Reaction, overlapping

You can interpret the stereochemistry and rates of many reactions involving soft electrophiles and nucleophiles—in particular pericyclic reactions—in terms of the properties of Frontier orbitals. This applies in particular to pericyclic reactions. Overlap between the HOMO and the LUMO is a governing factor in many reactions. HyperChem can show the forms of orbitals such as HOMO and LUMO in two ways a plot at a slice through the molecule and as values in a log file of the orbital coefficients for each atom. 

Reactivity may also depend on the C(l)-C(4) distance of the diene63. In a concerted [2 + 4] cycloaddition reaction, overlap has to be achieved between the lobes of the n orbital at C(l) and C(2) of the dienophile, lying about 1.3 A apart, and the lobes of the TT-orbital at C(l) and C(4) of the diene, lying about 3 A apart. This means that the shorter the C(l)-C(4) distance is, the more efficient the overlap will be in the transition state. This is translated into a higher resonance integral fi. It was shown for a series of equally substituted rigid 1,3-dienes that their reactivity in the cycloaddition reactions... 

The influences of gravitational contraction and thermonuclear reactions depend primarily on the mass of the stars. The phases of contraction and thermonuclear reactions overlap and determine the nucleogenesis as well as the fate of the stars. 

Writing the equilibrium constant as a cumulative equilibrium constant does not indicate that two or more ligands are added simultaneously. All association reactions occur in a stepwise manner, although in a few cases the successive reactions overlap extensively. 

In free-radical reactions, odd-electron species abound. Not all reactions involving free radicals are chain reactions, and not all chain reactions involve free radicals, but the set of free-radical reactions and the set of chain reactions overlap enough that the two subjects are almost always discussed together. 

The final stages of the above reaction overlapped with the onset of the nucleation and growth process that continued to complete the dehydration. Growth of three dimensional nuclei was confirmed microscopically. This second rate process was well described by the Avrami-Erofeev equation with = 2 and E, for crystals was 175 30 kJ mol (with a considerable scatter of data) below 460 K and a more reproducible reaction rate, with E, = 153 10 kJ mol, for powder. Above about 450 K there were some indications of intracrystalline melting of single crystals and the value of , increased markedly to 350 50 kJ mol (again with significant scatter of data). 

Mohamed and Galwey [71], in contrast, obtained evidence that copper underwent stepwise reduction when a was less than 0.5, from titration measurements of the copper(II) content (Cu " + KI — y in samples of partially decomposed salt. This is consistent with the behaviour foimd for other copper carboxylates [72]. Moreover, it was shown that the reactivities of both (Cu " and Cu" ) oxalates were similar and reactions overlapped. In agreement with the previous study [69], ur-time curves for the overall decomposition of copper(II) oxalate were sigmoidal and, in this slightly higher temperature interval (515 to 550 K), was increased to 180 7 kJ mol". ... 

De Kepper and Epstein (1982) identified ten reactions in the Briggs-Rauscher (B-R) system. These reactions overlap those proposed by Noyes and Furrow (1982) and by Cooke (1980-2). The regions of oscillations for the initial values of I2 and 10 j" were identified. By numerical simulations oscillations were obtained. 

Simulations demonstrate, however, that variations in kinetic parameters of reactions under consideration lead to substantial consequences. Figure 15 shows how relatively small variations in the rate constant for reaction (30) influence the SID in methane-ethane mixtures. In such a reaction system (which models real compositions of natural gas) competition of different channels of ethyl-oxygen reaction overlaps (and very probably interferes) with methyl-oxygen chemistry. The latter is even somewhat qualitatively different there are no variations in mono-molecular reactions of methylperoxy radicals at temperatures below 900 K (only dissociation to methyl and 02) and all their bi-molecular reactions lead to branching as a nearest consequence. As to the ethyl-oxygen chemistry, it is much more rich and much less definite at the same time. So in this particular case, small variations in kinetic parameters lead to very substantial consequences. 

An experiment is described to follow the mass losses when calcium oxalate monohydrate is heated from room temperature to 1000 °C. The object of the experiment is to find out how many steps of weight loss are seen and the temperatures at which the processes occur and if any reactions overlap. The % weight losses are to be calculated and compared with % losses from probable reaction mechanisms. 

Here decomposition is a mixture of the normal and autocatalytic reaction types, often with sequential reactions overlapping. At higher temperatures the overall pattern stays the same but it occurs more quickly and produces more heat. 

The reaction processes of substances cannot be analyzed by simple DTA/DSC or TG when thermal transition and the mass change due to reaction overlap. If DTA/DSC or TG is coupled with an evolved gas detector (EGD) and/or evolved gas analyzer (EGA), the reaction process can clearly be detected. Among various thermal analysis coupled simultaneous techniques [56], DTA/DSC or TG coupled with EGD and/or EGA is extensively used. TA-EGD-EGA coupled... 

If the electrode potential is different from (7op when an anodic and a cathodic reaction overlap, a net current / flows ... 

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