Solid decompositions, crystal structures

The alkylruthenium species obtained in eq. 5.1 is very stable in water, neither the addition of strong acids nor boiling for several hours lead to its decomposition. In aqueous solution it exists as a monomeric cation, however, it was isolated in solid state and characterized by X-ray crystallography as a dimer [ Ru(C2Hs)(CO)2(H20)2 2] - The stabiUty of this ruthenium alkyl is attributed to the stabihzation effect of strong hydrogen bonds which could be detected in the crystal structure and are postulated also in its aqueous solutions. Finally, elimination of propionic acid from the acyl could be induced by raising the temperature this reaction closes the catalytic cycle ... 

When the reactivity of a solid is controlled by the crystal structure, rather than by the chemical constituents of the crystal, the reaction is said to be topochemically controlled. The nature of products obtained in a decomposition reaction is frequently decided by topochemical factors, particularly when the reaction occurs within the solid without separation of a new phase (Thomas, 1974 Manohar, 1974). A topotactic reaction is a solid state reaction where the atomic arrangement in the reactant crystal remains largely unaffected during the course of the reaction, except for changes in dimension in one or more directions. Dehydration of Mo03-2H20 is a typical example of a topotactic reaction ... 

The only definite borate hydrates of cobalt are the CoO - 3B203 - 8H20 and CoO 3B203 10H2O compounds. The octahydrate is prepared by evaporation of acetic acid from cobalt acetate-boric acid mixtures, or by mixing aqueous solutions of cobalt chloride, borax, and boric acid (206). The 1 3 7.5 borate can form as a solid solution and, in the presence of 3% boric acid, affords the decahydrate (117). The crystal structure determination of this 1 3 10 compound shows it to possess the hexaborate ion (380). The IR spectra (402) and thermal decomposition (396) of these compounds have been determined. 

It should be noted that over the past 50 years studies of solid decomposition kinetics have progressed from the application of equations which were originally derived for gases to more and more detailed studies on molecular events occurring at the reactant-product interface. This point has been emphasized by Boldyrev28 who, with his co-workers, has amassed a large body of evidence that shows the importance of defect structure on kinetics. This structure may be affected by radiation or by mechanical forces, such as grinding, which affect both the surface and the internal structure of crystals. 

MacLean et al. (2000) have recently smdied the dimorphic behaviour of the pigment precursor ( latent pigment) derivative of 8-VI (R = COOr-but, R = H) (abbreviated DPP-Boc). The latency is due to the thermal decomposition reaction of both polymorphs resulting in the commercially important pigment DPP. The a form of DPP-Boc contains three half molecules in the asymmetric unit (see also Ellern et al. 1994) while the form contains one half molecule per asymmetric unit. Hence, they are easily distinguishable by solid state NMR as well as by X-ray powder diffraction. The crystal structure solution from powder data and Rietveld refinement of both polymorphs is an exemplary smdy demonstrating the potential of these methods in determining the detailed crystal structure of these compounds which are often difficult to crystallize. 

At ambient conditions ammonium dinitramide has not been found to be polymorphic the crystal structure of the orthorhombic a form under those conditions has been reported by Gilardi et al. (1997). The pressure/temperature studies led to the discovery of a second monoclinic phase (/3) formed from the a phase at 2.0 0.2 GPa. As the pressure is further raised, a solid state rearrangement, melting, and thermal decomposition take place. 

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