γ-Phase p-dichlorobenzene at 100 K

Abstract

Abstract. C6H4C12, monoclinic y-phase, P2ffc, a = 8.624(2), b=6.02I (2), c=7.414 (2) A, fl= 127.51 (1)°, Z=2, R = 0.044 for 797 reflections. Crystals of y-DCB were grown from ethanol solution at 250 K and analyzed at 100 K. The benzene ring bond lengths are in good agreement (1.391 + 0.004 A)and the chlorine atoms are displaced 0.045 A from the benzene plane.Introduction. Hexagonal plates suitable for X-ray analysis were obtained from a saturated ethanol solution at -25°C. Evaporation of the solvent was aided by passing a gentle stream of nitrogen over the solvent. To prevent the y -+ e phase transition, the filtering and crystal mounting procedures were performed in a large freezer maintained at -20°C. A specimen 0.3 x 0.3 x 0-1 mm was selected and placed in a quartz capillary which had been previously attached to an Air Products cryo-tip goniometer. After the capillary was sealed and covered with a beryllium vacuum shroud, the cryostat was activated and the goniometer removed from the freezer. Cell dimensions and a total of 890 (797> 3aF) intensity data were determined at 100+ 5K on a Picker FACS-I diffractometer using Mo radiation and a graphite monochromator (2=0.71069 A). The data set included all reflections to 20=55 ° plus

Single-crystal structure analysis and energy minimizations of a MFI-type zeolite at low p-dichlorobenzene sorbate loading

“…The contact distances between centrosymmetrically related CI atoms are 4.16 (2) and 3.56(2)A. The third contact, between CI o atoms related by a twofold screw axis, is 3.81 (2)A, in good agreement with the CI--CI contacts in y-gdichlorobenzene crystals [3.789 (2) and 3.800 (2)A; Wheeler & Colson, 1975], although the packing in the latter crystal is quite different. The pdcb molecules form a chain in the straight channel.…”

“…A double 10-ring of the straight channel is indicated. Wheeler & Colson (1975;W&C) The final difference electron-density map for model I shows peaks (1.26 and 1.17e,~, -3) around the CI atoms, whereas for model II the map is flat and does not show any significant features at the CI positions. In I no C atoms can be located, whereas one would expect the rotation of the pdcb molecule around the C1--C1 axis to be considerably hindered in this section of the straight channel; the wR factor slightly increases during a restrained refinement.…”

“…The parameters describing the interaction between the zeolite and the molecules (Coulomb and 6-9 Lennard-Jones terms) are taken from the separate parts of the force field without introduction of new quantum chemical or experimental data for this interaction. In Table 3 the energy optimized geometry of an isolated pdcb molecule, obtained with the CFFgl force field, is compared with the experimental geometry for a molecular crystal (Wheeler & Colson, 1975).…”

“…We therefore continued our efforts to refine II. A group refinement of a rigid pdcb molecule, using the geometry of Wheeler & Colson (1975;see Table 3) with individual isotropic displacement parameters for the C atoms and anisotropic parameters for the CI atoms, was carried out. H atoms are added at calculated positions (C--H = 1.00,~,, C--C--H= 120.5 °) and included in the group refinement.…”

Vibron band structure in chlorinated benzene crystals: lattice dynamics calculations and Raman spectra of 1,4-dichlorobenzene

“…Moreover, the internal modes of the free t c b molecule were calculated for molecules with different 35C1/37C1 isotopic mass distributions and the isotope shifts were compared with the calculated 0953 In this contribution, we present similar calculations of the dispersion of the coupled internal vibrations in the a -, ¡3-and y-phase of 1,4-dichlorobenzene ( d c b ) . This crystal was chosen because it also exhibits a one-dimensional stacked crystal structure [6][7][8][9][10][11] and because investigations on the triplet electronic excitation by von Borczyskowski and co-workers [12,13] show that the interaction between equivalent molecules in the stacks (7||) is about two orders of magnitude larger than the interaction between inequivalent molecules. Calculations of the lattice modes (phonons) of a-and /3 -d cb have been reported earlier, and in particular the frequencies at k = 0 and the dispersion of the phonons have been investigated [2][3][4]14,15].…”