Mechanisms, competing

Between 1923 and 1927, the concepts of quantum efficiency (number of photons emitted divided by number of photons absorbed by a sample) and quantum yield (fraction of excited molecules that emit) had been defined and values determined for many compounds by Vavilov (34). The quantum yield indicates the extent that other energy loss mechanisms compete with emission in an excited molecule. Although the quantum yield is influenced by the molecular environment of the emitter, for a given environment it depends on the nature of the emitting compound and is independent of concentration and excitation wavelength, at least at low concentrations (35). Tlius, it serves as another measurable parameter that can be used to identify the compounds in a sample and also, because of its sensitivity to the surroundings of the luminophore, to probe the environment of the emitter. 

The production of ozone in power-plant plumes has been suggested to explain ozone spatial distributions in nonurban areas.Comparison of oxidation mechanisms competing for sulfur dioxide suggests that three reactions—... 

A central tenet to the step-wise model is that the continental lithosphere deforms as a series of small plates, and that plate boundary stresses are sufficient to reactivate old suture boundaries as sites of crustal thickening and mantle subduction far from the plate boundary. The continental crust thickens homogeneously above internally undeformed mantle lithosphere that is partially subducted at these boundaries. Localized strain at sites of strike-slip and mantle subduction also requires a mechanically competent lower crust and mantle lithosphere such that stresses are efficiently transmitted from the mantle lithosphere through the crust. 

FIG. 9.—Two mechanisms compete an ionic mechanism prevalent in strong alkaline solutions, and a free-radical mechanism catalyzed by Fe(II). 

When other relaxation mechanisms compete with dipolar relaxation (often the case, as we see subsequently), the magnitude of rj is reduced according the fraction of the total relaxation rate represented by the dipolar contribution ... 

In fact, the cycloaddition of butadiene to ethylene, as well as cycloadditions of similar non-polar dienes to non-polar alkenes seem experimentally to be cases where concerted and stepwise (biradical or biradicaloid) mechanisms compete. We have recently discussed a number of cases, such as the dimerization of butadiene, piperylene, and chloroprene, the cycloadditions of butadiene or methylated dienes to halogenated alkenes, and others, where non-stereospecificity and competitive formation of [2 + 2] adducts indicate that mechanisms involving diradical intermediates compete with concerted mechanisms10). Alternatively, one could claim, with Firestone, that these reactions, both [4 + 2] and [2 + 2], involve diradical intermediates1 In our opinion, it is possible to believe that a concerted component can coexist with the diradical one , and that both mechanisms can occur in the very same vessel 1 ). Bartlett s experiments on diene-haloalkene cycloadditions have also been interpreted in this way12). 

S. W. Benson and R. Srinivassan, J, Ckem. Phys.y 23, 200 (1955), have shown that at temperatures above 600°K the chain mechanism competes with this direct reaction despite the higher activation energy of the chain. The reason is that kt = A A"1 2 and K 2 contributes a relatively large entropy term (Table XIII.6) of about 12.0 cal/mole- K, which represents a factor of about 420 in the rate constant. This has been verified experimentally by J. H. Sullivan, J, Chem, Phya., 80, 1292 (1959). 

Subsequent deposited Au atoms will adapt to this basic scheme, but clearly inducing the formation of different surface alloys. In the case of Cu(lOO), fliree different mechanisms compete the insertion of Au atoms in surface sites following a specific pattern, the formation of Cu islands in the overlayer as a result of the surface substitutions Au(S)Cu(0), and the optimum location of Cu(0) atoms... 

In general, on platinum, both mechanisms, direct 1-6 ring closure and the multistep mechanism, compete for aromatization of hydrocarbons. Their relative contributions do not change with temperature, because 1-6 ring closure has a higher activation energy than 1-5 ring closure and controls the rate of the overall process in the multistep mechanism. 

The rather small activation barrier of concerted cycloadditions involving bent dipoles leads us to exclude the two-step mechanism for these reactions. On the other hand, the quite large activation barrier of the cycloadditions involving linear dipoles would be in favor of the conclusion of Hyberty et al., i.e., for those reactions quite probably both mechanisms compete. . . . ... 

Scheme 4.2. The acyclic and cyclic mechanisms compete for the consumption of substrate.

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