Structure of Organic Crystals with Molecules Lying on Twofold Crystallographic Axes. Structural Class P21212, Z = 2(2)Zorkii, P.; Zorkaya, O.
doi: 10.1023/A:1010447418177pmid: N/A
Molecular arrangement in crystals of a rare structural class P21212, Z = 2(2), with molecules occupying a system of special positions on 2 axes is considered. The Cambridge Structural Database (CSD) contains 58 organic crystal substances belonging to this class; 54 of these have clear-cut chains Pc2, Z = 1(2) (“shashlyks”) and form two subclasses. In crystals of the first subclass (23 representatives), there are layers possessing one of two remarkable features: molecules in a layer have an extraordinarily high molecular coordination number (m.c.n. is 8 in a layer), or molecules in a layer are linked by halogen...halogen specific short contacts (m.c.n. in a layer equals 4). In crystals of the second subclass (31 representative), layers are absent, and the structure is formed directly from chains.
Molecular Dynamics Simulations for Ice Modifications II and IXZheligovskaya, E.; Malenkov, G.; Averkiev, A.
doi: 10.1023/A:1010403602247pmid: N/A
Molecular vibrations of water (H2O and D2O) in crystals of ice II and ice IX are studied by molecular dynamics in a rigid bond approximation with a fixed bond angle. Using an atom-atomic potential PM for describing the interactions between water molecules in ice II (N = 576 molecules) and ice IX (N = 768) in an NVE ensemble leads to reproduction of the structure of both types of ice. For all water molecules and separately for each system of crystallographically equivalent water molecules in ice crystals, we defined the time dependence of the mean-square displacement of the center of mass of the molecule, the autocorrelation function of velocity for the center of mass, and the autocorrelation function of velocity for hydrogen (deuterium) atoms. The densities of vibrational states are calculated as Fourier integrals of the corresponding autocorrelation functions. In the case of ice II, the densities of states agree well with the experimental incoherent inelastic neutron scattering spectra. In the case of ice IX, agreement is worse. For both polymorphs, the mean-square displacement and the densities of vibrational states of the center of mass of the molecule and the hydrogen (deuterium) atom differ slightly between molecules belonging to different systems of crystallographic positions. This is explained by the difference in their environments.
Isostructural and Nonisostructural Compounds in Series of Halogenated Organic Crystal Substances. Structure of Hal-AggregatesGrineva, O.; Zorkii, P.
doi: 10.1023/A:1010455619086pmid: N/A
The local characteristics and the form of intermolecular Hal aggregates (assemblies of contacting halogen atoms of neighboring molecules), existing in haloorganic crystal substances differing only in the nature of Hal atoms, are compared. Twenty three series of halogenated hydrocarbons involving 57 crystal structures are considered. Pronounced specifics of Hal-aggregates has been established for compounds with low and medium halogen contents. It is found that k(Hal) (coordination number of the Hal atom with respect to the neighboring Hal atoms) generally increases in the series F–Cl–Br–I; for constant k(Hal), <δ> = <ri - 2RHal> nearly always decreases (ri is the distance from the Hal atom in question to one of the k nearest Hal atoms, and R
Hal is the van der Waals radius).
Crystal-Chemical Classes of “Cambridge” Crystal Structures: Statistical Analysis of TopologyZorkii, P.; Oleinikov, P.
doi: 10.1023/A:1010407703156pmid: N/A
Crystals structures with “organic” carbon, i.e., organic and organometal compounds and coordination compounds with organic ligands are systematized based on the results of a statistical treatment of CSD (Cambridge Structural Database) data. The overwhelming majority of CSD structures are molecular crystals, which may be homomolecular or heteromolecular; the latter, in turn, are classified into molecular complexes, salts, crystal solvates, and crystal hydrates. Polymeric (nonmolecular) crystals occur much more rarely. For substances belonging to different crystal-chemical classes, distributions according to space groups and structural classes have been studied; considerable differences between the distributions have been found. In particular, it was established that chiral structures (with P212121, P21, etc. symmetry) are met much more rarely among inorganic CSD structures with metals and metalloids than among organic structures consisting exclusively of organogen elements; the most striking examples are hydrates and salts containing Cl–, Br–, and I– ions.
IR Spectra, Polymorphism, and Intermolecular Interactions in Carboxylic AcidsBabkov, L.; Vedyaeva, E.; Puchkovskaya, G.
doi: 10.1023/A:1010459719994pmid: N/A
This paper summarizes the results of our studies on the structural aspects of polymorphism and mesomorphous states in long-chain aliphatic compounds represented by the homologous series of alkyl-, alkoxybenzoic, and alkylcyclohexanecarboxylic acids and their completely or partially fluorinated analogs. The studies were carried out by vibrational IR spectroscopy (experiment, theory), calorimetry, polarization microscopy, and X-ray diffractometry. Conformational polymorphs have been found for the compounds, which differ in the bending of alkyl and fluoroalkyl radicals and in rotation of carboxyl groups and phenyl rings during phase transitions. Structural rearrangements and dynamic changes take place inside the nuclei of H-complexes. The dimers are partially dissociated, and open associates, chain associates, and monomers differing in their conformational composition are formed.
Synthesis and Investigation of Polysubstituted Triphenylenes with a Predictable Type of MesomorphismZemtsova, O.; Syromyatnikova, O.; Kotovich, L.; Akopova, O.
doi: 10.1023/A:1010411804065pmid: N/A
This paper reports on synthesis and investigation of columnar mesophases of the new members of the homologous series of nitro and amino derivatives of hexaalkoxytriphenylenes, whose mesomorphism was predicted earlier. The forecast is in good agreement with the experimental data. The effect of the introduction of the donor and acceptor groups into the central fragment of triphenylene on the phase transition temperature, texture, and range of existence of columnar mesophases is examined. The predicting ability of the molecular parameters suggested previously for the series under analysis is discussed. It is found that introduction of a nitro group, which is a strong electron acceptor, into the central fragment of triphenylene lowers the melting temperature of the compound versus its analog and does not promote columnar mesomorphism in the lower nonmesomorphous homologs. At the same time, this slightly expands the range of existence of the mesophase in higher homologs.
Forecasting Columnar Mesophases. Synthesis and Structure of Porphin DerivativesAkopova, O.; Zdanovich, S.; Akopov, D.; Aleksandrov, A.; Pashkova, T.
doi: 10.1023/A:1010463820903pmid: N/A
This paper examines the possibility of columnar mesophase (CM) predictions in a new series of porphin derivatives: polysubstituted phthalocyanines (I) and porphyrins (II, III). In order to reveal discogens, we have calculated the molecular parameters K, Kc, Ks, Kp, Mm, and Mr for 85 molecules, including compounds with few peripheral substituents. Introduction of a new electronic parameter, Ke, defining the electron density distribution from the center to the periphery of the discogen molecule, is discussed. Some of the compounds were synthesized, and their mesomorphism and structure were investigated in order to be able to verify the conclusions concerning CM prediction in the series of compounds under analysis.
Structural Specifics of the Concentration Dependence of Sound Velocity in Aqueous Solutions of ElectrolytesBalankina, E.; Lyashchenko, A.
doi: 10.1023/A:1010415904973pmid: N/A
The concentration dependences of sound velocity in aqueous solutions of electrolytes are analyzed over a wide range of concentrations (with extrapolation to 100% salt). For many systems, the concentration dependence of the square of sound velocity is linear up to the values corresponding to overcooled salt melts. For these solutions, an expression for partial molar compressibility of salts in infinitely dilute solutions
$$\bar K_{2s}^0 $$
is derived based on the additivity of the squares of sound velocities of the solution components, where the salt component is an overcooled melt. For alkali halides, there is good agreement between the values of
$$\bar K_{2s}^0 $$
calculated in this way and the values obtained by extrapolation of the apparent molar compressibility in the low-concentration region using the concentration dependences of density and the solution compressibility coefficient. For systems of this type, where hydration is the dominant process, sound velocity in hydration shells and complex ion groups is constant (i.e., independent of concentration). The effects are interpreted in terms of the concept of redistribution of the “fixed” structural groups in two concentration zones of a solution, where 1) the hydration groups are built in the structure of water in a complementary way; 2) the fragments of hydration spheres and the ionic clusters are united with each other in a complementary way.