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A new method for unravelling the intensities of severely or exactly overlapping reflections in a powder diffraction pattern has been developed. The fast iterafive Patterson squaring (FIPS) method involves an iterative procedure in which (1) a Patterson function is calculated using equipartitioned data (intensity ratio for overlapping reflections set to 1.0), (2) each point in the map is squared, (3) the new map is back-transformed to obtain new Fourier coefficients, and (4) these coefficients are then extrapo-~W 2 ow.,rl~tp lated to give a new set of ~ hkl values and a new intensity distribution for the overlapping reflections (nonoverlapping ones remain unaffected). The cycle is repeated until the intensity statistics of the overlapping reflections approximate those of the non-overlapping ones. Only after a redistribution of the intensities by the FIPS method was the ab initio structure solution possible for the molecular sieve SAPO-40 (structure-type code AFR), in which 65% of the reflections severely overlap.

A new method has been developed for the improved intensity assignment of severely or exactly overlapping Bragg reflections in a powder diffraction pattern. This fast iterative Patterson squaring (FIPS) method addresses, in particular, the systematic lack of small and large normalized intensities (IEI values) in severely overlapping powder data, which causes (a) the intensity statistics to be strongly acentric (even when the structure itself is centrosymmetric) and (b) the underestimation of strong structure-invariant relationships. Direct methods for structure determination are more likely to succeed with FIPS-improved data than with conventionally used equipartitioned data (ratio of lEI values for overlapping reflections set to 1.0). In the case of the molecular sieve AFR (in which 65 % of the reflections severely overlap), the ab initio structure solution was only possible after a redistribution of the intensities by the FIPS method.

The complex diffraction pattern of the heavily disordered cocrystals of perhydrotriphenylene and 1-(4-nitrophenyl)piperazine (5C 18 H 30 ÁC 10 H 13 N 3 O 2 ) has been investigated with synchrotron radiation and an area detector. Five (almost) complete, three-dimensional data sets have been obtained from the tips and the centre of a needle-like crystal at room temperature and 120 K. They revealed a rich variety of features including one,-two-and three-dimensional diffuse scattering, as well as incommensurate satellites. At the centre and one tip of the crystal the symmetry appears to be orthorhombic, whereas at the other tip the symmetry of the satellites and of some of the diffuse scattering is clearly monoclinic, indicating that the crystal is not homogeneous. Most of the scattering could be assigned to R/S occupational disorder of the chiral host molecules, to positional disorder of the guest molecules or to local distortions of the average structure. Assignments are based on the disorder deduced from the average structure and the molecular form factors of host and guest molecules which show characteristic patterns in reciprocal space. Two smaller, orthorhombic twin fragments and an additional phase with hexagonal symmetry have also been found.

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Starting from the octameric poly(triacetylene) (PTA) oligomer 1e as a large macromonomer, the monodisperse tetracosamer (24-mer) 1h was prepared by a previously introduced statistical deprotectionoligomerization sequence (Scheme). It is the longest known molecular rod featuring a fully conjugated, nonaromatic all-carbon backbone. Matrix-assisted laser-desorption-ionization time-of-flight (MALDI-TOF) mass spectrometry was particularly useful in the characterization of oligomer 1h and clearly demonstrated its monodispersity ( Fig. 1). In an effort to further clarify the conformational preferences of PTA oligomers, the Xray crystal structure of the 3.2-nm-long tetramer 1c was solved (Figs. 2 ± 4). In the solid state, the CC bonds in 1c all adopt the s-trans conformation with respect to the buta-1,3-diynediyl moieties. The p-conjugated system is perfectly planar, with the squared sum of the deviations of the backbone C-atoms from the best plane amounting to 0.077 2 . Analysis of the crystal lattice revealed a layered structure, in which the p-conjugated backbone of one oligomer is insulated by the trialkylsilyl groups of adjacent oligomers in neighboring layers.1. Introduction. ± Poly(triacetylene)s [À (CCÀCRCRÀCC) n À] are a new class of linearly p-conjugated polymers in the progression that starts with polyacetylene [À (CRCR) n À] and poly(diacetylene) [À (CCÀCRCR) n À], and ultimately leads to carbyne [À (CC) n À] [1] [2]. In recent work, we focused on the preparation and study of monodisperse oligomers as models for the corresponding infinite PTA polymers [3] [4]. Thus, we prepared the series 1a ± g, which extends up to a 11.9-nmlong 16-mer and investigated the evolution of the physicochemical properties into the higher oligomeric regime, where saturation of the properties becomes apparent [5]. In good agreement with previous extrapolative studies [6], evaluation of linear and nonlinear optical properties, Raman scattering, and electrochemical data supported an onset of saturation at ca. n 10 monomeric units, corresponding to 60 CÀC bonds. Based on the spectroscopic data, we proposed a preference of the molecular rods for adopting a planar geometry of the p-conjugated backbone, with adjacent CC bonds adopting an s-trans conformation with respect to the bridging buta-1,3-diynediyl linkers. Here, we report the X-ray crystal structure of the 32--long tetrameric rod 1c, which strongly supports such conformational preference in the solid state (for the X-ray analysis of PTA dimers, see [6]). We also describe the synthesis of the monodisperse tetracosamer (24-mer) 1h, which is the longest known monodisperse molecular rod featuring a fully conjugated, non-aromatic all-carbon backbone (for other multinanometer-long monodisperse oligomers, see [3] [4] [7] [8]).

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