Bond, Link hydrogen

In the initial state, the bonds are between the hydrogen and iodine atoms, H-I. In the activated complex, four weak bonds link the four atoms into a deformed square. In the final state the bonds link hydrogen to hydrogen, H-H, and iodine to iodine, I-I. 

FlC. 32.—Antiparallel packing arrangement of the 2-fold helices of calcium chondroitin 4-sulfate (35). (a) Stereo view of two unit cells approximately normal to the he-plane. The two comer chains, drawn in filled bonds are hydrogen bonded to the antiparallel center chain (open bonds). Calcium ions (crossed circles), associating with sulfate and carboxylate groups and water molecules link adjacent antiparallel chains, which ate also directly hydrogen bonded. 

The molecules (or atoms, for noble gases) of a molecular solid are held In place by the types of forces already discussed In this chapter dispersion forces, dipolar interactions, and/or hydrogen bonds. The atoms of a metallic solid are held in place by the delocalized bonding described in Section 10-. A network solid contains an array of covalent bonds linking every atom to its neighbors. An ionic solid contains cations and anions, attracted to one another by electrical forces as described in Section 8-. 

Sila-biperidin rac-(SiRS, C2 SR), 35, can be prepared by hydrolysis of the corresponding methoxysilane and behaves in a similar way to sila-procyclidine The exo form has centrosymmetric dimers formed via a pair of intermolecular OH---N hydrogen bonds (0---N distance 2.810 A), while the endo form has intermolecular OH---N hydrogen bonds linking the molecules into infinite chains (222). 

The crystal structures of both the cis and the trans isomers of 2,8-dihydroxy-2,4,4,6,6,8,10,10,12,12-decamethylcyclohexasiloxane have been determined. In this case (unlike the cyclotetrasiloxane in Fig. 26), the cis isomer does contain an intramolecular hydrogen bond, and intermolecular hydrogen bonds link the molecules into cyclic pairs 57. The trans isomer cannot form intramolecular hydrogen bonds, but forms cyclic tetramers which are further hydrogen-bonded to form infinite sheets 58 (280). 

The simple diols R2Si(OH)2 (R = Pr , Bul, c-C6Hn, and o-tolyl) form similar, but not identical, hydrogen-bonded chains. The structures of the Pr1 (305), Bu (40, 278), and c-C6Hn (292) compounds are very similar in that they comprise ladder chains of the general type 65, in which pairs of molecules are linked to form dimers and then further hydrogen bonds link the dimers to form the chains. [It has also been... 

These exceptional properties, exclusive to water, result from the structure of its molecule and the nature of the bonds (links) between its constituent atoms. The molecule of water is made up of two atoms of hydrogen bonded to one of oxygen, so that its formula is H20. The composition gives no clue as to its unusual properties, however. It is the special architecture... 

N,A -bis(2-phyidylmethyl)pyrazine-2,3-dicarboxamide was crystallized in two orthorhombic forms, which differ from each other in the conformational arrangements of the compound [44], A new polymorph of 2-bromo-5-hydroxybenzaldehyde has been reported where a pair of hydrogen bonds linked molecules related by a center of inversion [45], One of the intermediates important in the synthesis of an antituberculosis drug, methyl p-aminobenzoate, was found to crystallize in a new monoclinic polymorph where molecules were arranged in head-to-tail linear ribbon arrays [46]. 

Two molecules with comparable geometry in an asymmetric unit were found for 3,4-bis(4-fluorophenyl)-l,2,5-oxadiazole 2-oxide. The bond length of the dipolar N-O bond is 1.107 (7) A <2006AXEo4827>. In the molecule of 5-(6,7-dimethoxy-l,2,3,4-tetrahydroisoquinolin-2-yl)-4-phenyl-l,2,5-oxadiazole Ar-oxide, the six-membered heterocyclic ring has a flattened boat form. Intermolecular C-H- O hydrogen bonds link the molecules into dimers, which may be effective in the stabilization of the crystal structure <2006AXEo3130>. 

During the course of the last century, it was realized that many properties of solids are controlled not so much by the chemical composition or the chemical bonds linking the constituent atoms in the crystal but by faults or defects in the structure. Over the course of time the subject has, if anything, increased in importance. Indeed, there is no aspect of the physics and chemistry of solids that is not decisively influenced by the defects that occur in the material under consideration. The whole of the modem silicon-based computer industry is founded upon the introduction of precise amounts of specific impurities into extremely pure crystals. Solid-state lasers function because of the activity of impurity atoms. Battery science, solid oxide fuel cells, hydrogen storage, displays, all rest upon an understanding of defects in the solid matrix. 

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