Planar array

Orthoboric acid, B(OH)3, is the normal end product of hydrolysis of most boron compounds and is usually made ( 160 000 tonnes pa) by acidification of aqueous solutions of borax. Price depends on quality, being 805 per tonne for technical grade and about twice that for refined material (1990). It forms flaky, white, transparent crystals in which a planar array of BO3 units is joined by unsymmetrical H bonds as shown in Fig. 6.25. In contrast to the short O—H O distance of 272 pm within the plane, the distance between consecutive layers in the ciystal is 318 pm, thus accounting for the pronounced basal cleavage of the waxy, plate-like ciystals, and their low density (1.48 g cm ). B(OH)3 is a very weak monobasic acid and acts exclusively by hydroxyl-ion acceptance rather than proton donation ... 

The molecular structure of yellow crystalline Xe(OTep5)4 has been determined by X-ray analysis (see Pig. 18.3b) / the Xe atom is surrounded by a square-planar array of four O atoms, with the adjacent TeFj groups pointing, curiously, pair-wise up and down from this plane (Xe-O 203.9(5) and 202.6(5) pm, Te-O 188.5 pm). 

Cytochromes. A cytochrome is a protein containing a heme with an iron cation bonded to four donor nitrogen atoms in a square planar array. Figure 20-29a shows the structure of cytochrome c, in which a histidine nitrogen atom and a cysteine sulfur atom occupy the fifth and sixth coordination sites of the octahedral iron center. 

Since modern FTIR spectrometers can operate in a rapid scan mode with approximately 50 ms time resolution, TRIR experiments in the millisecond time regime are readily available. Recent advances in ultra-rapid scanning FTIR spectroscopy have improved the obtainable time resolution to 5 ms. Alternatively, experiments can be performed at time resolutions on the order of 1-10 ms with the planar array IR technique, which utilizes a spectrograph for wavelength dispersion and an IR focal plane detector for simultaneous detection of multiple wavelengths. ... 

The l,5-dibora-2,5-cyclohexadiene system (7) is of unusual interest because such a planar array of atoms is predicted to be a Hiickel antiaromatic system, similar to boroles themselves. Indeed, when 103 is generated at low temperatures, it readily rearrange into the nWo-2,3,4,5-tetracarba-hexaboranes6 (8)" (Eq. 33). 

Additional neutron scattering studies on different polymer systems could prove very important. Strobl [31,32,47,103] provides evidence that, for some polymers, lamellar crystallization is preceded by pre-ordering of the melt followed by formation of planar arrays of blocks. Investigating crystallization from the melt, Kaji and coworkers [25] find pre-ordering phenomena relating to orientational fluctuations of stiff polymer segments which, under appropriate conditions, determine phase separation prior to crystallization. 

Heterocyclic carbenes have been applied as ligands to gold(i) and gold(m) with a wide variety of heteroatoms and substitution patterns.256,257 Gold(i) forms 1 1 and 1 2 complexes with a linear coordination geometry, while at gold(m) centers, carbenes appear only in combination with other ligands in a square-planar array. 

Ligand (69) coordinates to nickel such that the four donors and the metal ion form a planar array whereas (68) coordinates around one face of an octahedral arrangement. Each complex type exhibits a characteristic kinetic inertness which no doubt arises from the operation of the macro-cyclic effect. Indeed, because of the inertness of the cation [Ni(tri)(H20)3]2+, its resolution into optical isomers has been possible... 

Ligand (119) yields nickel complexes of type [Ni2L](BF4)4 (low spin) and [Ni2L(NCS)4].2H20 (high spin). For each of these complexes, physical studies indicate that the macrocycle circumscribes the two nickel ions such that each ion is surrounded by four sulfur donors in a planar array. 

Water on Hallovsite. Central to the controversy is the observation that clay crystals present a planar array of oxygens (and hydroxyls in the case of kaolinite) which have hexagonal (or nearly) symmetry with a periodicity similar to that found in the crystal structure of ice. Because of this geometric similarity, it has frequently been assumed that water adsorbed on a clay surface will preferentially adopt an ice-like configuration. When looked at in detail, it is difficult to find unequivocal evidence to support this. 

The practice of considering the catalyst as a featureless surface or a planar array of atomic centers deprives theory of an adequate concern for the geometry of the transition from reactants to products. Balandin (23) recognized the importance of the concept of a transition state to the development of a mechanistic theory of catalysis, and in his hands the multiplet theory proved fruitful. However the directional properties of binding orbitals, a subject of more recent development, apparently has not been incorporated into his theory. 

The disadvantage of the nitrocellulose (NC) slide relates to resolution because biomolecules more easily diffuse from the surface than with planar arrays. Also, membranes suffer from considerable light scatter and higher intrinsic fluorescence, which are problematic for increased sensitivity. Membranes are better suited for detection of colorimetric or chemiluminescence reporters. Pall introduced a similar product, the Vivid GeneArray slide, based upon nylon-impregnated slides. 

The sp hybrid orbitals are distributed in a planar array around the atom this spacing minimizes any interactions. The 2p orbital is then located perpendicular to this plane. Such information is again obtained from the mathematical analysis, but simple logic would lead us to predict that this is the most favourable arrangement to incorporate the components. The sp orbital will be similar in shape to the sp orbital, but somewhat shorter and fatter, in that it has more character and less p character (Figure 2.14). 

Thus, pyrrole and acetone react as shown above. This involves pyrrole acting as the nucleophile to attack the protonated ketone in an aldol-like reaction. This is followed by elimination of water, facilitated by the acidic conditions. This gives an intermediate alkylidene pyrrolium cation, a highly reactive electrophile that reacts with another molecule of nucleophilic pyrrole. We then have a repeat sequence of reactions, in which further acetone and pyrrole molecules are incorporated. The presence of the two methyl substituents from acetone forces the growing polymer to adopt a planar array, and this eventually leads to a cyclic tetramer, the terminal pyrrole attacking the alkylidene pyrrolium cation at the other end of the chain. 

Besides electron-phonon coupling, the shape of the Fermi surface is important in deciding CDW formation. Fermi surfaces of systems containing linear or planar arrays of atoms reflect their one- or two-dimensional character. Since the Fermi surfaces connect many states with the same wave vector, Q, a periodic distortion having the wave vector Q will produce gaps at those portions connected by Q. The energy gained... 

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