Aggregate reactivity determination

Part, but not all, of the effect of these solvents is due to deaggregation of the organolithium. The importance of aggregation in determining reactivity is illustrated by methyllithium, which, as a monomer, should be more basic than phenyllithium by about 10 / Ka units. However, a 0.01 M solution of MeLi in THF is only three times more reactive than PhLi, and at 0.5 M concentration in THF PhLi is more reactive than MeLi.13... 

The degree of aggregation of polystyryl alkali salts in hydrocarbons, as well as the reactivity of their respective unassociated pairs, decrease along the series Li +, Na+, K +, Cs+ (Ref.Il, pp. 20 21). For example, the propagation constant of the lithium pair in benzene at 25 °C is estimated to be greater than 100 M "1 sec- while those of K +, Rb+, and Cs+ were determined as 47, 24, and 18 M-1 sec-1, respectively. Such a gradation contrasts with that of the reactivities of tight pairs in ethereal solvents,... 

Mortar specimens were prepared to determine the effectiveness of MRI in a time resolved lithium penetration experiment [15]. This work used a non-reactive aggregate and commercially available LiN03 solution to simulate topical treatments to concrete. These results will aid the development of a more general measurement of concrete core extracted from a lithium treated structure suffering from ASR. 

Practical metal catalysts frequently consist of small metal particles on an oxide support. Suitable model systems can be prepared by growing small metal aggregates onto single crystal oxide films, a technique whereby the role of the particle size or of the support material may be studied. [37] A quite remarkable example of the variation of the catalytic activity with particle size has recently been found for finely dispersed Au on a Ti02 support, which was revealed to be highly reactive for combustion reactions. [38] On the basis of STM experiments it was concluded that this phenomenon has to be attributed to a quantum size effect determined by the thickness of the gold layers. 

As noted above, Sumner and co-workers were unable to determine the diffusion coefficient of urease unless they added Na2S03 and NaHSO-to the phosphate buffer (40) used. Nichol and Creeth, employing identical concentrations (60), measured both the sedimentation coefficient and the electrophoretic mobility of sulfite-modified urease. They concluded that sulfite contributed to the formation of -S-S03 groups attached to the (16n) species. Some of these groups they ascribed to the scission of intermolecular disulfide bonds of aggregated forms others, they suggested, arose from the 22 reactive sulfhydryl groups that react with 02 (air) to form transitory disulfides that can, in turn, react with sulfite. 

In an effort to more fully elucidate the structure and reactivity of metal oxide pillared clays, we have been investigating the structure-reactivity properties of chromia-pillared derivatives (17). In the following sections, we provide an example of the structure-catalytic reactivity properties of chromia-pillared montmorillonites. Also, we report our initial efforts to structurally characterize the intercalated chromia aggregates by Extended X-ray Absorption Fine Structure (EXAFS) Spectroscopy. Unlike previously reported metal oxide pillared clays, chromia-pillared clay exhibits strong K-edge absorption and fine structure suitable for determination of metal-oxygen bond distances in the pillars. 

The similarities in catalytic reactivity between Cr3 53-montmorillonite and chromia supported on alumina suggest that the structure of the intercalated chromia particles may resemble the structure of the bulk oxide. In order to obtain structural information for the chromia aggregates in pillared clays, we have initiated structural studies of these materials. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy is being recognized as an effective tool for determining the local structure of a variety of materials. The basic principles and utility of this technique have been discussed elsewhere (18.). ... 

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