Tungsten activation parameters

In the meantime temperature-dependent stopped-flow measurements were conducted on the latter complex in order to determine the activation parameters of the N-N cleavage reaction (24). Plots of the absorption intensity at 418 nm vs. time at T — —35 to +15°C indicate biphasic kinetics with two rate constants 0bs(p and obs(2)> in analogy to our measurements of the tungsten complex. This time, however, both rates depended upon the acid concentration. Interestingly much smaller rate constants 0bs(i) and 0bs(2)> were found for all acid concentrations than given by Henderson et al. for his (single) rate constant kobs (up to 1 order of magnitude). Furthermore plots of 0bs(i) and kohs(2) vs. the acid concentration showed no saturation behavior but linear dependencies with slopes k and k and intercepts k und k, respectively (s — acid dependent and i — acid independent), Eq. (2) ... 

Equation (89) shows that the allowance for the variation of the charge of the adsorbed atom in the activation-deactivation process in the Anderson model leads to the appearance of a new parameter 2EJ U in the theory. If U — 2Er, the dependence of amn on AFnm becomes very weak as compared to that for the basic model [see Eq. (79)]. In the first papers on chemisorption theory, a U value of 13eV was usually accepted for the process of hydrogen adsorption on tungsten. However, a more refined theory gave values of 6 eV.57 For the adsorption of hydrogen from solution we may expect even smaller values for this quantity due to screening by the dielectric medium. 

Tungsten Disulfide and Molybdenum Disulfide. Arrhenius parameters reported for reactions on both these catalysts (222- 225) were close to two compensation lines, characteristic of cracking and exchange processes (Table II, O and P). Since the reactants studied in this work were different from those used in kinetic measurements on tungsten metal, no direct comparison of activities is appropriate. Data for reactions of thiophene did not correlate with either line. 

It is worthwhile to note that in the reaction of the group 6B metal hexacarbonyls with the azide anion to provide isocyanatometallates, a concerted mechanism is proposed based on kinetic parameters which involves a three-centered transition state (7), and that the activation enthalpy is some 22.6 kJ lower for tungsten than for chromium (52). This reactivity sequence correlates with an increase in M-CO bond distances going from chromium to tungsten hexacarbonyl (53-55). 

Values at 1000 K of the pre-exponential factor A2 [eqn. (20)] for localised and mobile adsorption. The parameters for the localised state relate to tungsten, and p has been calculated assuming that the activation energy for surface diffusion (Em) is one-fifth the energy of desorption of atoms (— AUX i) [107] (K0)i 2 has been assigned the value 0.5 and Ns the value 1015 cm 2. 

Molybdenum and tungsten complexes as models for oxygen atom transfer enzymes have been deployed in the full catalytic cycle from Scheme 4.3 predominantly in the early days of this field of research. A selection of the respective determined Michaelis-Menten parameters were expertly reviewed by Holm et al. Since in some cases both forms of model complexes (M and M mimicking the fully reduced or fully oxidized active sites, respectively) are isolable and available in a sufficient amount, the isolated half-reactions are much more often investigated than the whole catalytic cycle. This means that either the reduced form of the enzyme model is oxidized by an oxygen donor substrate like TMAO or the oxidized form is reduced by an oxygen acceptor substrate like triphenylphosphine (PhgP). The observed kinetic behaviour is in some cases described to be of a saturation type. An observation which... 

Experimentally obtained data on the thermodynamic properties of the C-Fe-W alloys are very scarce. The effect of tungsten on flie activity of carbon in liquid iron alloys has been studied by [1959Fuw, 1960Fuw, 1960Mor]. The interaction parameter obtained at 1560°C in low-carbon melts by [1959Fuw] was reported as = -2.3. This value was not referred to a fixed carbon concentration. The summarized results of [1959Fuw] were mentioned in the review of [1963Ell]. 

Mor] studied the effect of tungsten on graphite solubility in liquid iron at 1550°C and used the obtained data for the verification of the general approximate equation for calculation tire activity coefficient of carbon in a multicomponent solution deduced by them. The value of interaction parameter = -1.31 at xc = 0.207 was presented. The influence of the tungsten content (up to about 12 mass%)... 

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