Pressure-dependent competition

When the substrate contains two double bonds, either di- or tetra-carboxylatkm can be achieved.96 The reaction of 1,5-hexadiene at 5 bar CO gives the diester, while at 1-3 bar the tetraester is obtained (equation 72). These somewhat surprising results are consistent with a pressure-dependent competition of the alkene and carbon monoxide for coordination sites on palladium. Since 1,5-hexadiene is not easily displaced by CO, even at higher pressures, the carboxylation of one double bond is ensured. Competition of the resulting monoalkenic diester with carbon monoxide is dependent on the CO pressure, lower pressures allowing monoalkene coordination and thus dicarboxylation. 

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. 

The experimental procedures were adapted to allow the application of a number of physical and chemical techniques, including the use of radical scavengers, ionic interceptors, deuteriated reagents, pressure-dependence studies, competition kinetics, etc. A serious problem, already encountered in the study of the carbonium ions directly formed from the decay of tritiated hydrocarbons (vide supra), arises from the possibility that radiolytic processes, promoted by the j8-radiation of... 

The fall-off of the unimolecular rate constant as a function of pressure arises because of the way in which the competition between reaction and deactivation depends on pressure. The rate of reaction is equal to /t2[A ], and so its pressure dependence follows the pressure dependence of [A ], given by Eq. (8). As the pressure is reduced, the importance of k2 in the denominator relative to A i[M] increases, and so the steady-state concentration [A ] is reduced. A useful way of looking at this is that [A ] is depleted by reaction, and is lower than its Boltzmann value, A i[A]/fc i. This reactive depletion is most important at low pressures, where the collisional activation is too slow to replenish it. 

Reactions (68) and (69) are chain branching and so the system is potentially explosive. However, the pressure dependent reaction, (70), is in direct competition with the H + O2 chain branching reaction leading to a complex pressure and temperature dependence of the explosion limits as shown in Fig. 2.40. 

CH3 + OH is distinctly pressure dependent. A computer simulation of the reaction kinetics, taking into account the competitive reactions... 

Migliorini et al. [65] used both the IAS and the RAS theory to accoimt for the experimental binary competitive isotherm data of the Troger s base enantiomers on microcrystalline triacetate cellulose (CTA), using ethanol as the solvent. For the calculations of the RAS theory, they used the Wilson model for the solution, including the empirical spreading pressure dependence [66]. They concluded that the IAS model imderestimates the extent of the competition between the two enantiomers in this system while the RAS model accormts accurately for the complex competitive adsorption behavior exhibited by these enantiomers. 

In this section we have examined the situation where NO preferentially adsorbs on Rh and CO on Pt. Right now one can discard the requirement of a vacant Rh site for the dissociation of adsorbed NO since Eq. (14) is unable to take into account the CO partial pressure dependency of the reaction rate. Figures 3 and 4 show linear plots of P o/r v.s. P o or Pqj req)ectively on R-Rh/D and R-Rh/R However the negative intercept of the linear plot in Figure 3-a allows to discard the assumption of non-competitive adsorptions of NO and CO on R-Rh/D. 

The partial pressure dependencies of the CO+NO reaction rate on monometallic Pt/AljO, Rh/AljOj and bimetallic Pt-Rh/R, Pt-Rh/D catalysts can be discussed in terms of competitive as well as non-competitive adsorptions of the reactants (NO and CO). In the case of bimetallic Pt-Rh/AljOj catalysts these two kinetic models have been discriminated using graphic and mathematical methods. The comparison between kinetic and thermodynamic constants obtained from these two methods with those previously determined on monometallic Pt/Al203 and Rh/AljOj catalysts allows us to state on the role of RIi incorporation to kinetic behaviour of Pt. 

For all investigated reactions, the photosubstitution quantum yield decreased significantly with increasing pressure. Under the assumption that nonradiative deactivation is relatively independent of pressure, the pressure dependence of /(l — < ) represents that of the photochemical reaction [Eq. (27)]. The positive volumes of activation fit well into the picture of a dissociative mechanism, that is, release of CO. This model cannot account for the observed trends in AF ( /(1 — < )) especially as a function of solvent. For this reason, a second way to account for the observed data was presented [100] according to which CO dissociation leads to a trigonal bipyramidal M(CO)5 fragment with dissociated CO within the solvent cage. The latter species can either recombine with CO, be trapped by solvent, or bind to the nucleophile L, which results in a competition between these reaction paths. The difference in the pressure dependence for the recombination with CO or combination with L can be used to account for the observed activation volumes. 

Kim et al.f studied the effect of gas pressure on the nucleation behavior of diamond on a Si(lOO) substrate in HFCVD. The pressure was varied from 2 to 50 torr, while a filament temperature of2200°C, a substrate temperature of 850°C, a total flow rate of 20 seem and a CH4 concentration of 0.8 vol.% were used. The characterization of diamond deposits using micro-Raman spectroscopy, SEM and OM revealed that the maximum nucleation density of diamond parades on the unscratched Si substrate occurred at a pressure of 5 torr. The pressure dependence of the nucleation density was explained by the competition effect between P-SiC formation, which increases the diamond nucleation density, and atomic-hydrogen etching, which decreases the nmnber of nucleation sites. On the basis of this finding, a new fabrication approach for high-quality diamond films without... 

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