Chain generation

Side chain generation is often a source of error. It will be most reliable if certain rules of thumb are obeyed. Start with structurally conserved side chains and hold them fixed. Then look at the energy and entropy of rotamers for the remaining side chains. Conventional conformation search techniques are often used to place each side chain. 

When Mitchell first described his chemiosmotic hypothesis in 1961, little evidence existed to support it, and it was met with considerable skepticism by the scientific community. Eventually, however, considerable evidence accumulated to support this model. It is now clear that the electron transport chain generates a proton gradient, and careful measurements have shown that ATP is synthesized when a pH gradient is applied to mitochondria that cannot carry out electron transport. Even more relevant is a simple but crucial experiment reported in 1974 by Efraim Racker and Walther Stoeckenius, which provided specific confirmation of the Mitchell hypothesis. In this experiment, the bovine mitochondrial ATP synthasereconstituted in simple lipid vesicles with bac-teriorhodopsin, a light-driven proton pump from Halobaeterium halobium. As shown in Eigure 21.28, upon illumination, bacteriorhodopsin pumped protons... 

Along these lines, both chains generate approximations on the solution u = u x,t) of the original problem (5). Indeed, it is straightforward to verify for problem (6)-(7) with the aid of the relation Va u = Va that... 

At the same rate of polymerization, the kinetic chain length for vinyl acetate is over a hundred times that for styrene on account of the greater speed of propagation relative to termination for vinyl acetate. At a convenient rate of 10 moles/liter/sec., for example, each radical chain generated consumes on the average about 5X10 vinyl acetate units under the conditions stated. 

Branched and crossiinked condensation polymers are produced when the reaction mixture includes tri-functional monomers as well as bi-functional ones. The incorporation of a single tri-functional monomer into a chain generates a branch point. As we increase the fraction of tri-functional monomers, branching becomes more prevalent and the resulting molecules more complex. When sufficient tri-functional monomers are present we create a three-dimensional crossiinked network. Figure 1.12 shows the general outline of the effects of tri-functional monomers on condensation polymers. 

Chain Generation by Reaction of Hydrocarbon with Dioxygen... 

The reaction of chain generation PhCH=CH2 + 02 —> free radicals was experimentally proved A. A. Miller and F. R. Mayo [54]... 

Interaction of hydroperoxide with ketones with rapid chain generation was studied. The following mechanism R00H + 0=CR R2 Peroxide — Free radicals was proposed E. T. Denisov [72]... 

Chain generation by reaction ROOH with R CiOjOH was observed L. G. Privalova, Z. K. Maizus, and N. M. Emanuel [75]... 

One of the important problems of the chain oxidation of organic compounds was the problem of chain generation in the absence of hydroperoxide and other initiating agents. These reactions should be very slow due to their endothermicity. Two most probable reactions were predicted [56,57] ... 

CHAIN GENERATION BY REACTION OF HYDROCARBON WITH DIOXYGEN... 

The IPM as a semiempirical model of an elementary bimolecular reaction appeared to be very useful and efficient in the analysis and calculation of the activation energies for a wide variety of radical abstraction and addition reactions [108-113]. As a result, it became possible to classify diverse radical abstraction reactions and to differentiate in each class the groups of isotypical reactions. Later this conception was applied to the calculations of activation energies and rate constants of bimolecular reactions of chain generation [114]. In the IPM, the radical abstraction reaction, for example,... 

The polar media influences the reaction of peroxyl radicals with ether as on the reaction of two polar reagents [10] and on chain generation by the reaction of ether with dioxygen as reaction with the polar TS. 

Chain generation by the reaction of diacetals of different structures with dioxygen was studied by the method of free radical acceptors. Free radical generation was found to occur with the rate constant [68]... 

In thermal oxidation, chain initiation takes place by the reaction of the aldehyde with dioxygen. Two reactions of chain generation in autoxidized aldehydes, namely, bimolecular and trimolecular, were proved [25]. 

Chain generation in autoxidized ketones proceeds via the bimolecular reaction [4]. The BDE of the a-C—H bonds of the alkyl and benzyl ketones are higher than 330 kJ mol 1 and, therefore, the bimolecular reaction should prevail as the main reaction of radical generation (see Chapter 4). 

Chain generation by the reaction with dioxygen was studied for cyclohexanone by the acceptor radical method and was proved to proceed through the bimolecular reaction with the rate... 

In oxidized nonsaturated esters the chain generation proceeds via the reaction of formed hydroperoxides with the double bond of ester (see Chapter 4). 

GV Butovskaya. Chain Generation in Oxidized Oxygen-Containing Compounds. Ph.D. Thesis, Institute General and Inorganic Chemistry, Minsk, 1982 [in Russian]. 

Cobalt catalysis of chain generation was found in the system styrene-dioxygen-acetyla-cetonate cobalt(II) [169]. Free radicals are generated in this system with the rate... 

The measurement of the chain generation rate vi0 in oxidized ethylbenzene with bis(acetyla-cetonate) nickel showed that vi0 is sufficiently higher in the presence of the catalyst than in the noncatalyzed reaction (T = 393 K, [171]). 

Such a chain generation could be expected to proceed by the reaction of polymer C—H bonds with dioxygen. The values of ki0 per C—H bond should be close in the polymer and the model hydrocarbon. The results of comparison at T= 403 K are given below [12]. 

The chain generation in polymers in the solid phase and solution is by several orders magnitude higher than that in pattern hydrocarbon. Hence, other fast reactions of dioxygen generate the chains more rapidly than that in hydrocarbons. The comparison of the ki0 values in PE with the percentage of ash showed the correlation (T = 391 K, LDPE [6]). 

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