Precursor structure effects

Precursor Structure Effects. The precursor structure can impact a broad range of properties, including crystallization temperature, the formation of intermediate phases during thermal treatment and film density, among other properties. Table 2.4 reports some of the key precursor properties that may affect densification and crystallization behavior, as well as the final film microstructure. 

These techniques have very important applications to some of the micro-structural effects discussed previously in this chapter. For example, time-resolved measurements of the actual lattice strain at the impact surface will give direct information on rate of departure from ideal elastic impact conditions. Recall that the stress tensor depends on the elastic (lattice) strains (7.4). Measurements of the type described above give stress relaxation directly, without all of the interpretational assumptions required of elastic-precursor-decay studies. 

To assess the trapping of biological nucleophiles, the pyrido[l,2-a]indole cyclopropyl quinone methide was generated in the presence of 5 -dGMP. The reaction afforded a mixture of phosphate adducts that could not be separated by reverse-phase chromatography (Fig. 7.16). The 13C-NMR spectrum of the purified mixture shown in Fig. 7.16 reveals that the pyrido [1,2-a] indole was the major product with trace amounts of azepino[l,2-a] indole present. Since the stereoelec-tronic effect favors either product, steric effects must dictate nucleophilic attack at the least hindered cyclopropane carbon to afford the pyrido[l,2-a]indole product. Both adducts were stable with elimination and aromatization not observed. In fact, the pyrido [1,2-a] indole precursor (structure shown in Scheme 7.14) to the pyrido [l,2-a]indole cyclopropyl quinone methide possesses cytotoxic and cytostatic properties not observed with the pyrrolo [1,2-a] indole precursor.47... 

Numerous investigators have attempted to control the precursor structure and related solution chemistry effects with varying degrees of success, to influence subsequent processing behavior, such as crystallization tempera-ture.40-42,78,109 110 Particular attention has been given to precursor characteristics such as structural similarity to the desired product and the chemical homogeneity of the precursor species. For multicomponent films, this latter factor is believed to influence the interdiffusional distances associated with the formation of complex crystal structures, such as perovskite compounds. Synthetic approaches have been geared toward the preparation of multimetal species with cation stoichiometry identical to that of the desired crystalline phase.40 42 83 84... 

Rather early it became evident that the NIS glycosy-lation of an axial 4-OH group, even in a blociced 3-amino sugar li)ce daunosamine or rhodosamine, could not be effected efficiently 6 ). Consequently, a trisaccharide synthesis was required that allowed facile inversion of a precursor structure subsequent to the advantageous use of NIS glycosylation steps. 

The most drastic effect on the losses of the thermal energy is due to dissociation of molecular hydrogen. According to Fox and Wood (1985) as much as a half of the thermal energy behind the shock front is absorbed due to dissociation of Hg molecules. At the same time photodissociation of Hg molecules in the precursor causes retardation of the collisional ionization in the relaxation zone, whereas the precursor structure is very sensitive to the radiation flowing from the wake (Gillet and Lafon 1983 1984). So, the self-consistent model of the radiative shock is urgently needed. 

Structure-reactivity relationship in polyarylcarbocation systems 334 Conformations of carbocations 334 Reactivity-conformation relationship 337 Stabilities of carbocations in the gas phase 343 Structural effects 343 Tlie resonance demand parameter 355 Theoretically optimized structures of carbocations 362 Reaction mechanisms and transition-state shifts 365 Extended selectivity-stability relationships 365 Ground-state electrophilic reactivity of carbocations 366 Sn2 reactions of 1-arylethyl and benzyl precursors 372 Concluding remarks 378 Acknowledgements 379 References 379... 

In this work the performance of Cr-Co spinels as catalysts for total combustion of methane is studied. The spinels were prepared from nitrate precursors. The effect of temperature and time of calcining was studied using x-ray diffraction (XRD) in order to check the crystalline structure and the absence of other phases, N2 physisorption (BET) in order to study the porous structure of these solids and Temperature Programmed Reduction (TPR) in order to determine the presence of different metallic species. 

The present study deals with the incorporation of zirconium sulfate hydroxyl complex in Na-montmorillonite using zirconium acetate as a precursor. The effect of the preparation parameters on the textural and structural properties of the resulting materials will be discussed. 

All carbon materials, including those used for catalytic apphcations, are formed in either the gas, liquid, or solid phase and these conditions to a large extent dictate the variabilities possible in their physicochemical properties. The range of hydrocarbon feedstocks used as carbon precursors is also dictated by these conditions and seemingly subtle changes often produce profound structural effects. These are discussed briefly below. 

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