摘要:

An overview is given of two methods developed recently for crystal structure solution from powder diffraction data, based on the techniques of Monte Carlo sampling and Genetic Algorithms. Both methods operate in direct space, and the specific advantages of this approach in the case of structure solution from powder diffraction data are highlighted. The fundamental principles underlying the Monte Carlo and Genetic Algorithm techniques are described, and selected case studies highlighting different aspects of the application of these techniques are presented.

ISSN:1058-725X    

DOI:10.1080/10587259808044255

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Most [2+2] photodimerization reactions proceed heterogeneously. At a small conversion phase separation leads to the destruction of the parent crystal. The crystal fragmentation can be avoided by irradiation in the long wavelength tail of the absorption. The intermediate mixed crystalline states as well as thermally reversible systems have been studied.

ISSN:1058-725X    

DOI:10.1080/10587259808044256

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The question of the quantitative energetics of organic reactions in solids is addressed by studies of the solid state reaction of 2,5-distyrylpyrazine (DSP). Crystal electronic spectra permit the role of excitons to be analyzed. Experiments are described which elucidate the evolution of lattice vibrational modes as a function of extent of the reaction and the relative motions of the molecules associated with those modes. A general theory of solid state reactivity is employed to interpret the structural and spectroscopic results to produce a coherent, comprehensive picture appropriate for solid state reactions

ISSN:1058-725X    

DOI:10.1080/10587259808044257

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The basic principals underpinning the technique of polarised Near Edge X-ray Absorption Fine Structure (NEXAFS) Spectroscopy using synchrotron radiation are described with particular relevance to the technique's application in the structural characterisation of organic surfaces and interfaces. Experimental data, recorded in the ultra-soft spectral region close to the C and N K-edges (ca. 200eV to 500eV) on beamline U1 A at the Brookhaven National Synchrotron Light Source (NSLS) is presented including applications to thin films of long chain hydrocarbons, LB films, anti-corrosion coatings and organic single crystals.

ISSN:1058-725X    

DOI:10.1080/10587259808044258

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A new set of donor-acceptor (DA) complexes between azoalkanes and 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) is reported. Bis(cis-azoalkane)s are found to bind DDQ in solution, with complex formation constants in CH2Cl2that range from 22–76 M−1. Bis(azo)/DDQ crystalline solids are isolable and their crystal structures have been determined. Short intermolecular contacts in the crystals between the azo nitrogens and quinone ring carbons of 2.9–3.1 Å denote D-A “bonds” whose geometry is consistent with azo HOMO (n°)/DDQ LUMO (π*) overlap. The DA “bonding” promotes one-dimensional supramolecular ordering in the cocrystal which is organized as parallel (-DD-A-)nstrands. Thus, the azo/DDQ DA “bond” serves as a new crystal engineering synthon for preparing one-dimensional, two-component solids.

ISSN:1058-725X    

DOI:10.1080/10587259808044259

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Preparation, structure and discrimination of chiral two-component molecular crystals are described by connecting flexible achiral molecules such as diphenylacetic acid indole-3-propionic acid with aza aromatic compounds through hydrogen bonding. The chirality generation mechanisms are discussed.

ISSN:1058-725X    

DOI:10.1080/10587259808044260

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The edge to edge aryl C—H···N dimer is a previously unrecognised intermolecular hydrogen bonded lattice packing motif; a search of the Cambridge Structural Database revealing that it is widespread amongst many classes of aromatic nitrogen-containing solids. This weak attractive interaction can be retained throughout crystal engineering studies as typified by the systematic modification of the diquinoline1into host-guest compounds formed by its dibromo derivative2.The latter is a molecule designed to form multimolecular inclusion compounds specifically with small polyhalocarbon guests.

ISSN:1058-725X    

DOI:10.1080/10587259808044261

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

X-Ray single crystal study, molecular mechanics calculations and quantumchemical calculations of the static nonlinear optical (NLO) polarizabilities (β) were performed for a large series of dicyanovinylaromatic derivatives in order to make conclusions about the relationship between their molecular geometry, crystal structure and NLO properties. For some compounds studied EFISH measurements of the P values in solutions were made and good correlation was found between the calculated and experimental values. X-Ray data and optimal molecular associate calculations revealed the factors responsible for formation of centric/acentric crystal structures. This approach might be useful for prediction of possible crystal structures for simple organic chromophores. In the series studied only three acentric crystal structures were found, and in agreement with their molecular, electronic and crystal packing characteristics all were found to be active in second harmonic generation (SHG) in the solid state. For the known NLO crystal of dicyanovinylanisole (DIVA), a high-resolution low-temperature (153K) multipole X-ray diffraction analysis of the electron density distribution has been performed, and these data were used for an estimation of the molecular dipole moment and p values directly from the X-ray diffraction data.

ISSN:1058-725X    

DOI:10.1080/10587259808044262

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The cobaloxime complex with the β-cyanoethyl group and isonicotinic acid as axial ligands forms host-guest complexes with several kinds of host molecules. Whe the crystals of the host-guest complexes were irradiated with visible light, the β-cyanoethyl group of the guest molecule was changed to the α-cyanoethyl group. The rate of the photoisomerization was controlled by the shape of the host molecule. Designing the host structures, the isotructural host-guest crystals were prepared and the controlled reaction rates were obtained.

ISSN:1058-725X    

DOI:10.1080/10587259808044263

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

This study deals with the preparation and structural characterization of crystalline assemblies based on the tetra(4-cyanophenyl), tetra(4-nitrophenyl) and tetra(4-amidophenyl) derivatives of metallated porphyrin as building blocks, toward the design of novel coordination polymers and microporous organic solids. The cyanophenyl derivative forms under selected conditions two-dimensional coordination polymers through direct ligation of the-CN functions to the metal centers of neighboring porphyrins. In most other cases it self-assembles into layered multiporphyrin arrays which consist of uniquely structured supramolecular chains and networks with large cavities. Similar intermolecular architectures were observed in solids based on the nitrophenyl and amidophenyl porphyrin entities. The layered organization in these materials is dominated to a large extent by molecular shape and aromaticity of the porphyrin frameworks, while its porous nature is sustained and controlled by non-covalent interactions between the amidophenyl, cyanophenyl or nitrophenyl sensor groups of adjacent metallomacrocyclic units.

ISSN:1058-725X    

DOI:10.1080/10587259808044264

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor