Substituted systems crystal structure

As an example of the effect of solid solution substitutions on crystal structure and properties, consider the PbZrOj-PbTiOj system (PZT materials). Several compositions within this system are important for piezoelectric transducers, and, with fairly large concentrations of La ions (PLZT materials), are of potential for imaging and storage devices. As mentioned earlier, PbZr03... 

Examples of this ring system were synthesized by nucleophillic addition of 613 to dimethyl acetylenedicarboxylate in moist solvents to afford the ylidene-substituted oxazolo[3,2-rf][l,2,4]triazine 614. Its X-ray crystal structure has been described [90JCR(S)354] (Scheme 126). 

Thus, l,6-methano[10]annulene (77) and its oxygen and nitrogen analogs 78 and 79 have been prepared and are stable compounds that undergo aromatic substitution and are diatropic. For example, the perimeter protons of 77 are found at 6.9-7.3 5, while the bridge protons are at —0.5 5. The crystal structure of 77 shows that the perimeter is nonplanar, but the bond distances are in the range 1.37-1.42A. It has therefore been amply demonstrated that a closed loop of 10 electrons is an aromatic system, although some molecules that could conceivably have such a system are too distorted from planarity to be aromatic. A small distortion from planarity (as in 77) does not prevent aromaticity, at least in part because the s orbitals so distort themselves as to maximize the favorable (parallel) overlap of p... 

In contrast to 1, the related pure host 7 may be obtained in crystalline form 68). The crystal structure of 7 is built via helical chains of alternating intra- and inter-molecular H-bonding through the carboxyl functions. This structure supplies the information that the carboxyl groups are therefore already positioned in an appropriate way to facilitate analogous H-bonding in the known inclusions of 7. As discussed later (Sect. 4.2.2), these are exclusively salt-type associates and as such, intimately interact with the carboxyl groups. Hence one may infer that displacement of the carboxyl functions from position 2 in 1 to position 8 in 7 reduces the ability of inclusion formation. Similar reasons such as the solid-solubility differences observed in the classical naphthalene/chloronaphthalene systems (alpha- vs. beta-substituted derivatives, cf. Ref. 28 may also be applied here. 

On reaction with MeMgCl, the lithium triazine Li(TF[F)2 N[C(Ph)=N]2 CBut(Bun) yields the methylmagnesio-dihydrotriazine 86 (Equation (18)). An X-ray crystal structure determination revealed that the compound closely resembled its lithio precursor the formal substitution of MeMg+ (Mg-C = 2.164(3) A) for Li+ did not produce the substantial structural differences often observed between isoleptic organolithium and organomagnesium systems.267... 

Mutual solid-state solubility a simple structural representation - order/ disorder. In a number of systems such as the previously described V-Mo and Cs-Rb, continuous solid solutions are formed in the whole range of compositions, characteristics and structures of which will be discussed in more detail in Chapter 3. These result from two metals having the same crystal structure, which is maintained for all the intermediate compositions, due to a continuous random substitution of the atoms of one kind for another and vice versa. 

This is not a trivial problem, and has important implications for the mechanism of the reaction. However, the bulk of the evidence is for centrosymmetric rings, which would be in keeping with our experience in small-molecule systems. For the present purposes we assume this to be the case. On this basis DSP is one of a class of monomers of crystal structural type 100 that polymerize to polymers 101. Note that, as is typical of topochemical reactions, there are cases of polymorphism of the monomers, in which only those of structure 100 are reactive. Also small changes in the substitution of this molecule frequently result in changes in crystal structure and reactivity. 

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