摘要:

Numerous gas/solid reactions are studied on microscopically smooth surfaces by atomic force microscopy (AFM). Five basic types for solid phase to solid phase transformations have been found, as well as mixed types. These are discussed in some detail, exemplified, and compared to some solid state photoreactions in order to provide a basis for a real understanding of non is typical solid state transformations, which are preparatively useful.

ISSN:1058-725X    

DOI:10.1080/10587259208025800

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Conventional crystal structure analysis using Bragg diffraction data reveals only average one-body structural information, such as atomic positions, thermal ellipsoids and site occupancies. Diffuse scattering, on the other hand, gives two-body information and is thus potentially a rich source of information of how molecules interact. However, since typical diffuse scattering intensities are ∼lO3-lO4down on Bragg intensities, obtaining such information is still a far from routine process and quantitative studies are rare. In this paper we describe some of the concepts and difficulties involved, with particular reference to the example of 1,3-dibromo-2,5-diethyl-4,6-dimethyl benzene (BEMB2), which is disordered as a result of the fact that the molecule can take up either one of two different orientations in a given molecular site.

ISSN:1058-725X    

DOI:10.1080/10587259208025801

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

White Beam Synchrotron X-ray Topography has been used to characterize defects in solution-grown p-terphenyl single crystals. Defects observed include growth dislocations, mechanically introduced dislocations, macroscopic twins, twin lamellae, and dislocations emitted from twin boundaries. Line direction and Burgers Vector analysis of the growth and mechanically introduced dislocations, using projective geometry calculations and g. b analysis, respectively, are presented, along with complete analysis of the twin operation, obtained from a combination of analysis of optical micrographs and of orientation contrast on synchrotron topographs. The relationship between the Burgers vector of the dislocations generated at the twin boundary and the strain associated with the twins is discussed. Interesting interactions between mechanically introduced dislocations and twin boundaries are presented. Defect analyses are discussed in the context of results obtained previously using transmission electron microscopy.

ISSN:1058-725X    

DOI:10.1080/10587259208025802

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

In situobservations have been made, using synchrotron white beam X-ray topography in conjunction with a specially designed liquid nitrogen cold-finger cryostat, of single crystals ofp-terphenyl as they undergo a phase transition at ≈l30K. The transformation involves a monoclinic to tri-clinic structural change. Direct observation was made of two kinds of domains in the low temperature phase. The original microstructure of the crystal appeared unperturbed despite the considerable strain which develops during the transition. Comparison is drawn between the X-ray results presented here and those obtained previously using transmission electron microscopy.

ISSN:1058-725X    

DOI:10.1080/10587259208025803

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

A new technique for the unambiguous determination of dislocation line directions in macroscopic single crystals is presented. The technique is based on analysis of the projected directions of the images of the dislocations on synchrotron white beam topographs. Calculation is carried out in a pseudo-cubic axis system which is related to the crystal axis system by a transformation matrix. Final line directions are presented referred to the crystal axes. An example of the application of this technique to the analysis of growth dislocations in pyrene single crystals is presented.

ISSN:1058-725X    

DOI:10.1080/10587259208025804

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

The phase behavior of quadricyclane has been investigated by differential scanning calorimetry and vibrational spectroscopy. An order-disorder phase transition occurs at 153 K on cooling and at 180 K on heating.

ISSN:1058-725X    

DOI:10.1080/10587259208025805

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

The C-H…O interaction, though weak, is not van deer Wails in nature but electrostatic and resembles O-H…O and N-H…O hydrogen bonds in its geometrical properties. Spectroscopic evidence hints at some degree of covalent bonding between H and 0 atoms. The long-range, electrostatic character of the C-H…O interaction determines its important role in crystal engineering. Planar aromatic hydrocarbons adopt herringbone structures but if a critical number of O atoms is present, a planar molecule will cross the structural threshold from a C…H stabilised herringbone structure to a C-H…O stabilised layer structure wherein adjacent molecules are parallel and highly overlapped. These ideas have been used to design a family of crystal structures of alkoxyphenylpro-piolic acids that participate in an intermolecular, solid state Diels-Alder reaction. When the number of C-H…O interactions in a structure is unusually large or small, there is a possibility of distortions in the O-H…O network.

ISSN:1058-725X    

DOI:10.1080/10587259208025806

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

The surroundings of functional groups in molecules within a crystal can be analyzed from data in crystallographic databases. A method is described here for preparing contoured maps of the probabilities of interactions of functional groups in various directions. The results of such analyses can provide probability plots of orientational preferences of binding which can be used for model building involving the interaction of two different molecular species. Examples chosen for description here include hydrogen bonding to oxygen-containing functional groups, the surrounding of C-F bonds and the manner by which carboxyl groups bind metal ions.

ISSN:1058-725X    

DOI:10.1080/10587259208025807

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

In this paper the effect of fluoro substitution on the packing of the 2-benzyl-5-benzylidene- and 2, 5-dibenzylidene-cyclopentanone frameworks will be discussed. We chose the fluoro substituent because it has a much smaller van der Waals radius than other substituents used hitherto, while it is very similar to that of hydrogen. This allows a method to distinguish between substituent effects due to size and those due to non-bonded interactions on the crystal packing. It can be shown that intermolecular atom-atom interactions involving the fluoro groups hold sway over the effect of the volume of the subtituents upon crystal packing. A comparison is made between the F…F contacts in these structures and others published in the literature.

ISSN:1058-725X    

DOI:10.1080/10587259208025808

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

In a molecular crystal, intermolecular interactions will correspond to specific symmetry elements. If one chooses molecules carefully, one can reliably predict specific intermolecular interactions and the corresponding symmetry operations. The symmetry operations of one and two dimensional networks of molecules can be combined to form rod and layer groups respectively. In many cases of chemical interest the sequence of moving from a molecule to a one dimensional array, then on to two and three dimensions corresponds directly to the symmetry combinations leading from the point group to rod group, to layer group and on to the space group. The structures of a number of disubstituted urea derivatives were determined and are used to illustrate these ideas of molecular design.

ISSN:1058-725X    

DOI:10.1080/10587259208025809

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor