An X-ray single-crystal study of the molecular system C₆F₆.C₆D₆

Abstract

An X-ray diffraction study of the molecular complex system hexaflurobenzene-deuterobenzene at 279K is reported. The structure has been determined for the first time as rhombohedral, space group R3m or R3m, and of unit-cell dimensions a = 7.310 (19)/k and a = 109.67 (43) °, giving a unit-cell volume of 299 (5) A ~. R = 0.098 for 150 independent reflections. There is one molecule of each species within the unit cell and they are alternately arranged in infinite stacks along the unique [ 111 ] crystallographic direction, the molecular planes being perpendicular to this direction. Both constrained and unconstrained refinements have been done and the results of both are presented. The constraints consisted of rigidly fixing the bond lengths and geometry of the molecules such that the resulting molecular hexad symmetry axis was coincident with the unique axis of the structure. The unconstrained model relaxes the fixed bond lengths but the hexad symmetry is retained. The possibility of pairing of the molecules along the unique direction is considered for this phase, and crystallographic evidence is given for at least two other solid-state phases of this system.

Supramolecular Assemblies of Hydrogen‐Bonded Carboxylic Acid Dimers Mediated by Phenyl–Pentafluorophenyl Stacking Interactions

“…Thus, this interaction has emerged as a very important synthon in crystal engineering that has a utility comparable to that of many robust hydrogen‐bonding synthons. For example, the aryl–perfluoroaryl stacking interaction has been used to induce the crystal packing of monomers suitable for solid‐state photopolymerization2 and to stabilize liquid‐crystalline phases 3. It also seems to offer new possibilities for the modification of biologically important structures including peptides and oligonucleotides 10…”

“…In contrast to the crystals of the individual components, which show a herringbone packing, this complex consists of face‐to‐face stacks of alternating benzene and hexafluorobenzene molecules. Crystallographic studies of several 1:1 complexes of perfluorinated aromatic compounds with non‐fluorinated arenes have revealed similar stacking motifs of alternating component molecules 2–7. This kind of arrangement can be attributed to a quadrupolar interaction between electron‐rich and electron‐deficient aromatic rings 8.…”

Interactions with Aromatic Rings in Chemical and Biological Recognition

“…Hints of the unusual properties of perfluoroaromatic compounds came four decades ago from formation of a crystal from an equimolar mixture of hexafluorobenzene and benzene: A solid was formed with a melting point of 23.7 °C, approximately 19 °C higher than either of the two components 329. Unlike the crystal‐packing patterns of benzene and hexafluorobenzene, which feature herringbone arrangements arising from T‐shaped interactions (see Section 3), the C 6 H 6 ⋅C 6 F 6 cocrystal shows nearly parallel molecules stacked alternately in infinite columns (Figure 28) with an interplanar distance ( r ) of about 3.4 Å and an intercentroid distance ( d ) of 3.7 Å 330. 331 Weak C aryl H⋅⋅⋅F contacts help to stabilize neighboring columns (see above).…”

Interpreting geometric preferences in π‐stacking interactions through molecular orbital analysis

“…Experimental evidence has been found for parallel π‐stacking in liquid and solid phases of Bz and F 6 Bz mixtures . The Bz‐F 6 Bz dimer was examined to determine if similar trends in monomer MO mixing could be found in heterodimers.…”