Pressure dependence, mechanism

Matheu, D.M., Grenda, J.M. A systematically generated, pressure-dependent mechanism for high-conversion ethane pyrolysis. 1. Pathways to the minor products. J. Phys. Chem. A 109, 5332-5342 (2005a)... 

The correct treatment of the mechanism (equation (A3.4.25), equation (A3.4.26) and equation (A3.4.27), which goes back to Lindemann [18] and Hinshelwood [19], also describes the pressure dependence of the effective rate constant in the low-pressure limit ([M] < [CHoNC], see section A3.4.8.2). 

Figure A3.4.9. Pressure dependence of the effective unimolecular rate constant. Schematic fall-off curve for the Lindemaim-FIinshelwood mechanism. A is the (constant) high-pressure limit of the effective rate constant...

The last phase transition is to the soHd state, where molecules have both positional and orientational order. If further pressure is appHed on the monolayer, it collapses, owiag to mechanical iastabiHty and a sharp decrease ia the pressure is observed. This coUapse-pressure depends on the temperature, the pH of the subphase, and the speed with which the barrier is moved. 

By way of example, Volume 26 in Group III (Crystal and Solid State Physics) is devoted to Diffusion in Solid Metals and Alloys, this volume has an editor and 14 contributors. Their task was not only to gather numerical data on such matters as self- and chemical diffusivities, pressure dependence of diffusivities, diffusion along dislocations, surface diffusion, but also to exercise their professional judgment as to the reliability of the various numerical values available. The whole volume of about 750 pages is introduced by a chapter describing diffusion mechanisms and methods of measuring diffusivities this kind of introduction is a special feature of Landolt-Bornstein . Subsequent developments in diffusion data can then be found in a specialised journal. Defect and Diffusion Forum, which is not connected with Landolt-Bdrnstein. 

The development of devices that provide a direct measure of stress or particle velocity led to observations of new rate-dependent mechanical responses and showed the power of such time-resolved measurements. The quartz gauge was the first of these devices with nanosecond time resolution, but its upper operating limit of 4 GPa limited its application. The development of the VISAR has had the most substantial impact on capabilities. VISAR systems, with time-resolution approaching 1 ns and the ability to work to pressures of 100 GPa, provide capabilities that have substantially altered the scientific descriptions of shock-compressed matter. 

The question of which channels account for the difference between the observed CH5 + cross-section and the CH4 + loss is illuminated by studying the isotopic system CH4-CD4. When mixtures of CH4 and CD4 were subjected to electron impact, a pressure dependent yield of CH2D+ was observed which established the reaction mechanism ... 

The kinetic data fit a mechanism of successive reactions sequent to only one primary ion equally well, provided that the first step can yield 1.37 methyl radical/100 e.v. and is pressure dependent and that the succeeding pressure independent step yields methyl radicals with a lesser efficiency and leads to a pressure independent yield of 0.58 methyl radicals/100 e.v. If the first step is either Reaction 9a or Reaction 17b, one can once more use the rate constant ratios given earlier to estimate the yields of the possible primary precursor ions. Hence, either G-(C2H2+) = 1.9 ions/100 e.v., or G(C2H4+) = 1.52 ions/100 e.v. The... 

Luo and Domfeld [110] introduced a fitting parameter H , a d5mamical" hardness value of the wafer surface to show the chemical effect and mechanical effect on the interface in their model. It reflects the influences of chemicals on the mechanical material removal. It is found that the nonlinear down pressure dependence of material removal rate is related to a probability density function of the abrasive size and the elastic deformation of the pad. 

S-layer ultrafiltration membranes (SUMs) are isoporous structures with very sharp molecular exclusion limits (see Section III.B). SUMs were manufactured by depositing S-layer-carrying cell wall fragments of B. sphaericus CCM 2120 on commercial microfiltration membranes with a pore size up to 1 pm in a pressure-dependent process [73]. Mechanical and chemical resistance of these composite structures could be improved by introducing inter- and intramolecular covalent linkages between the individual S-layer subunits. The uni-... 

