Stopped-flow method high-pressure

In the case of solvent-exchange reactions, including those pertaining to aqueous metal ions, accurate mechanisms are particularly difficult to ascertain with standard experimental methods, particularly since rate constants span nearly 20 orders of magnitude. However, variable-pressure techniques (e.g., stopped-flow kinetics, high-pressure NMR spectroscopy) and the measurement of activation volumes (Af ) have often shed light on such systems. ... 

In kinetically labile simple systems it may be assumed that the regularity reflected by the above examples (i.e., that the rate-determining step is the solvent substitution) is of fairly general validity. The high rates of most of these reactions make their investigation difficult this is the reason why reliable experimental data have become available for the interpretation of the solvent effect in this field only since the spreading of the fast kinetic methods (stopped flow, T-jump, pressure jump, etc.). 

Hydraulic fracturing is a method of stimulating production of oil or gas from rock formations. A fluid is pumped under conditions of high pressure and high rate Into the formation to fracture it. The fluid also carries sand or a similar proppant material into the fractures. When the pumping is stopped and the hydraulic pressure is released at the wellhead, the fracture partially closes on the sand leaving a highly permeable channel for the oil or gas to flow back to the well. 

The systems that we investigated in collaboration with others involved intermolecular and intramolecular electron-transfer reactions between ruthenium complexes and cytochrome c. We also studied a series of intermolecular reactions between chelated cobalt complexes and cytochrome c. A variety of high-pressure experimental techniques, including stopped-flow, flash-photolysis, pulse-radiolysis, and voltammetry, were employed in these investigations. As the following presentation shows, a remarkably good agreement was found between the volume data obtained with the aid of these different techniques, which clearly demonstrates the complementarity of these methods for the study of electron-transfer processes. 

The AV data of Fig. 5.1 that are satisfactorily accounted for by Eqs (5.5)-(5.8) are fewer in number than the anomalous cases of Table 5.1. This is a rather unsatisfactory situation, even though most of the anomalies can be explained away - indeed, deviations from the predictions of Eqs (5.5)-(5.8) can often provide important mechanistic information. More AV data are clearly desirable, but the prospects for further successful experiments are poor. The measurements of AV summarized in Fig. 5.1 and Table 5.1 were obtained at high pressures by radiochemical tracer methods for the slowest reactions [12, 17, 25], NMR linebroadening techniques for the faster cases [11, 13, 15, 19-22, 34], and stopped-flow circular dichroism [13, 14, 18] for moderately rapid reactions of reactants that could be prepared as resolved enantiomers. There are, however, many self-exchange reactions that are inaccessible to these techniques. For example, rates of electron transfer in couples where both reactants have unpaired electrons generally cannot be studied by NMR methods, while other couples that undergo electron transfer at intermediate rates may not be resolvable into optical isomers or be amenable to radiochemical sampling procedures under pressure. 

The study of the dynamics of host-guest interaction can be subdivided into three aspects the kinetics of complexation, the flexibility of the host, and the dynamics of the guest. A method based on high-pressure stopped-flow kinetics/UV-Vis absorption spectroscopy has been described for the investigation of the kinetics and thermodynamics of the formation of inclusion complexes between phenylazo dyes and a-CyD [49]. Time-resolved measurements of excited-state evolution could be... 

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