L. N. Kuleshova

New similarity index for crystal structure determination from X-ray powder diagrams

A new similarity index for automated comparison of powder diagrams is proposed. In contrast to traditionally used similarity indices, the proposed method is valid in cases of large deviations in the cell constants. The refinement according to this index closes the gap between crystal structure prediction and automated crystal structure determination. The opportunities of the new procedure have been demonstrated by crystal structure solution of un-indexed powder diagrams of some organic pigments (PY111, PR181 and Me-PR170).

Polymorphism and design of noncentrosymmetric crystals of 4-hydroxybenzaldehyde-4-nitrophenylhydrazone and N′-(2-phenyl-1H-indole-3-aldehyde)-4-nitrophenylhydrazone

The products of crystallization of 4-hydroxybenzaldehyde-4-nitrophenylhydrazone (I) and N′-(2-phenyl-1H-indole-3-aldehyde)-4-nitrophenylhydrazone (II) from different solvents are studied by X-ray diffraction with the aim of examining the polymorphism of hydrazone derivatives. The structural features of the crystals prepared are analyzed. It is shown that, in the case when molecules of organic compounds with a high molecular hyperpolarizability are capable of forming stable acentric supramolecular associates in the crystal, knowledge of their polymorphism offers strong possibilities for designing the noncentrosymmetric crystal structure necessary for a manifestation of the nonlinear optical activity.

New force field for molecular simulation and crystal design developed based on the “data mining” method

The Data Mining algorithm is used to analyze the information of the Cambridge Structural Database (CSD) with the aim to develop a force field which describes intermolecular interactions. The force-field parameters obtained are successfully tested in calculations of sublimation energy; in construction of polar crystals; in prediction of crystal packings, including predictions based on X-ray powder diffraction data; and in prognosis of protein-ligand interactions and stability of polymorphs. The parametrization developed may also be used in other programs. The parameters are given for all the atomic species encountered in the CSD together with the instructions for their use.

General structural features of centric and acentric polymorphic pairs of organic molecular crystals: I. Polymorphic pairs P21/c-P212121

A systematic investigation into the general structural features of centric and acentric polymorphic pairs of organic molecular crystals is performed using the data available in the Cambridge Structural Database. The main regularities of the formation of these modifications are revealed. The inference is made that the intermolecular association plays an important role in the formation of crystal packing.

Formation of crystal structure of cyanovinylbenzene derivatives: X-ray structure investigation of ethyl esters of 3,4-dimethoxy-, 3,4,5-trimethoxy-, and 4-fluorobenzylidenecyanoacetic acids

Ethyl esters of 3,4-dimethoxy-, 3,4,5-trimethoxy-, and 4-fluorobenzylidenecyanoacetic acids are investigated by X-ray diffraction analysis with the aim of elucidating the factors responsible for the formation of crystal structures of cyanovinylbenzene derivatives. The crystal data for other known derivatives of this series are analyzed. It is revealed that, in the absence of strong hydrogen bonding, the main structure-forming function in crystals is fulfilled by intermolecular contacts of two types, namely, staking contacts of parallel and antiparallel molecules. The staking contacts of antiparallel molecules are most frequently realized. It is assumed that their presence in the structure determines the prevailing formation of centrosymmetric packings in the compounds under consideration.

X-ray structure investigation of 1-acetoxy-1-cyano-2-naphthylethylene and 1,1-dicyano-2-naphthylethylene

The crystal structures of 1-acetoxy-1-cyano-2-naphthylethylene (I) and 1,1-dicyano-2-naphthylethylene (II) are determined by X-ray structure analysis. Crystals I are monoclinic; at 25° C, the unit cell parameters are as follows: a = 17.308(6) Å, b = 4.507(1) Å, c = 17.845(5) Å, β = 107.90(2)°, V = 1324.7(7) Å3, dcalcd = 1.260 g/cm3, Z = 4, and space group P21/n. Crystals II are monoclinic; at 25°C, the unit cell parameters are a = 3.827(1) Å, b = 15.784(4) Å, c = 17.226(2) Å, β = 91.22(2)°, V = 1040.3(4) Å3, dcalcd = 1.304 g/cm3, Z = 4, and space group P21/n. It is revealed that, in crystal structures of I and II, the molecular stacks characteristic of compounds of this series are formed through stacking contacts along the direction of the smallest lattice parameter.

A method for automated determination of the crystal structures from X-ray powder diffraction data

An algorithm is proposed for determining the crystal structure of compounds. In the framework of this algorithm, X-ray powder diffraction patterns are compared using a new similarity index. Unlike the indices traditionally employed in X-ray powder diffraction analysis, the new similarity index can be applied even in the case of overlapping peaks and large differences in unit cell parameters. The capabilities of the proposed procedure are demonstrated by solving the crystal structures of a number of organic pigments (PY111, PR181, Me-PR170).

Conformational polymorphism of N-(4-butoxyphenyl)-4-(4′-nitrophenyl)-2-thiazolamine

The molecular and crystal structures of three concomitant polymorphic modifications of N-(4-butoxyphenyl)-4-(4′-nitrophenyl)-2-thiazolamine are investigated by X-ray diffraction. These polymorphs are simultaneously formed upon recrystallization from toluene. It is established that different colors of the polymorphs are associated with the different conformations of the molecules in the crystal. The relative energy gain of the conformers and the energy of crystal packings are evaluated, and the possibility of occurring the structural phase transition between the polymorphic modifications is discussed.

Ferrocenylbiphenylyl- and ferrocenylterphenylyl-containing liquid crystals: Solid-phase precursors, structure, and properties

This paper reports on the results of investigations into the liquid-crystal properties and structure of compounds belonging to a new class of mesogens containing monosubstituted ferrocenyl fragments, such as 4′-ferrocenyl-1,1′-biphenyl- and (terphenyl)-4-yl 4-alkoxybenzoates of the general formula Fc-(C6H4)m-O2C-C6H4-O-CnH2n+1 (m = 2, 3; n = 6, 8, 10, 12) and 4-alkoxyphenyl 4′-ferrocenyl-1,1′-biphenylyl-4-carboxylates of the general formula Fc-(C6H4)2-CO2-C6H4-O-CnH2n+1 (n = 10, 12). The crystal packings of the ferrocene-containing mesogens are theoretically predicted and calculated, and the results obtained are discussed.

General structural features of centric and acentric polymorphic pairs of organic molecular crystals: II. Polymorphic pairs P21/c-P21, P21/c-Pc, and P21/c-Pna21

A systematic investigation into the general structural features of centric and acentric polymorphic pairs of organic molecular crystals is performed using the data available in the Cambridge Structural Database. The structural aspects of the P21/c-P21, P21/c-Pc, and P21/c-Pna21 polymorphic pairs are considered. The most important inferences made in this study are as follows: (i) the centric and acentric modifications rather frequently (∼50% of all the cases) crystallize simultaneously under the same conditions (concomitant polymorphism); (ii) the unit cell parameters of these modifications are found to be either close in magnitude or multiples; (iii) in the centric and acentric modifications, identical or very similar types of chiral or acentric stable molecular associates are formed through hydrogen bonds or other specific intermolecular interactions; and (iv) the centric and acentric modifications more often than other compounds are characterized by polysystem unit cells (Z′ > 1).