The existence and nature of C-H‚‚‚F-C interactions in crystalline fluorobenzenes 1-3 and 7-10 are discussed. These compounds were chosen because they contain only C, H, and F atoms; this is necessary in the evaluation of the weak acceptor capabilities of the C-F group. All of these compounds are liquids at room temperature, and single crystals for X-ray diffraction were grown in situ. The analysis of the C-H‚‚‚F interactions that are found in all of these crystal structures takes the form of comparisons with related C-H‚‚‚O/C-H‚‚‚N analogues. Fluorobenzene, 1, bears a close relationship to pyridinium fluoride, pyridine 1-oxide, and benzonitrile at the level of individual interactions, showing that the character of the structuredetermining intermolecular interactions in these four crystal structures are the same. Similarly, 1,4-difluorobenzene, 3, and 1,4-benzoquinone are related, the C-H‚‚‚F interactions in the former playing the same structural role as the C-H‚‚‚O interactions in the latter. A comparison of 3 with the unsymmetrical 1,4-dihalogenated benzenes, 4-6 indicates the importance of C-H‚‚‚F interactions in these structures. With an increase in the F content of the molecules, the C-H acidity also increases and the C-H‚‚‚F interactions in 1,3,5-trifluorobenzene, 7, and 1,2,4,5-tetrafluorobenzene, 8, become stronger and more important. Compounds 7 and 8 are structurally very similar to 1,3,5-triazine and 1,2,4,5-tetrazine, and this similarity further strengthens the argument that C-H‚‚‚F interactions resemble C-H‚‚‚N interactions and provides evidence for their description as weak hydrogen bonds. 1,2,3,4-Tetrafluorobenzene, 9, is polymorphic but the role of the C-H‚‚‚F interactions in the two forms is similar. A comparison of the C-H‚‚‚F geometries in compounds 1-10 with other C-, H-, and F-containing compounds in the Cambridge Structural Database reveals that the hydrogen bond properties are more pronounced in 1-10. It is concluded that only when the carbon acidity is enhanced to the levels of the compounds in the present study, is the hydrogen-bond nature of the C-H‚‚‚F interaction even revealed. This study also demonstrates that the C-F group prefers to form C-H‚‚‚F interactions rather than F‚‚‚F contacts. The behavior of organic fluorine in crystal packing is therefore quite different from the heavier halogens.

A less dense packing is observed in the odd-numbered n-alkanes compared to the even-numbered members, which consequently lowers melting temperatures. The reason for this is that the even-numbered n-alkanes have optimal intermolecular interactions at both ends (see the picture on the left), while the odd-numbered ones possess these only at one end-at the other end the intermolecular distances are longer (right).

Single-crystal X-ray diffraction analyses of α,ω-alkanedicarboxylic acids (HOOC−(CH2) n - 2−COOH, n = 2−10) have been carried out at 130 and 298 K. Dimorphism is prevalent in odd carbon members, and the crystal structures of α- and β-forms of C7-diacid have been determined. Diacids show an alternation in their melting points with members containing an even number of carbon atoms exhibiting systematically higher melting points compared to odd ones. On the contrary, the solid-state densities of odd members with C n ≥ 5 are higher than those of even members. Closest packing is therefore not the reason for alternating melting points in diacids. Diacids with C n ≥ 5 show distinct packing regularities within even series and also within α- and β-series of odd members. The gross structural features are similar in even and (both forms of) odd diacids:  (a) carboxy groups form hydrogen bonded dimers at both ends of the molecules, leading to infinite chains, and (b) methylene chains stack into columns through hydrophobic interactions. However, there are certain differences within these similar packing patterns that are important in the context of melting point alternation:  (a) molecules are offset along their length within the columnar stacks in even members, whereas such an offset is absent in both forms of odd members, and (b) molecules in both modifications of odd members exhibit twisted molecular conformations with severe torsions as opposed to the non-twisted all-trans conformation in the even members. Energies of the ideal and observed conformations have been computed with the hybrid-DFT method B3LYP and 6-31G* basis set. A simple geometrical model has been developed wherein the even and odd members are described as modified parallelograms and trapezoids, respectively. It is shown that, whereas the packing of parallelograms allows an offset which reduces the repulsions between the carboxy dimers of adjacent hydrogen bonded chains, a similar offset is forbidden for the packing of trapezoids. The model also suggests the reason for the prevalence of dimorphism in odd diacids. Because the twisted molecular conformations in odd diacids are associated with high energy, they have lower melting points. The melting point alternation in diacids is therefore attributed to the geometry-allowed or -forbidden attainment of an offset packing with a non-twist molecular conformation.

Die Reduktion von C6H5)3PAuCl mit B2H6 in Benzol ergibt Au9,2[P(C6H5)3]2Cl, das mittels Molmassebestimmungen als Au55[P(C6H5)3]12Cl6 charakterisiert wurde. Ein einfaches Modell, beruhend auf einer Anordnung dichtest gepackter Goldatome, führt zu einem Goldcluster, dessen Aufbau mit der ungewöhnlichen chemischen Zusammensetzung in guter Übereinstimmung ist. Das Mößbauer‐Spektrum der Verbindung zeigt vier Sorten von Goldatomen: einen metallischen Anteil (Au13‐Kern), durch P(C6H5)3‐bzw. Cl‐Liganden Koordinierte Au‐Atome, sowie unkoordi‐niertes Oberflächengold. Mit Brom und lod läß sich der Komplex zu (C6H5)3PAuCl, (C6H5)3 PAuBr(I) und metallischem Gold abbauen. Die Thermolyse bei 50°C führt quantitativ zu [(C6H5)3P]2AuCl und Gold.

Both X-ray crystallography and electronic structure calculations using the cc-pVDZ basis set at the DFT B3LYP level were employed to study the explosive properties of triacetone triperoxide (TATP) and diacetone diperoxide (DADP). The thermal decomposition pathway of TATP was investigated by a series of calculations that identified transition states, intermediates, and the final products. Counterintuitively, these calculations predict that the explosion of TATP is not a thermochemically highly favored event. It rather involves entropy burst, which is the result of formation of one ozone and three acetone molecules from every molecule of TATP in the solid state.