Skeleton crystals

Figure 2. An octahedron resulting from rapid growth perpendicular to faces of cube and a skeleton crystal resulting from rapid growth along diagonals of cube...

When growth in the direction perpendicular to the face of the cube is rapid, the cube face disappears and an octahedron results. Salt octahedron shape can be obtained under certain conditions—for example, when grown in the presence of urea. When growth in the direction of the diagonal of the cube is rapid, the edges and comers of the cube grow in preference to the center of the cube face and a skeleton crystal results. One common table salt is composed largely of crystals of this type. 

Watson EB, Cherniak DJ (1997) Oxygen diffusion in zircon. Earth Planet Sci Letters 148 527-544 Weis PL (1980) Graphite skeleton crystals—A newly recognized morphology of crystalline carbon in metasedimentary rocks. Geology 8 296-297... 

SiOs Skeleton / Crystal Structures / Dynamic Behavior / Chirality / Isomerization... 

Figure 1.6 P-SijN morphology (a, b) "skeleton" crystals and (c) holohedral crystals.

The morphological pattern of the products of silicon vapor combustion in gaseous nitrogen at condensation synthesis with skeleton crystal formation as well as denchite growth of silicon nitride crystals in melted metal salts proves the existence of the nonequilibrium mechanism of structure formation in the case of SHS. The mechanism appears to be the basis of the conception of nanodispersed particle formation under the combustion mode [28]. 

For aromatic hydrocarbon molecules, in particular, the main acceptor modes are strongly anharmonic C-H vibrations which pick up the main part of the electronic energy in ST conversion. Inactive modes are stretching and bending vibrations of the carbon skeleton. The value of Pf provided by these intramolecular vibrations is so large that they act practically as a continuous bath even without intermolecular vibrations. This is confirmed by the similarity of RLT rates for isolated molecules and the same molecules imbedded in crystals. 

Figure 4.17 Crystal and molecular structure of (LiMe)4 showing (a) the unit cell of lithium methyl, (b) the LijCj skeleton of the tetramer viewed approximately along one of the threefold axes, (c) the 7-coordinate environment of each C atom, and (d) the (4 -I- 3 -I- 3)-coordinate environment of each Li atom. After ref. 93, modified to include Li—H contacts.

We reacted 2 first with bromine in chloroform at 10 C. iH NMR studies have revealed that the reaction mixture was very complex and consisted of six products. This mixture was submitted to silica gel column chromatography. Careful repeated chromatography followed by fractional crystallization allowed us to isolate ten products (Scheme 3). IR analysis indicated that a hydroxyl group was incorporated in compounds lfi-19. Therefore, we assume that these products have been formed by partial hydrolysis of compounds lfl-14. Structural determination of compounds lfl-19 revealed that the barrelene skeleton was rearranged completely. 

Next we studied high temperature bromination of benzobarrelene at 150 C. NMR analysis indicated that the reaction mixture was very complex and consisted of at least ten products. After repeated column chromatography combined with fractional crystallization we have been able to separate 18 compounds (Scheme 6). Four of them were bromoalcohol compounds 18, 12, 22 and 2fl. After high temperature bromination we expected three isomeric non-rearranged products with benzobarrelene skeleton and isolated 22, 22, and 24 in yields of 34, 9.3, and 6.2 %, respectively. Because of the very close structural similarity we were not able to make a clear-cut differentiation between the stereochemistry of 22 and 24-Therefore, we carried out an X-ray analysis (ref. 9) of the isomer 22-... 

A systematic approach to the crystal chemistry of borides is possible on the simple basis of atom size considerations, as well as the tendency of B to form covalent skeletons. 

Figure 1. The crystal structure of ThB4-type tetraborides. (a) The covalent boron skeleton, (b) Atomic arrangement in MB4 (projected along 5).

In the crystal structure of these phases with tetragonal symmetry (P4/mbm, D h) the boron covalent sublattice is formed by chains of octahedra, developing along the c axis and by pairs of B atoms, bonding the octahedra in the xOy plane (see Fig. 1). The resulting three-dimensional skeleton contains tunnels parallel to the c axis that are filled by metal atoms . ... 

Hexaborides of a CaBg type are formed by K, the alkaline earths, Y and the larger lanthanides, as well as Th and some actinides ". The crystal structure of these compounds with cubic symmetry (Pm3m, O, ) (see Fig. 1) is characterized by a three-dimensional skeleton of Bg boron octahedra, the interstices of which are filled by metal atoms. The connection between two octahedra is by a B—B bond of length 1.66 X 10 pm, whereas the B—B bond lengths in one octahedron are 1.76 X 10 pm. ... 

With the chemical structure of PbTX-1 finally known and coordinates for the molecule available from the dimethyl acetal structure, we wanted to return to the natural product crystal structure. From the similarities in unit cells, we assumed that the structures were nearly isomorphous. Structures that are isomorphous are crystallographically similar in all respects, except where they differ chemically. The difference between the derivative structure in space group C2 and the natural product structure in P2. (a subgroup of C2) was that the C-centering translational symmetry was obeyed by most, but not all atoms in the natural product crystal. We proceeded from the beginning with direct methods, using the known orientation of the PbTX-1 dimethyl acetal skeleton (assuming isomorphism) to estimate phase... 

The crystal structures of PbTX-1 dimethyl acetal, PbTX-1, and dihydro PbTX-1 provide a total of four independent pictures of the same brevetoxin skeleton. It is rare that this quantity of structural data is available for a natural product of this size. A comparison of torsional angles shows that all four molecules have approximately the same conformations in all rings, except, of course, for the aldehyde side chain and the E-ring in one of the independent molecules of PbTX-1. Least squares superposition fits among the four molecules gave the following average distances ... 

Therefore we have two extremes in crystal habit in sodium chloride, the octahedron and the skeleton, the prevailing shape depending upon the relative rates of growth in different directions in the crystal lattice. The common cube shape is formed when a balance in the two rates of growth prevails. 

In the crystal, 17+ exhibited no interaction with either counterion or solvent molecules its Si3 skeleton represents a nearly regular triangle with Si-Si bond distances intermediate between the Si=Si and Si-Si bonds of the precursor 18. All NMR chemical shifts of 17+ were practically solvent and counterion independent, which proved its freedom in the solution. The Si NMR resonances of the skeletal Si atoms in 17+ are expectedly strongly deshielded due to the distributed positive charge on them 284.6 and 288.1 ppm. ... 

Figure 9. SEM photographs of polished, etched thin sections of modem Acropora palmata coral (after Edwards 1988). The scale bar in a is 100 microns. Visible in a are large macroscopic pores in the skeleton and well as the textnre of the very fine aragonite crystals, b is the same section as a bnt at higher magnification. The scale bar is 10 microns. Individnal aragonite crystal fibers are visible in b. ...

Because of the superior inclusion properties exhibited by 1 2), one may infer that the positional isomer with reference to the carboxylic group, 7, also behaves as a good inclusion host. However, this is not true. So far, we have not succeeded in isolating any inclusion compound of an uncharged molecule using 7, except those of a salt-like nature (see Sect. 4.2.2.). Obviously the functional groups of 7 are located in the molecule in a way that works against the net bulkiness of the skeleton (connected with the crystal build-up) in 1 they cooperate. 

Special features which characterize the prototypical roof-shaped compound 26 are a rigid molecular skeleton and the type of sensor groups used. They render 26 a potential coordinatoclathrand 50). The capability of 26 in forming inclusion compounds is evident from Table 5. Here, nearly thirty different crystal inclusions of 26 are specified including as guest molecules various alcohols, acids, aprotic dipolar, and rather apolar species. 

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