Figure 3.5.2 shows the results obtained using M-5 and TS-500 samples with S/V values of 3.03 x 107 and 3.28 x 107 m 1, respectively, and porosities of 0.936 and 0.938, respectively. Note the significant deviation of the relaxation behavior from that ofbulk CF4 gas (dotted lines in Figure 3.5.2). The experimental data were first fitted to the model described above, assuming an increase in collision frequency due purely to the inclusion of gas-wall collisions, assuming normal bulk gas density. However, this model merely shifts the T) versus pressure curve to the left, whereas the data also have a steeper slope than bulk gas data. This pressure dependence can be empirically accounted for in the model via the inclusion of an additional fit parameter. Two possible physical mechanisms can explain the necessity of this parameter.

The simplicity and accuracy of such models for the hydration of small molecule solutes has been surprising, as well as extensively scrutinized (Pratt, 2002). In the context of biophysical applications, these models can be viewed as providing a basis for considering specific physical mechanisms that contribute to hydrophobicity in more complex systems. For example, a natural explanation of entropy convergence in the temperature dependence of hydrophobic hydration and the heat denaturation of proteins emerges from this model (Garde et al., 1996), as well as a mechanistic description of the pressure dependence of hydrophobic... 

The observation that the transition state volumes in many Diels-Alder reactions are product-like, has been regarded as an indication of a concerted mechanism. In order to test this hypothesis and to gain further insight into the often more complex mechanism of Diels-Alder reactions, the effect of pressure on competing [4 + 2] and [2 + 2] or [4 + 4] cycloadditions has been investigated. In competitive reactions the difference between the activation volumes, and hence the transition state volumes, is derived directly from the pressure dependence of the product ratio, [4 + 2]/[2 + 2]p = [4 + 2]/[2 + 2]p=i exp —< AF (p — 1)/RT. All [2 + 2] or [4 + 4] cycloadditions listed in Tables 3 and 4 doubtlessly occur in two steps via diradical intermediates and can therefore be used as internal standards of activation volumes expected for stepwise processes. Thus, a relatively simple measurement of the pressure dependence of the product ratio can give important information about the mechanism of Diels-Alder reactions. 

Measuring the pressure dependence of the exchange rate constant leads to activation volumes, AV and this technique has become a major tool for the mechanistic identification of solvent exchange mechanisms (8,16,17). In the last 25 years high-pressure, high-resolution NMR probes were developed which allow the application of all NMR techniques described to pressures up to several hundreds of mega Pascals (18). 

Iron(III)-catalyzed autoxidation of ascorbic acid has received considerably less attention than the comparable reactions with copper species. Anaerobic studies confirmed that Fe(III) can easily oxidize ascorbic acid to dehydroascorbic acid. Xu and Jordan reported two-stage kinetics for this system in the presence of an excess of the metal ion, and suggested the fast formation of iron(III) ascorbate complexes which undergo reversible electron transfer steps (21). However, Bansch and coworkers did not find spectral evidence for the formation of ascorbate complexes in excess ascorbic acid (22). On the basis of a combined pH, temperature and pressure dependence study these authors confirmed that the oxidation by Fe(H20)g+ proceeds via an outer-sphere mechanism, while the reaction with Fe(H20)50H2+ is substitution-controlled and follows an inner-sphere electron transfer path. To some extent, these results may contradict with the model proposed by Taqui Khan and Martell (6), because the oxidation by the metal ion may take place before the ternary oxygen complex is actually formed in Eq. (17). 

A review regarding experimental findings, which seemingly speak for the alternatively discussed dihydride mechanism, can be found in I.D. Gridnev, T. Imamoto, Acc. Chem. Res. 2004, 37, 633. However, it must be stressed that verified results such as the pressure dependence of enantioselectivity cannot be described by this model. Models related to the dihydride mechanism and developed... 

A challenging question concerns the feasibility of the application of high-pressure kinetic and thermodynamic techniques in the study of such reactions. Do long-distance electron-transfer processes exhibit a characteristic pressure dependence and to what extent can a volume profile analysis reveal information on the intimate mechanism ... 

The chemically realistic simulations we are discussing have been performed using a united atom representation of PB, which leads to the question How does one actually measure a CH vector reorientation for such a model The answer to this question is to use the trick we discussed in the analysis of the pressure dependence of the melt structure factor of PB. Hydrogen atoms are placed on the backbone carbons at their mechanical equilibrium positions for each structure that has been sampled along the MD trajectory. The CH vector dynamics we are showing in Figure 16 is solely from the backbone reorientations of the chain. 

In Ottawa I would pick out the work with Currie in 1978 on pressure dependence of electrode reactions. It gave a new mechanism criterion. 

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