Powder diffraction techniques are becoming increasingly popular as tools for the determination of crystal structures. The authors of this paper have developed a software package, named PowderSolve, to solve crystal structures from experimental powder diffraction patterns and have applied this package to solve the crystal structures of organic compounds with up to 18 variable degrees of freedom (de®ned in terms of the positions, orientations, and internal torsions of the molecular fragments in the asymmetric unit). The package employs a combination of simulated annealing and rigid-body Rietveld re®nement techniques to maximize the agreement between calculated and experimental powder diffraction patterns. The agreement is measured by a full-pro®le comparison (using the R factor R wp ). As an additional check at the end of the structure solution process, accurate force-®eld energies may be used to con®rm the stability of the proposed structure solutions. To generate the calculated powder diffraction pattern, lattice parameters, peak shape parameters and background parameters must be determined accurately before proceeding with the structure solution calculations. For this purpose, a novel variant of the Pawley algorithm is proposed, which avoids the instabilities of the original Pawley method. The successful application and performance of PowderSolve for crystal structure solution of 14 organic compounds of differing complexity are discussed.

Racemic perhydrotriphenylene (PHTP) forms a polar inclusion compound with 1-(4-nitrophenyl)piperazine (NPP) as a guest molecule. Homochiral stacks of PHTP molecules surround polar chains of hydrogen-bonded NPP molecules in a channel-type, honeycomb architecture. The NPP chains are arranged in an all-parallel (rather than an antiparallel) fashion. Crystals of [PHTP]5[NPP] show second harmonic generation for incident light of wavelength 1064 nm, an electro-optical and a pyroelectric effect. The X-ray diffraction pattern exhibits Bragg-like reflections, interspersed with planes of diffuse scattering and weak satellite reflections superimposed on these planes, features which are indicative of extensive disorder. In spite of this, an orthorhombic average structure model could be deduced from the Bragg-like reflections (space group Cmc21, a = 15.023(2), b = 23.198(2), and c = 4.730(1) Å (T = 100 K), wR 2 = 0.088, goodness of fit 1.38). A qualitative interpretation of diffuse and satelite scattering is given. Crystals of [PHTP]5[NPP] are approximately hexagonal prisms. If the tailor-made additive 1-(p-tolyl)piperazine (TP) is present during crystallization, the habit changes to approximately hexagonal plates and TP is incorporated in small amounts. Crystals of the composition [PHTP]5[NPP]0.93[TP]0.07 lose the ability of second harmonic generation. The observation of polar properties for a crystal structure whose polar building blocks, the -NO2···HN- hydrogen-bonded NPP chains, are 14−15 Å apart and separated by PHTP hydrocarbon molecules is surprising, but can be explained in terms of a Markov chain model of crystal growth. The same simple model accounts for the loss of polarity in the presence of the tailor-made additive TP. The knowledge gained during the present analysis provides a rational tool for the engineering of polar properties of PHTP and similar channel-type inclusion compounds.

Host lattices based on piperazines, which can include a large variety of linear “rod‐shaped” π systems, are presented. One example is the inclusion of efficient nonlinear optical molecules (e.g. see Figure, and the cover picture this month) which leads to materials exhibiting attractive electro‐optical properties .

A prerequisite for the design and synthesis of materials with nonlinear optical (NLO) properties is a molecular building block that can be incorporated into a solid in an appropriately functionalized and oriented manner. To obtain new, single-crystal materials that exhibit electrooptical (EO) effects more pronounced than those of the currently known organic NLO crystals,[' -41 stable molecules showing hyperpolarizabilities B, of up to 10000 x m 4 V 1 and. having dipole moments that can be aligned in a parallel manner within the solid are required. While a large number of target compounds can be envisioned based on these known criteria,I3. ' 1 the probability is low that during the crystallization of pure NLO compounds a crystal structure with a parallel arrangement of the b, axes is realized (about 25% for an acentric space group, less than 5 % for a nearly parallel Hence, the question of alternatives for the formation of crystalline and structurally optimized EO materials arises. In this study it is shown that perhydrophenylene (PHTP)I6] forms inclusion compounds with electronically optimized NLO species, in which the guest molecules can attain a parallel arrangement; materials with polar properties on a macroscopic level are formed about 90% of the time.

How can the efficiency of formation of polar materials be improved? The work presented ehre makes use of the concept of supramolecular synthons, resulting in an inclusion formation approach that produces a nearly four times higher yield of polar crystal structures than the crystallization of dipolar molecules alone. The xample of co‐crystallization of racemic all‐trans perhydrotriphenylene (PHTP) with linear acceptors or donors is analyzed, and it is suggested that thie “Property‐directed” synthesis has a potentially high degree of predictability.