ISSN:0889-311X    

DOI:10.1080/08893119608035388

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Modulated structures can be found in organic, inorganic and even quasicrystalline structures. They are generally detected by diffraction from the presence of satellites surrounding the Bragg reflections. Their positions may vary continuously with temperature or pressure so that the phase can be considered as incommensurately modulated. Incommensurate phases usually transform to a periodic high symmetry phase by increasing temperature and to a commensurate lock-in phase by lowering the temperature. Their domain of stability varies with temperature, pressure and with the type of compound. Examples have been identified where the interval of equilibrium vary from a few tenths to a few hundreds of degrees. Recent progress in the field of incommensurate structure analysis has been greatly favoured by the superspace group approach which is now almost exclusively applied. In many cases, the resolution of the modulated phases by diffraction methods has contributed towards the understanding of the phase changes. In addition, microscopic models for the transition mechanisms have been developed to understand small organic systems. The methods of molecular dynamics have been able to explain the formation of various sequences of commensurate and incommensurate phases observed experimentally.

ISSN:0889-311X    

DOI:10.1080/08893119608035389

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A brief account of transformations of internal structure of hydrogen bonds, and transformations involving the breaking and formation of hydrogen bonds is highlighted for a selected group of crystals. The main stress is laid on the short bistable -OH‒O= hydrogen bonds, illustrating several general properties, often blurred for weaker hydrogen bonds. A set of parameters relating the electronic structure of the oxygen atoms and geometry of hydrogen bonds is described. This allows one to combine the hydrogen bond dimensions with changes in the electronic structure of the oxygen atoms, accompanying any transformations of hydrogen bonds. Several resultant effects are described: coupling of displacements of molecules or ions with the H-atom transfer, the isotope effect of deuteration on Tcin ferroelectric crystals, a skew trajectory of a disordered H-atom with respect to the O…O direction, or shortening of the O…O distance when the H-atom becomes disordered. It is shown that hydrogen-bond patterns, molecular arrangements and thermodynamic properties of crystals are interdependent. Several examples and possible applications of the presented model are given.

ISSN:0889-311X    

DOI:10.1080/08893119608035390

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Although there are good reasons to believe that second-order phase transitions occur widely among crystalline mixed-stack π-molecular compounds, only three examples (pyrene-pyromellitic dianhydride (PYRPMA;Tc= 160 K), naphthalene-1,2–4,5-tetracyanobenzene (NAPTCB; T, = 73 K), and anthracene-1,2–4,5-tetracyanobenxene (ANTCYB;Tc= 213 K)) have been studied intensively. In PYRPMA the space group changes fromP21/atoP21/n, with doubling of one cell edge, while in the other two examples the space group changes fromC2/mtoP21/a, without change of cell dimensions. For all three systems there are experimental measurements (to different degrees of completeness) ofCp, cell dimensions and intensities of superlative reflections, and of NMR and ESR spectra as functions of temperature. The crystal structures of ordered and disordered phases have been determined. These results have been analysed in terms of current descriptions of phase transitions (Ehrenfest order of transition, Landau theory, determination of critical exponents) by various authors and the results are critically compared here at the phenomenological and molecular levels. All indications are that these three phase transitions, although showing many differences of detail, resemble one another in the sense that subtle intermolecular packing interactions are the driving force for the transitions rather than electronic interactions.

ISSN:0889-311X    

DOI:10.1080/08893119608035391

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The analysis from X-ray or neutron diffraction data, of the unusually high librational motion around the long molecular axis of thep-terphenyl central ring, reveals a doubly-peaked probability density function (p.d.f.), associated with a double-well potential function between two twisted conformations ( = ± 13.3° on either side of the average molecular plane). This p.d.f. is the pretransitional effect of an order-disorder transition which takes place at low-temperature. With a simple model averaging the intermolecular interactions, it is possible to separate in the overall libration on each site, at the bottoms of the double well, the contribution of the torsionalgmode (2) = 35 deg2, responsible for the jumps above the barrier, from the contribution of the other modes (2) = 17.5 deg2. Thus it becomes possible to determine the parameters of the model describing the intermolecular, as well as the intramolecular interactions and to specify the non-sinusoidal shape of the intramolecular potential between two adjacent phenyl rings. It is straightforward to extend the model to anyp-polyphenyl molecule with any number of phenyl rings, by considering that the molecular packings are the same in all the family. The model gives an estimate of the reorientational potential barrier heightsV= 4.57 kJ mol−1inp-terphenyl,V= 7.91 kJ mol−1inp-quaterphenyl, in good agreement with NMR results; the calculated p.d.f. are doubly peaked andV < kTat room temperature, which characterizes order-disorder transitions. On the other hand, the model gives a singly peaked p.d.f. andV= 1.25 kJ mol−1<kTin crystalline biphenyl, which agrees with the displacive nature of the phase transition. Combined with the features of the incommensurate phase of biphenyl atT= 20K, it gives an estimate of the interactions involved in the modulated phase, predicting a large increase of intermolecular interactions in biphenyl at low-temperature. Finally it is applied to poly(p-phenylene), predicting a twisted conformation in agreement with recent diffraction results and high reorientational barrier potentials.

ISSN:0889-311X    

DOI:10.1080/08893119608035392

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

This is a scanned image of the original Editorial Board page(s) for this issue.

ISSN:0889-311X    

DOI:10.1080/08893119608035387

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor