Crystal Engineering最新文献

Pictorial molecular orbital theory, in a qualitative approach, is used in order to identify possible structures of the red chromophore in ultramarine red pigments. The known blue S₃^−⋅ molecule is assumed to be the starting reagent in a concerted reaction to form the S₄ molecule. Different proposed structures for the S₄ species are tested. Applying the frontier orbital symmetry rules of Woodward and Hoffmann eliminates the tetrahedral S₄ structure as a possibility.

Crystal structures, absolute configurations, and crystalline packing features of nine complexes, namely, cis-α-Λ-(RR)(δλδ)-[Co(trien)(D-histidinato)](ClO₄)₂·2H₂O 1, cis-β₁-Δ-(RR)(λδδ)- [Co(trien)(L-asparaginato)](ClO₄)₂ 2, cis-β₂-Λ(SS)(λλδ)-[Co(trien)(L-valinato)](ClO₄)₂·H₂O 3, cis-β₂-Δ-(RR)(δδλ)[Co(trien)(D-methioninato)](ClO₄)₂ and its enantiomer 4, trans(N,t-N)-[Co(tren)(DL-leucinato)]X₂, (X=ClO₄⁻ 5, BF₄⁻ 6, PF₆⁻ 7), and trans(N,t-N)-[Co(tren)(DL- methioninato)]X₂ (X=Br⁻ 8, Cl⁻(BF₄⁻) 9) (trien=triethylenetetramine, tren=tris(2-aminoethyl)amine), have been determined. All compounds were prepared from racemic DL-amino acid. 1–3 crystallize as conglomerates. 5–7 are isomorphous and crystallize as the so-called conglomeratic solids. While 4, 8 and 9 undergo racemic crystallization. In 1–7, the carboxylic oxygen of the amino acid forms double hydrogen bonds with the amino hydrogen atoms of N4 and/or amino acidato of an adjacent cation. By these hydrogen bonds, cations having the same chirality are linked together into helical string. In the isomorphous 8 and 9, the cation containing L-methionine interacts with a cation containing D-methioninine, through hydrogen bonds, to form a racemic pair, and no spiral string arrangements are observed.

The structure of [NH(CH₃)₃]₃Sb₂Cl₉, tris(trimethylammonium) nonachlorodiantimonate(III) (TMACA) has been determined at 295 K and 373 K, below and above the high temperature ferro-paraelectric phase transition. In both phases the anionic sublattice of TMACA is built of characteristic two-dimensional (Sb₂Cl₉³⁻)_n polyanionic layers lying in the bc plane. In room temperature, ferroelectric phase (monoclinic, Pc space group) there are three crystallographically non-equivalent trimethylammonium [NH(CH₃)₃]⁺ cations. Two of them are located between polyanionic layers and the third one, disordered, inside the cavity formed by six SbCl₆³⁻ octahedra. In the high temperature paraelectric phase (monoclinic, P2₁/c space group) there are only two independent trimethylammonium cations in the structure. One of them is located between the inorganic layers, while the other one is placed inside the polyanionic cavity. Both cations are disordered. Temperature dependencies of the lattice parameters, determined between 295 and 375 K, confirmed the presence of phase transition(s) in TMACA in the high temperature region. It was found that the ferro-paraelectric phase transition is associated with the triggering of the overall reorientation of trimethylammonium cations located inside the polyanionic cavities.

Two new copper complexes with triethanolamine (H₃L) have been prepared using copper powder and characterized by X-ray crystallography, IR and EPR spectroscopy. These are [Cu^I(H₃L)Cl]_x⋅[Cu^II(H₂L)Cl]_1−x (X=0; 2/3) (1) and [Cu^II(H₂L)SCN] (2). Both complexes are neutral species and contain copper atoms in trigonal bipyramidal environments. The important dimensions in 1 are Cu-N 2.011(5), 3Cu-O 2.073(3), Cu-Cl 2.239(2) Å and N-Cu-Cl=180.0°; in 2 Cu-N_NCS 1.918(3), Cu-N_H2Tea 1.983(3), Cu-O 2.004(2), 2Cu-O 2.13(1) Å and N_H2Tea-Cu-N_NCS=174.6(1)°. 2 has all the copper atoms in the oxidation state II, while the three-fold symmetry in 1 would suggest that the metal atoms are formally in the oxidation state I. Results of magnetic measurements of several samples of 1 are rationalized by assuming that a continuous range of composition for [Cu^I(H₃L)Cl]_x⋅[Cu^II(H₂L)Cl]_1−x can occur. In the case of x=0, 1 becomes a polymorph of the reported compound [Cu^II(H₂L)Cl].

