Preparation and Structural Evaluation of the Conformational Polymorphs of -[(4-Methoxyphenyl)methylene]-4-nitrobenzeneacetonitrile
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Crystals of MG SRC="/images/gifchars/alpha.gif" BORDER=0>-[(4-methoxyphenyl)methylene]-4-nitrobenzeneacetonitrile (C16H12O3N2) are shown byX-ray crystallography to exist in three polymorphic trans forms (I-III) and one cis configuration (IV). Calorimetricstudies show that at low temperature form I is thermodynamically the most stable phase, followed by form III andform II. On heating, form III undergoes a phase transformation to form II. There is no direct thermal transformationbetween forms II and I. Form I melts at 164.5 mages/entities/deg.gif">C. Mechanical damage of form II initiates a phase transformationinto form III, and structural studies confirm that forms II and III, both of which show attractive second-order nonlinearoptical behavior, are closely related. Though clearly conformational polymorphs, they also exhibit the essence oforientational polymorphism. Although form I is the most stable of the three trans isomers, nevertheless forms IIand III are indefinitely stable and may be useful for development for NLO purposes. Crystal growth studies showthat growth from the melt and vapor phases could be the only routes for the preparation of specimens for opticalexamination. The results show well the difficulties involved in the preparation and isolation of the differentpolymorphic forms of such materials.
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