Monomer hydrolysis species

Figure V-11 Stability fields of monomer and polymer hydrolysis species of Zr in 1 m NaC104. Stability field boundaries between two solution species are defined here by equal Zr-concentrations bound to the two species. It is important to note that these boundaries are, in most cases, not equal to the boundaries at which both species have the same concentration.

The experiments indicated that in 2 M HCIO4 the most likely zirconium species are Zr" and ZrOH ". The few experiments performed in 2 m HCIO4 containing 0.1 M Zr showed the presence of polymers as evidenced by the persistence of the tyndall effect for many months. Any complexing of species in the aqueous phase is quantitatively reflected in a decrease of the extraction coefficient D. From the slope d wD d W, in principal, it is possible to determine the degree of hydrolysis of monomer aqueous species and corresponding stability constants using the equation ... 

Stability fields of monomer and polymer hydrolysis species of Zr... 

One can immediately see the inductive or catalytic role that protons play in dissociation depends upon the relative Br0nsted acidities of the dissociating dimer and the stable monomer. If the hydrated, dissociating dimer is a stronger acid than the reaction products, protons are retained and the reaction is induced. If the dissociating dimer is a weaker acid than the monomeric reaction products (as in the example of silica, above) the protons are catalytic. Because protonation reactions are so much more rapid that depolymerization, the actual protonation state of the immediate reaction product is difficult to ascertain. How would one know if the dissociation step released fully hydrated metals or partly hydrolyzed metals and protons if the hydrolysis species equilibrate in 10 s ... 

However, the presence of dimers and trimers of the hydrolysis species cannot be excluded. Polymerization of the monomer to produce the hydrated oxide is represented by... 

Monomer-cluster growth, on the other hand, requires a continuous source of monomers. Hydrolysis is the rate-determining step under basic conditions. The hydrolyzed species are immediately consumed by reaction with existing clusters because of the faster condensation reactions. Furthermore, the rate of hydrolytic cleavage of (terminal) Si—O—Si bonds is much higher than that under acidic... 

AAB sols were prepared by a three-step process involving acid-catalyzed hydrolysis of aged stock solution (r = 2.5) followed by a base-catdyzed hydrolysis step (r s= 3.7). The final hydrolysis ratio, pH, and silicon concentration were identical to those of B2 sols. Comparing the NMR spectra of the stock solution and the AAB sol, we observed that the effect of the second (acid-catalyzed) hydrolysis step was to promote extensive hydrolysis and condensation. Monomer and species are consumed to produce primarily a variety of and species. Various cyclic tetramers are prominent sol species after the second hyckolysis step as was evident from a strong q2 resonance observed near -95 ppm. 

S.3.2 Sol-Gel Encapsulation of Reactive Species Another new and attractive route for tailoring electrode surfaces involves the low-temperature encapsulation of recognition species within sol-gel films (41,42). Such ceramic films are prepared by the hydrolysis of an alkoxide precursor such as, Si(OCH3)4 under acidic or basic condensation, followed by polycondensation of the hydroxylated monomer to form a three-dimensional interconnected porous network. The resulting porous glass-like material can physically retain the desired modifier but permits its interaction with the analyte that diffuses into the matrix. Besides their ability to entrap the modifier, sol-gel processes offer tunability of the physical characteristics... 

In situ SAXS investigations of a variety of sol-gel-derived silicates are consistent with the above predictions. For example, silicate species formed by hydrolysis of TEOS at pH 11.5 and H20/Si = 12, conditions in which we expect monomers to be continually produced by dissolution, are dense, uniform particles with well defined interfaces as determined in SAXS experiments by the Porod slope of -4 (non-fractal) (Brinker, C. J., Hurd, A. J. and Ward, K. D., in press). By comparison, silicate polymers formed by hydrolysis at pH 2 and H20/Si = 5, conditions in which we expect reaction-limited cluster-cluster aggregation with an absence of monomer due to the hydrolytic stability of siloxane bonds, are fractal structures characterized by D - 1.9 (Porod slope — -1.9) (29-30). 

Komiyama at al. have prepared two oligonuclear Zn(II) complexes (22 and 23) and tested their hydrolytic activity toward different diribonucleotides [45,46] (catalytic turnover was not demonstrated). The dimer and trimer structures of the active species were confirmed by measuring the hydrolytic activity as a function of Zn/L ratio, which show sharp maxima at the expected 2/1 and 3/1 ratios, respectively. The oligomer complexes have high ribonuclease activity (e.g. the hydrolysis of UpU is accelerated more than 4 and 5 orders of magnitude by 22 and 23, respectively), whereas the effect of the monomer complex 24 was not... 

Komiyama et al. have described [65] enhanced hydrolysis of ApA by La(III) ion in presence of the tetrapyridine ligand 44. The complexation was followed by H NMR spectroscopy. Besides a 1/1 (La(DI)/L) complex, a 2/1 species is formed in the presence of excess metal. Both the 1/1 and 2/1 complexes are efficient promotors of ApA cleavage the monomer complex being ca. 7, the dimer ca. 70-fold more reactive than the metal ion alone under the same conditions (pH = 7.2, T = 323 K). 

After isolation, the intermediate products were reacted with zinc in acetonitrile or a glyme solvent to provide the trifluorovinyl ether monomers. This reaction with zinc is also sensitive to the presence of protic substances in the reaction mixture, and forms the same tetrafluoroethyl ether by-product in an undesirable side reaction. The reaction of the 2-bromotetrafluoroethyl ether reactants with zinc involves the formation of zinc organometallic species as an intermediate, and this species is sensitive to hydrolysis by water or acidic substances (Figure 3.6). 

At higher temperatures the monomer is the predominant species although the rate of hydrolysis to U03 is increased. U03 dissolves in uranyl solutions to give U02OH+ and polymerised hydroxo-bridged species. Polynuclear species could arise from U4+ as it hydrolyses in dilute acid solutions. Complex ions are formed with thiocyanate, phosphate, citrate and anions of other organic acids. 

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