Different substances can phenomenologically be defined by use of Gibbs' phase rule. In quantum mechanics, on the other hand, the notion of a substance is not clearly understood: The thermal density operator D_β corresponding to some Hamiltonian and some chosen inverse temperature β is uniquely defined, which implies that different isomers exhibit the same thermal density operator. Coexistence of isomers, substances or phases at some temperature, on the other hand, would need different thermal density operators, one for each isomer, substance or phase. Here we try to understand this problem for the classical van der Waals gas using large deviation statistics. We show that the gaseous and liquid phase of the van der Waals gas emerge with increasing numbers of particles. Intermediate states — neither gaseous nor liquid — exist but die out with increasing number of particles. Extension of our method to quantum mechanics is not straightforward, but looks promising.

The importance of obtaining reliable detection systems for enantiomers' assays increases with the necessity of chiral discrimination between the enantiomers of raw materials from the pharmaceutical industry. The utilization of electrochemical sensors in molecular recognition of chiral substances becomes a very accurate and precise alternative for the structural analysis as well as for the chromatographic techniques. The reliability of the response characteristics as well as of the analytical information obtained by using electrochemical sensors is strictly correlated with the design of the sensors. The most reliable design is that of carbon paste based sensors; this design was adopted for the construction of potentiometric, enantioselective membrane electrodes as well as for the construction of the amperometric biosensors, and immunosensors. However, it is also necessary to look for more reliable chiral selectors.

The Raman spectra of G-9 garnets from kimberlites and their relationship to compositional changes, and therefore changes in their cell edges, are evaluated. Increasing CaO and Cr₂O₃ cause the cell constant to become larger and positions of Raman bands to shift. G-9 garnets have characteristic Raman spectra but the application of Raman spectroscopy as sole identification tool, without the need for micro-analytical techniques, requires further research.

The 173K structures of the α, β and γ phases of o-ethoxy-trans-cinnamic acid (OETCA), which crystallize in $\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>̄</mtext>}}$, $\text{R}{}_{\mathrm{<mtext>3</mtext><mtext>̄</mtext>}}$ and C_2/c respectively, are reported here. The common building block in all the structures is hydrogen bonded OETCA R²₂(8) centrosymmetric dimer pairs. The α polymorph forms a layered structure and the γ polymorph a herringbone structure. Both of these phases are made up of hydrogen bonded OETCA dimers that are further assembled through C-H· · · O interactions to form layers or ribbons of OETCA molecules. The main difference between these structures is the 3-D assembly of the layers. The β phase is actually a benzene solvate or host–guest complex (1:6 benzene: OETCA). Benzene, which occupies the $\text{3}\text{̄}$ site at (0,0,1/2) in the trigonal unit cell, is essential for the stabilization of this phase. It provides a template for the spiral arrangement of the OETCA molecules required for the formation of the β-photo-dimerization product.

The same structural principles based on scale-rotational symmetries observed in snow crystals also occur in nucleic acids with helical structure. Due to the axial symmetry, a point group characterization implies a 2-dimensional description in the projected axial view. This allows a comparison with snow crystals. Three aspects are involved in the present approach: a geometric one, based on the polygrammal symmetries of scaled forms (growth forms and enclosing forms, respectively); an arithmetic one, allowing an assignment of rational indices to planes (or lines) and to special points of the scaled forms and to a selected set of atomic positions; and an algebraic group-theoretical one, by means of a scale-rotation group relating the planes (or lines) and the special points of the scaled forms and the selected atomic positions involved in the structural relations.

In most III–V and II–VI compounds with the $\text{F}\text{4}\text{̄}\text{3}\text{m}$ zinc-blende structure, such as GaAs, CdTe, InP, and also in fcc alkali halides and solid Xenon, the experimental reflectivity peaks near the first Λ and L-transitions are enhanced and probably caused by excitons. In CdTe the reflectivity drops sharply at 3.46 and 4.03 eV and this is to be associated with the electron-hole bound state at the L point and the near by Λ symmetry line. The total symmetry of L and Λ excitons are fully determined by the irreducible representations contained in L₆×L_4,5 and Λ₆×Λ_4,5, and these follow from momentum conservation and selection rules. We have determined the allowed L and Λ exciton momenta. The proper exciton wave function symmetries needed in the variational method for exciton binding energies have been determined by Vector-Coupling Coefficients (VCCs), (known also as the Clebsch–Gordan Coefficients (CGCs)), for the direct non parabolic excitons of the L₆×L_4,5 symmetry. The CGCs can be helpful in the evaluation of intensity of optical transitions, Raman scattering tensors and effective Hamiltonians.