Molecular crystals, synthesis

Due to their demanding synthesis, diamondoids are helpful models to study structure-activity relationships in carbocations and radicals, to develop empirical computational methods for hydrocarbons, and to investigate orientational disorders in molecular crystals as well [5,32]. 

J.E. Fischer and a team of researchers (University of Pennsylvania) report, The recent discovery of an efficient synthesis of C and C711... has facilitated the study of a new class of molecular crystals Cfullcrilcs ) based on these molecules ( fullerenes ). In the Fischer reference listed, a study of the compressibility of solid is described. 

A few years after these events, a review article by McCleverty4 in 1968 provided (in over 170 pages) the first thorough overview of this field. This review contains a multitude of details which still merits its mention as a valuable source of information on this class of compounds. Several other reviews dealing with different aspects of dithiolene chemistry have appeared in the meantime.5"11 The field has expanded in different directions, spurred by the synthesis of several structurally unique compounds and of materials of particular use in quite diverse areas such as highly conducting molecular crystals and as infrared dyes for various applications. Dithiolenes thus represent a class of materials which are not only of significance on their own, but which also find some applications in which their unique electronic properties can be employed. 

The latter involves the synthesis of monomers containing LC (mesogenic) groups, with the subsequent homopolymerization or copolymerization with mesogenic or non-mesogenic compounds (Fig. 1), or the attachment of low-molecular crystal... 

T. Caronna, R. Sinisia, M. Catellani, S. Luzzati, L. Malpezzia, S. V Meillea, A. Melea, C. Richter, R. Sinisi, Molecular Crystal Architecture and Optical Properties of a Thiohelicenes Series Containing 5, 7, 9 and 11 Rings Prepared via Photochemical Synthesis, Chem. Mater. 2001, 33, 3906-3914. 

Chiral crystals generated from non-chiral molecules have served as reactants for the performance of so-called absolute asymmetric synthesis. The chiral environments of such crystals exert asymmetric induction in photochemical, thermal and heterogeneous reactions [41]. Early reports on successful absolute asymmetric synthesis include the y-ray-induced isotactic polymerization of frans-frans-l,3-pentadiene in an all-frans perhydropheny-lene crystal by Farina et al. [42] and the gas-solid asymmetric bromination ofpjp -chmethyl chalcone, yielding the chiral dibromo compound, by Penzien and Schmidt [43]. These studies were followed by the 2n + 2n photodimerization reactions of non-chiral dienes, resulting in the formation of chiral cyclobutanes [44-48]. In recent years more than a dozen such syntheses have been reported. They include unimolecular di- r-methane rearrangements and the Nourish Type II photoreactions [49] of an achiral oxo- [50] and athio-amide [51] into optically active /Mactams, photo-isomerization of alkyl-cobalt complexes [52], asymmetric synthesis of two-component molecular crystals composed from achiral molecules [53] and, more recently, the conversion of non-chiral aldehydes into homochiral alcohols [54,55]. 

In this section, I have included a few of my papers dealing with synthesis, defects and certain properties of oxidic materials and fullerenes, besides a general article dealing with important directions in materials chemistry. There is also an article dealing with experimental charge densities in organic molecular crystals. These articles should indicate the diversity and breadth of coverage in materials chemistry. 

Included among the interesting properties currently being pursued in some Zintl phases are the metal-nonmetal transition and colossal magnetoresistance. Zintl phases have also been used as precursors in the synthesis of novel solid-state materials. For example, afullerene-type silicon clathrate compound, Na2BagSi46 (clathrates are covalent crystals, whereas fullerides are molecular crystals), with a superconducting transition at... 

In this entry, the principal chemical features of defect populations (defect chemistry) will be described from the restricted viewpoint of crystalline inorganic solids. The influence of defects upon mechanical properties will be excluded and defects that may have greatest relevance to physical properties will be treated from the point of view of chemical importance. Defects in molecular crystals and amorphous and glassy solids will be omitted see Noncrystalline Solids), as will the important areas of alloys see Alloys), thin films see Thin Film Synthesis of Solids), and carbon nanotubes and related nanoparticles see Carbon Fullerenes). References to the literature before 1994 are to be found in the corresponding article in the first edition of this Encyclopedia. ... 

Luneau, D. and Rey, P. (1995) New manganese(II) complexes of nitronyl nitroxide radicals. Synthesis, structure and magnetic properties. Molecular Crystals and Liquid Crystals Science and Technology Section A, 273, 81-87. 

Supported zeolite membranes have been prepared using numerous procedures [4] such as alignment of crystals in electrical fields, electroplating, self-assembly, growth on organic molecular layers, covalent linkages, hydrothermal synthesis (in situ and ex situ), hydrothermal method microwave heating assisted, dry gel method (vapor-phase transport method and steam-assisted crystallization), synthesis at the interface between two fluid phases, etc. 

Ultimately the aim of molecular modeling is the design (before synthesis) of novel materials. In molecular crystal chemistiy, this requires the ab initio prediction of solid-state structures. Although much progress has been made and the potential of such an ability is enormous, progress in this field has been hindered by problems with global minimization and force-field accuracy. 

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