Carbon monoxide dissociation kinetics

The first and rate-determining step involves carbon monoxide dissociation from the initial pentacarbonyl carbene complex A to yield the coordinatively unsaturated tetracarbonyl carbene complex B (Scheme 3). The decarbonyla-tion and consequently the benzannulation reaction may be induced thermally, photochemically [2], sonochemically [3], or even under microwave-assisted conditions [4]. A detailed kinetic study by Dotz et al. proved that the initial reaction step proceeds via a reversible dissociative mechanism [5]. More recently, density functional studies on the preactivation scenario by Sola et al. tried to propose alkyne addition as the first step [6],but it was shown that this... 

Prior to 1970 our understanding of the bonding of diatomic molecules to surfaces, and in many cases the type of adsorption (i.e., molecular or dissociative) was almost entirely dependent on indirect experimental evidence. By this we mean that deductions were made on the basis of data obtained from monitoring the gas phase whether in the context of kinetic studies based on gas uptake or flash desorption, mass spectrometry, or isotopic exchange. The exception was the important information that had accrued from infrared studies of mainly adsorbed carbon monoxide, a molecule that lent itself very well to this approach owing to its comparatively large extinction coefficient. 

Rh > Ir > Ni > Pd > Co > Ru > Fe A plot of the relation between the catalytic activity and the affinity of the metals for halide ion resulted in a volcano shape. The rate determining step of the reaction was discussed on the basis of this affinity and the reaction order with respect to methyl iodide. Methanol was first carbonylated to methyl acetate directly or via dimethyl ether, then carbonylated again to acetic anhydride and finally quickly hydrolyzed to acetic acid. Overall kinetics were explored to simulate variable product profiles based on the reaction network mentioned above. Carbon monoxide was adsorbed weakly and associatively on nickel-activated-carbon catalysts. Carbon monoxide was adsorbed on nickel-y-alumina or nickel-silica gel catalysts more strongly and, in part, dissociatively,... 

Oxidations of the two molecules appear to proceed almost but perhaps not quite wholly independently of each other the dissociative chemisorption of hydrogen is not much affected by the carbon monoxide, which, in view of what was said in Chapter 5, is somewhat surprising, as there it seemed that both interacted with lowly coordinated gold atoms. The dependence of kinetic parameters on the hydrogen concentration has not been... 

Catalysis relies on changes in the kinetics of chemical reactions. Thermodynamics acts as an arrow to show the way to the most stable products, but kinetics defines the relative rates of the many competitive pathways available for the reactants, and can therefore be used to make metastable products from catalytic processes in a fast and selective way. Indeed, cafalysis work by opening alternative mechanistic routes with lower activation energy barriers than those of the noncatalyzed reactions. As an example, Figure 1 illustrates how the use of metal catalysts facilitates the dissociation of molecular oxygen, and with that the oxidation of carbon monoxide. Thanks to the availability of new pathways, catalyzed reactions can be carried out at much faster rates and at lower temperatures than noncatalyzed reactions. Note, however, that a catalyst can shorten the time needed to achieve thermodynamic equihbrium, but caimot shift the position of that equihbrium, and therefore cannot catalyze a thermodynamicaUy unfavorable reaction. ... 

Kinetic studies on the exchange of radioactive carbon monoxide with nickel carbonyl have shown that the first step in the dissociation may be represented by... 

The inability of gold to adsorb substantial amounts of oxygen at low temperatures is presumably a problem of kinetics rather than thermodynamics. The kinetic barrier probably arises from the inability of gold to catalyse the dissociation of oxygen molecules. Gold will however catalyse the oxidation of carbon monoxide in the temperature range 250-330 K with kinetics indicating the... 

The thermal dissociation of phosgene into carbon monoxide and elemental chlorine. Equation (8.2), is the reverse of the reaction used widely, either thermally, catalytically or photochemlcally promoted, for the synthesis of phosgene (see Section 5.1). Thus, much of the chemistry relevant to this Section has already been covered in Chapters 5 and 6. Much of the early work, both kinetic and thermodynamic, was marred by a lack of appreciation of the importance of high purity, and the fact that the system takes a very long time to come... 

These thermodynamic calculations would seem to rule out the possibility of preparing COIj by the simple combination of carbon monoxide and diiodine. However, if the oxidative decomposition of CHI 3 does proceed via the formation of COI3 as a primary product, then the kinetics of the dissociation of COIj becomes an important consideration in the question of the material s instability. 

The capped porphyrins prepared by Baldwin et al. [56, 57] are other model systems designed to test the consequences of steric hindrance on CO binding (Scheme 4). These compounds were reported to discriminate against dioxygen in favor of carbon monoxide [62, 119-121]. The CO affinity of the capped porphyrins differs by less than a factor of three from that of unprotected iron(II) tetraphenylporphyrin, while the dioxygen affinity is more than a factor of 100 lower. Kinetic studies of CO binding show that the CO dissociation rate constants are very similar to those of unprotected hemes. Recently, the X-ray crystal structure of a carbonylated complex of the smallest capped porphyrin was obtained [122]. The CO ligand is reported to deviate 7° and 4° from the heme normal, respectively, for each independent molecule present in the unit cell. 

In this reaction scheme, CH4 is produced by two steps of radiative association with slow rate constants. Because the destruction of C by electron capture (radiative) is four orders of magnitude slower than the destruction of a molecular ion by dissociative recombination, there is not a rapid loss of C , allowing production of a saturated hydrocarbon. We recall that chemical equilibrium arguments predict preponderant conversion of carbon monoxide to methane and water. There is little evidence for this, as stated earlier. The gas-phase production of CH4 from CO and H2 then proceeds by a very high-energy kinetic path, namely He + CO = C +... 

Extensive ion yield spectra, mass spectra, and ion-ion coincidence data have been acquired for carbon monoxide at both the carbon and oxygen K ionization edges. Dissociative multiple ionization efficiencies, ion branching ratios, and kinetic energy distributions were derived. The results were related to electron energy spectra and potential energy curves for states of (Hitchcock et al. 1988). 

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