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Single-crystal X-ray diffraction data for anthracene, flail10, have been measured at six temperatures from 94 to 295 K. Positional and displacement parameters for C and H atoms were refined at each temperature by conventional least-squares techniques. The derived anisotropic displacement parameters for the C atoms at each of the six temperatures were corrected for the contributions of the internal modes and were then analyzed to determine translational (T) and librational (L) tensors describing the rigid-body molecular motion. Although the resulting T and L tensors agree well with those measured and calculated previously at room temperature, the shrinkage of the L tensor with crystal cooling appears somewhat anomalous. The unexpected behavior is concentrated in the component of L associated with the long molecular axis, a component that is, as a consequence of the molecular geometry, relatively poorly determined. Therefore, while the possibility of some subtle, temperature-dependent structural change, such as a molecular reorientation, cannot be ruled out, neither can it be endorsed. This study underlines how the geometry of the molecule being studied can limit the quality of the rigid-body thermal-motion description determined by diffraction methods.

Single-crystal data for naphthalene have been measured at five temperatures between 90 and 240 K. Positional and thermal parameters for C and H atoms at each temperature were refined by conventional least-squares techniques. The effect of varying the weighting scheme was examined. Contributions of internal molecular modes to the motions of the atoms turn out to be important. They were estimated for the C atoms at each temperature from a standard force field and subtracted from the experimental U~/ values. The corrected UiTs were then analysed to determine rigid-body translational and librational tensors for the naphthalene molecule. The absolute magnitudes and temperature dependence of these quantities have been compared with values calculated from atom-atom potentials and from spectroscopic data.

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We report the preparation and solid-state characterization of the perchlorophenalenyl radical (1). The radical is initially obtained as a yellow-green solid by reduction of the perchlorophenalenium salt (12(+)). This solid sublimes in a sealed tube to give black shiny hexagonal crystals of the perchlorophenalenyl radical (1). The structure consists of 1-dimensional stacks of the monomeric radical. The peri-chlorine atoms force the phenalenyl system to be strongly nonplanar leading to a large separation between adjacent molecules within the stacks (3.78 A), and the molecules adopt two distinct stacking motifs (quasisuperimposed and rotated by 60 degrees with respect to neighbors). Because of the packing frustration in the lattice and the large intermolecular spacing, the solid shows Curie paramagnetism in the temperature range 100-400 K, before antiferromagnetic coupling sets in at low temperatures. Due to the narrow bandwidth that results from the isolation of the individual molecules, the solid is a Mott-Hubbard insulator, with a room-temperature conductivity of rho(RT) = 10(-10) S/cm.