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A single crystal to single crystal phase transformation in (3-cyclopentylpropanoyl)dipivaloylmethane

Over the previous decades the quantity of evidence of the mobility of molecules within the crystal phase increased dramatically. The extensive research of crystal disorder, phase transitions and chemical reactions in the crystalline state have almost completely done away with the classical prejudice of crystals as chemical graveyards, consisting of motionless molecules frozen in a stiff periodical array. (3-cyclopentylpropanoyl)dipivaloylmethane (dp(cpp)m) is a 1, 3, 3'-triketone which crystallises in the space group P-1 with two independent molecules in the asymmetric unit. One of these exhibits a statistical disorder of the cyclopentyl ring which is disordered over two orientations with approximately perpendicular mean ring planes. The molecules pack in columns with approximately collinear dipoles of the triketo groups. The ordered and the disordered molecules are arranged alternatively in the columns. Also alternating within the column is the chirality of the triketo group conformation, with each P-molecule neighboured by two M-molecules and vice versa. Upon gradual cooling of the crystal, a significant change in the unit cell parameters occurs between 265 and 235 K. The most striking difference is the reduction of one of the unit cell parameters by ca 50%, reducing the unit cell volume by a half. The data for structure determination was collected once more at 100 K. The low temperature polymorph is also of P-1 symmetry, but with only one molecule per asymmetric unit and with no structural disorder. More noteworthy, the chirality of all the molecules packed in a column became the same. The phase transition therefore included not only the ordering of the structure, but also the inversion of the conformation of every alternate molecule. The fact that the resulting material was still a single crystal provides evidence of an extensive vibrational mobility of molecules in the crystalline phase of dp(cpp)m. This is enabled by the lack of directed intermolecular interactions in the crystal phase of this compound, but also by the similarity of geometries of the two conformers, as well as the intermediate conformations. This demonstrates that aliphatic 1, 3, 3'-triketones are good model compounds for the study of the molecular mobility in the crystalline phase.

triacylmethanes; solid state phase transition; single crystal; conformational chirality

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