues concerning the development of this novel architecture remain largely open to future contributions.

High molecular mass and high regioregularity favor the more common polymorph (form I) of poly(3-alkylthiophenes) and substantially extend toward high temperatures the existence domain of the 2D mesomorphic phase, which these systems access above 70−80 °C. Morphological observations indicate that in thin films of form I the planar polythiophene main chains lie roughly edge-on parallel to the film surface, while side chains are approximately orthogonal to the substrate. Evidence relative to the second polymorph (form II), easily obtained with low-molecular-mass material, indicates that side-chain interdigitation is unlikely in this modification, just as in form I. Diffraction patterns in which the two crystalline phases coexist show that the longer axis directions in the two forms are approximately orthogonal, suggesting differences in the crystallization process. Very low-molecular-weight systems tend to crystallize preferentially in form II and give rise to a mesophase characterized by very low degrees of order, probably monodimensional. High regioregularity of the samples appears to affect both the intramolecular order and the packing.

We describe the results of quantum chemical calculations (DFT and MP2) on the intermolecular interactions involving ammonia and halofluoromethanes. The equilibrium C−X···N geometries are linear and the X···N distances are shorter than the sum of the van der Waals radii. The binding energies of CF3X···NH3 increase from 2 to 6 kcal/mol on following the sequence X = Cl, Br, I. Also, progressive introduction of F atoms in methyliodides raises the interaction energy from 2 kcal/mol for CH3I to 6 kcal/mol for CF3I. Therefore, halogen bonding involving perfluorinated alkylhalides and appropriate donors can be comparable in strength to strong hydrogen bonding. This agrees with recent experimental observations, that also the former can drive the construction of supramolecular edifices overcoming the low affinity between perfluorocarbons and hydrocarbons. Calculation of the atomic charges by the Atoms in Molecules method indicates that the charge-transfer contribution to the interaction energy is much less important for the present systems than for dihalogen−ammonia complexes.

We present the results of a thorough molecular modeling study of several alkylthiophene-based oligomers and polymers. In particular, we consider two polymers whose limit-ordered crystal structures have been recently reported by our group, on the basis of powder X-ray data analysis: poly(3-(S)-2-methylbutylthiophene) (P3MBT) and form I' of poly(3-butylthiophene) (P3BT). We first describe the development of a series general purpose force fields for the simulation of these and related systems. The force fields incorporate the results of ab initio calculations of the bond torsion energies of selected oligomers and differ in the set of atomic charges used to represent the electrostatic interactions. We then present the results of an extensive validation of these force fields, by means of molecular mechanics (MM) energy minimizations and molecular dynamics (MD) simulations of the crystal structures of these oligomers and polymers. While our "best" force field does not outperform the others on each of the investigated systems, it provides a balanced description of their overall structure and energetics. Finally, our MM minimizations and MD simulations confirm that the reported crystal structures of P3MBT and P3BT are stable and correspond to well-defined energetic minima. The room-temperature MD simulations reveal a certain degree of side-chain disorder, even in our virtually defect-free polymer crystal models.

Halogen bonds, attractive intermolecular interactions between perfluoroalkyl bromides and bromide ions, are present in cocrystals of (-)-sparteinium hydrobromide (1) and (S)-1,2-dibromohexafluoropropane (2; shown schematically), and result in enantiopure and infinite supramolecular helices. The perfluorocarbon-hydrocarbon self-assembly allows the resolution of racemic 2.