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The weak electrostatic and dispersive forces between C(δ+)—F(δ−) and H(δ+)—C(δ−) are at the borderline of the hydrogen-bond phenomenon and are poorly directional and further deformed in the presence of other dominant interactions, e.g. C—H...π. The title compound, C6H4F2, Z′ = 2, forms one-dimensional tapes along two homodromic C—H...F hydrogen bonds. The one-dimensional tapes are connected into corrugated two-dimensional sheets by further bi- or trifrucated C—H...F hydrogen bonds. Packing in the third dimension is controlled by C—H...π interactions.

In the title compound, C6H3F3, weak electrostatic and dispersive forces between C(δ+)—F(δ−) and H(δ+)—C(δ−) groups are at the borderline of the hydrogen-bond phenomenon and are poorly directional and further deformed in the presence of π–π stacking interactions. The molecule lies on a twofold rotation axis. In the crystal structure, one-dimensional tapes are formed via two antidromic C—H...F hydrogen bonds. These tapes are, in turn, connected into corrugated two-dimensional sheets by bifurcated C—H...F hydrogen bonds. Packing in the third dimension is furnished by π–π stacking interactions with a centroid–centroid distance of 3.6362 (14) Å.

The relative configuration of the title compound, C11H18O3, was corroborated by single-crystal X-ray diffraction analysis. In the crystal, molecules are linked via a O—H...O hydrogen bond and a chain of molecules is formed along [010].

The indole based zwitterion 2 forms stable dimers held together by H-bond assisted ion pairs. Dimerisation was confirmed in the solid state and studied in solution using dilution NMR experiments. Even though zwitterion 2 forms very stable dimers even in DMSO, their stability is lower than of an analogous pyrrole based zwitterion 1. As revealed by the X-ray crystal structure the two binding sites in 2 cannot be planar due to steric interactions between the guanidinium group and a neighbouring aromatic CH. Hence the guanidinium moiety is twisted out of planarity from the rest of the molecule forcing the two monomers in dimer 2·2 to interact in a non-ideal orientation. Furthermore, the acidity of the NHs is lower than in 1 (as determined by UV-pH-titration) also leading to less efficient binding interactions.

The crystal structure of 1,2,3,5-tetrafluorobenzene, 1, has been determined and is compared with those of other polyfluoro-substituted benzenes. Compound 1has a layered monoclinic structure in which the layers are held by bifurcated C–HF–C interactions. Short FF separations are also observed. The layers are stacked at van der Waals separation to give a 4 Å packing. This structure is adopted instead of an alternative tetragonal structure adopted by fluorobenzene and a number of related compounds such as benzonitrile, pyridine N-oxide and alloxan. Compound 1does not take the tetragonal structure mostly because this structure would demand the formation of a C–Fπ interaction, which appears improbable for the molecule. The role of weak intermolecular interactions in the crystal packing of predominantly non-polar compounds is highlighted. [ABSTRACT FROM AUTHOR]

A new approach (MMH-2) was applied and tested for the prediction of intermolecular interactions in the crystal packing of acetamide. In MMH-2, energies of random molecular interaction configurations are computed. It uses molecular association quantities from statistical thermodynamics in order to obtain intermolecular interaction motifs that follow a ranking process. The most important motifs are optimized. Here, the AM1 semiempirical Hamiltonian was applied for the calculation and optimization of each obtained configuration and a comparison to MP2 results is provided. Such a stepwise procedure follows the assumed genesis of crystal growth without using experimental input. For evaluation purposes, graph set analysis was used to classify the structural patterns of both acetamide polymorphs. It was also necessary to introduce a new geometrical similarity index for the comparison of calculated and experimental motifs. As a result, all experimental hydrogen bond patterns were found and molecular synthons in both polymorphic acetamide structures were predicted as local minima. This suggests a new strategy for crystal structure prediction of flexible molecules with a possible subsequent progress in crystal engineering in silico. [ABSTRACT FROM AUTHOR]

Abstract  Cyclocondensation reaction of ethyl 7,8-diamino-1-cyclopropyl-6-fluoro-4-oxo-1,4-dihydroquinoline-3-carboxylate with 1-methylisatin produced a separable mixture of the corresponding indolo[3,2-b]- and [2,3-b]pyrido[2,3-f]quinoxaline-3-carboxylates, of which the latter isomer predominates. On the other hand, interaction with 1H-isatin or 5-chloroisatin gave the respective indolo[2,3-b]pyrido[2,3-f]quinoxaline-3-carboxylates as the sole regiospecific products. The structures of these new pentacyclic derivatives are based on microanalytical, spectral (IR, MS, and NMR) and X-ray crystal structure data. [ABSTRACT FROM AUTHOR]

Trichloromethanesulfenyl acetate, CCl 3SOC(O)CH 3, belongs to the family of sulfenic esters. This molecule has been characterized by vibrational spectroscopy. The conformational and geometrical properties of this species have been determined by IR and Raman spectroscopy, X-ray diffraction, and quantum chemical calculations. Geometry optimizations of the most stable forms were performed with ab initio (HF, MP2) and density functional theory (B3LYP) methods. According to our data, this compound results in a gauche−syn conformer with C1symmetry (gauche orientation around the S−O bond and syn orientation of the CO double bond with respect to the S−O single bond) for the most stable geometry, and trans−syn conformer with C ssymmetry (trans orientation around the S−O bond and syn orientation of the CO double bond with respect to the S−O single bond) for the second stable conformer (1.1 and 0.53 kcal/mol higher in energy than the most stable C1form according to the matrix FTIR spectroscopy and MP2/6-31G* level of the theory, respectively). The crystalline solid (monoclinic, P2 1/ n, a= 8.0152(17) Å, b= 5.7922(13) Å, c= 17.429(4) Å, α = γ= 90°, β = 100.341(3)°) consists exclusively of the main form. The geometrical parameters (X-ray diffraction) are dC−Cl= 1.767(19) Å, dC−S= 1.797(2) Å, dS−O= 1.663(14) Å, dCO= 1.189(2) Å, dO−C= 1.389(3) Å, dC−C= 1.483(3) Å, angles Cl−C−Cl = 110.3(11)°, Cl−C−S = 111.8(12)°, C−S−O = 97.4(8)°, S−O−C = 116.7(11)°, O−CO = 122.8(19)°, OC−C = 127.1(2)°, and the main torsion angles are δ(CSOC) = 105.9(15)° and δ(SOC(O)) = 7.6(3)°. The geometrical data calculated with B3LYP/6-31G++(3df,3pd), B3LYP/6-311G++(3df,3pd), B3LYP/aug-cc-pVTZ, and MP2/6-31G* are in good agreement with diffraction data. [ABSTRACT FROM AUTHOR]

The title system yields polymorph and bi-component crystals readily under cryocrystallisation conditions; the present study clarifies inconsistencies in the literature and defines a new structural landscape. [ABSTRACT FROM AUTHOR]