Crystal structures, sketched from

This sketch from Pauling s research notes shows a proposed structure for a silicate crystal—the type of complex molecular structure impossible to solve before Pauling developed a new approach. 

Figure 6 (a) Sketch of the chain arrangement in shish-kebab structures. Reproduced from [67] with permission, (b) Polyethylene shish-kebabs crystalled from solution [68]. Reproduced with permission from H. D. Keith, 1996 American Institute of Physics, (c) Mannan-lamellae, epitaxially grown on cellulose fibres [69]. Reproduced from H. Chanzy, reprinted with permission from John Wiley Sons, Inc. 

Figure 33 (a) Sketch of a pseudo-polyrotaxane from a bis(biphenyl-phenylene) cyclophane threaded on phenylene-l,4-bis(oxy(oligoethyleneoxy)carbonic acid), (b) Space filling model, calculated from crystal structure data [385], With permission of Wiley-VCH... 

Figure 3.66. Sketch of a kaolinlte crystal, (a) the basic double sheet of A1 octaihedrons and SI tetrahedrons, (b) The crystal structure, as built up of such double sheets. (Redrawn from R.J. Hunter, Foundations oj Colloid Science, Vol. 1, Oxford Univ. Press (1987).)...

In the remainder of this section, three different types of chemical reaction will be considered in more detail nucleophilic addition, nucleophilic substitution and an electron transfer reaction via change of coordination state. Structural evidence from protein structure determinations is complemented by results of structure correlations, derived from small molecule crystal data, to get a more detailed picture of the elementary steps during enzyme reaction and to sketch possible modes of inhibition. 

Fig. 7. Sketch of the copper coordination polyhedron obtained from the 2SOD crystal structure. The copper and zinc atoms and the water molecule are represented by spheres of arbitrary radius.

In general, stereochemical models have been extrapolated from crystal structures of catalyst precursors, usually formulated in the chemist s mind on the basis of observation and intuition, and then sketched with pencil and paper. Experiments were then designed to test the model, and the model was refined in light of the new experiments. Simply seeing the active site and iteratively adding steric encumbrances to achieve a desired effect have been enough to advance the field for nearly two decades. 

A half century after the discovery of Hofmann s benzene compound. Powell verified Hofmann s view by determining the crystal structure of the complex,a modernized sketch of which is shown in Fig. 1. The host has a layer structure involving stacking of 2D networks linked by cyano ligands, in which a network of square planar [Ni(CN)4] anions play the role of an N-connected cross-shaped linker to four octahedral Ni(II) cations at the respective N ends a pair of the ammine ligands protrude up and down from the octahedral Ni(II) cation in the... 

Fig. 2.12 The hydrogen bonding and lithium ion coordination of the CO2 radical in the crystal structure of HC02Li-H20, in a projection viewed down . The lithium ions and the protons giving rise to the hyperfine couplings reported by Vestad [29] are explicitly marked. AU distances are in A (10 ° m). Tlie dipolar hfc is composed of contributions from spin densities at three atoms as indicated by the the sketch to the right. The figure is reproduced from [29(c)] with permission from Dr. T.A. Vestad...

The diacetylene with R = R = paratoluylsulfonyloximethylene is termed TS6 (sometimes TS). It was shown by G. Wegner and his co-workers in a series of works, pubhshed in the early 1970s [1, 7], that large molecular crystals can be grown from a solution of TS6, e.g. in acetone, and that these monomer diacetylene crystals can be converted by a topo-chemical (or solid state) 1,4-addition reaction to the polydiacetylene single crystals (Fig. 9.2). The crystal structures of TS before and after the reaction have been investigated in detail by Kobelt and Paulus [8], Bloor et al. [8], and Enkelmann [9] and are sketched in Fig. 9.3 and in Tab. 9.1. 

While from a structural point of view metal/solution and metal/vac-uum interfaces are qualitatively comparable even if quantitatively dissimilar, in the presence of ionic adsorbates the comparability is more difficult and is possible only if specific conditions are met.33 This is sketched in Fig. 7. A UHV metal surface with ions adsorbed on it is electrically neutral because of a counter-charge on the metal phase. These conditions cannot be compared with the condition of a = 0 in an electrochemical cell, but with the conditions in which the adsorbed charge is balanced by an equal and opposite charge on the metal surface, i.e., the condition of zero diffuse-layer charge. This is a further complication in comparing electrochemical and UHV conditions and has been pointed out in the case of Br adsorption on Ag single-crystal faces.88... 

The formalism sketched above has been used in the literature in more or less the same detail by many authors [87-92]. The predicted membrane structure that follows from this strategy has one essential problem the main gel-to-liquid phase transition known to occur in lipid membranes is not recovered. It is interesting to note that one of the first computer models of the bilayer membrane by Marcelja [93] did feature a first-order phase transition. This author included nematic-like interactions between the acyl tail, similar to that used in liquid crystals. This approach was abandoned for modelling membranes in later studies, because this transition was (unfortunately) lost when the molecules were described in more detail [87]. 

Polymer crystals most commonly take the form of folded-chain lamellae. Figure 3 sketches single polymer crystals grown from dilute solution and illustrates two possible modes of chain re-entry. Similar structures exist in bulk-crystallized polymers, although the lamellae are usually thicker. Individual lamellae are held together by tie molecules that pass irregularly between lamellae. This explains why it is difficult to obtain a completely crystalline polymer. Tie molecules and material in the folds at the lamellae surfaces cannot readily fit into a lattice. 

Figure 20. Sketch of the real structure of a Frenkel disordered crystal (only the affected sublattice is shown). The increase in temperature corresponds to an increase in defect concentration. Interaction leads to a narrowing of the spacing of the energy levels (electrochemical potentials minus configurational term) and eventually to a transition into the superionic state.12 (Reprinted from J. Maier and W. Munch, Z. Anorg. Allg. Chem., 626,264-269, Copyright 2000 with permission from WILEY-VCH Verlag GmbH.)...

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