Competing reactions, density effects

In order to implement the PDF equations into a LES context, a filtered version of the PDF equation is required, usually denoted as filtered density function (FDF). Although the LES filtering operation implies that SGS modeling has to be taken into account in order to capture micromixing effects, the reaction term remains closed in the FDF formulation. Van Vliet et al. (2001) showed that the sensitivity to the Damkohler number of the yield of competitive parallel reactions in isotropic homogeneous turbulence is qualitatively well predicted by FDF/LES. They applied the method for calculating the selectivity for a set of competing reactions in a tubular reactor at Re = 4,000. 

These effects can be attributed mainly to the inductive nature of the chlorine atoms, which reduces the electron density at position 4 and increases polarization of the 3,4-double bond. The dual reactivity of the chloropteridines has been further confirmed by the preparation of new adducts and substitution products. The addition reaction competes successfully, in a preparative sense, with the substitution reaction, if the latter is slowed down by a low temperature and a non-polar solvent. Compounds (12) and (13) react with dry ammonia in benzene at 5 °C to yield the 3,4-adducts (IS), which were shown by IR spectroscopy to contain little or none of the corresponding substitution product. The adducts decompose slowly in air and almost instantaneously in water or ethanol to give the original chloropteridine and ammonia. Certain other amines behave similarly, forming adducts which can be stored for a few days at -20 °C. Treatment of (12) and (13) in acetone with hydrogen sulfide or toluene-a-thiol gives adducts of the same type. 

In the second case, the phosphorus atom in the phosphine also has empty d orbitals that can accept electron density donated from the Pt2+. In fact, it is more effective in this regard than is the sulfur atom in SCN-. This results in the more stable bonding to SCN- being to the nitrogen atom when PR3 is in the trans position. In essence, the presence of tt bonding ligands in trans positions that compete for back donation leads to a complex of lower stability. As will be discussed in Chapter 20, this phenomenon (known as the tram effect) has a profound effect on the rates of substitution reactions in such complexes. 

Electron-withdrawing m-substituents will decrease the electron density at the aromatic terminus of the [3,3]-rearrangement, thereby retarding the reaction. This allows polymerization of the terminal alkyne to compete successfully with the cyclization. However, the presence of a m -substituent has a much more significant effect the cyclization of m-substituted aryl propargyl ethers can lead to two isomeric products (46a and b). 

Electrochemical studies with a stepped surface in which the terrace size and the step symmetry can be controlled are analyzed in [111]. This can be achieved by introducing a series of stepped surfaces with different step densities to study the behavior of the oxygen electroreduction on these stepped-surface electrodes. In that paper, it was concluded that oxygen must compete for the adsorption sites with anions and oxide precursors, and the two effects together contribute to the structure sensitivity of this reaction. 

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