Reaction mechanisms symmetry parameters

The parameters aa and ac contain useful information about the reaction mechanism and their analytical computation using the symmetry factor, /3 ( 0.5) the... 

One important catalytic reaction cycle which starts from a primary gas mixture of carbon monoxide and hydrogen is the Eischer—Tropsch synthesis. Depending on the reaction parameters (temperature of the catalytic surface, gas pressure and composition of the gas mixture) a great variety of aliphatic, aromatic and even oxygen-containing compounds can be obtained. The understanding of reaction mechanisms in terms of the appearance of intermediates on the surface, their structure and symmetry, is of fundamental interest for the development of well-defined reaction pathways. The frequency of the C—H stretching Raman band is a measure of the state of hybridization of the adsorbed molecule. 

However, even for the simple methyl transfer reactions, there is considerable confusion and some disagreement about the details of the mechanism. Some authors (Sneen, 1973) have suggested that ionization of RX always precedes attack by the nucleophile, while others have maintained that the nucleophile attacks the covalent substrate. Extensive references to both points of view are given by McLennan (1976). In the present review the application of the Marcus theory of atom transfer (Marcus, 1968a) allows us to deduce values of the parameter a which describes the symmetry of the transition state. We shall compare this information about the transition state with that from changing the solvent, from isotope effects, and from Hammett relations. We shall then attempt to deduce a model for the transition state which is consistent for all the different types of data. 

Two simplest intermediate states of C2 and C3t> symmetry with identical geometrical parameters for H and D in a reaction complex (Figs. 5a,b) can be suggested for the synchronous mechanism of H/D exchange between a water... 

It is important that the acceleration of the H-D exchange, observed experimentally on the replacement of the PtCl/ or Pt(H20)4 catalysts by PtClaCH O), could not be related to any calculated quantum-chemical parameters. It was therefore postulated that an optimum combination of the donor-acceptor and dative mechanisms of the transfer of electron density is essential for the effective alkane-platinum(II) complex interaction. Since the contribution of the former inaeases and that of the latter diminishes with the increase of the positive charge on the complex, the optimum combination can be achieved for the complex with zero charge. Somewhat earlier, the presence of a catalytic activity maximum for the complex PtCl2(H20)2 was attributed to the influence of the symmetry of the ligand environment [32]. On the basis of the hypothesis that the formation of a bond with the activated alkane takes place mainly on interaction with the drf AO of the complex, the rates of reaction were correlated with the contribution by this orbital to the LUMO of the platinum(II) complex. Symmetry considerations led to the conclusion that, for n = 0 and n = 4, the LUMO of the complex is formed solely by the AO and cannot contain an admixture of the dfl state. As a consequence of the decrease of symmetry for n =1, 2, 3, such an admixture exists but the n = 2 complex occupies a special place (thus the... 

The transfer coefficient a is distinctly different from the symmetry factor p which has been used in some of the previous equations. The former is an experimental parameter derived from the observed i-V relationship while the latter is a quantity of fundamental importance, the magnitude of which depends on the shape of the potential energy barrier for the rate determining step in the reaction sequence. In general P has a value close to 0.5 while ol P, and it may reach high values of 3-4 for certain mechanisms. 

The first step is a disrotatory cyclohexadiene-hexatriene isomerization. Its product, cf5-dihydrobenzocyclooctatetraene, is less stable than the trans dimer and is known to isomerize to it, [27] the isomerization presumably taking place via an a" displacement that reduces symmetry to Ci, in which no reaction is forbidden. At the higher temperatures at which fragmentation occurs, the first product should be in equilibrium with the reactant, and its eight-membered ring is sufficiently flexible that a similar desymmetrization would allow it to serve as an unstable intermediate. The activation parameters cited above, which - for the postulated mechanism - measure the enthalpy and entropy differences between the reactant and the transition state of the second step, are not inconsistent with concerted electrocyclic rupture of both bonds via a relatively unconstrained transition state. 

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