Initial stages of reaction

In the crystal of 1,4-dicinnamoylbenzene (1,4-DCB) (see Fig. 12), the distances between the intermolecular photoadductive carbons are 3.973 and 4.086 A for one cyclobutane ring, and 3.903 and 3.955 A for the other. The two topochemical pathways may occur competitively in a single crystal of 1,4-DCB at the initial stage of reaction. Then, both intramolecular photodimerization and intermolecular photopolymerization of the diolefinic mono-cyclobutane intermediate occur competitively to give tricyclic dimer 21,22,23,24-tetraphenyl-l,4,ll,14-tetraoxo-2(13),12(13-diethanol, [4.4] para-cyclophane or oligomers (Hasegawa et al., (1985). On photoirridation at room temperature the 1,4-DCB crystal gives >90% of the tricylic... 

The rate of reduction of Tl(III) by Fe(II) was studied titrimetrically by John-son between 25 °C and 45 °C in aqueous perchloric acid (0.5 M to 2.0 M) at i = 3.00 M. At constant acidity the rate data in the initial stages of reaction conform to a second-order equation, the rate coefficient of which is not dependent on whether Tl(III) or Fe(II) is in excess. The second-order character of the reaction confirms early work on this system . A non-linearity in the second-order plots in the last 30 % of reaction was noted, and proved to be particularly significant. Ashurst and Higginson observed that Fe(III) retards the oxidation, thereby accounting for the curvature of the rate plots in the last stages of reaction. On the other hand, the addition of Tl(l) has no significant effect. On this basis, they proposed the scheme... 

Table 2. H-NMR data (400 Hz, D2O) for the anomeric and C-1 protons in products formed in the initial stages of reaction by endopolygalacturonase I (PGI) from A. niger from reduced pentagalacturonic acid (pentaGalU-ol).

These ESR spectra are in good agreement with ESR spectra of ozonized PP published previously (30) The rapid formation of peroxy radicals indicates that ozone reacts with PP without induction period. In the initial stage of reaction the hydroperoxide groups (POOH) concentration increases and the rate of POOH formation is linearly dependent on the ozone concentration (Fig.2). After prolonged ozonization the concentration of POOH remains almost constant. 

At the initial stage of reactions, the produced intermediate species such as the cation radical and the electron exist in a narrow space, the so-called spur. After the electron thermalization process, a pair of a cation radical and a thermalized electron remain in a spur. The geminate ion recombination of the cation radical and the electron occurs before these ionic species diffuse and spread uniformly in the media. Therefore the geminate ion recombination takes place in the spur. On the condition of a so-called single pair model,... 

When t — 0, x — 0, and therefore in the initial stages of reaction the expression simplifies to ... 

The calculations of a mean square particle displacement, (r2(t)), have demonstrated that at the initial stage of reaction (small t) the conventional expression is valid,... 

During classical asymmetric synthesis, the amplitude of these fluctuations are expected to decrease during the course of the reaction because more and more chiral molecules are formed and eeeXp declines. However, in the presence of chiral autocatalysis, the small ee caused by such fluctuations can be amplified. In such cases, the system is likely to be most sensitive in the initial stage of reaction when the concentration of chiral molecules is still small. If the autocatalytic species are concentrated they can be either in a racemic or optically active state but if they are highly diluted, as at the beginning of the reaction, statistical fluctuations can become significant so that the state... 

In the present study, the value of y- which designates the distribution of initiation rates in respective regions is assumed to remain constant in the course of reactions. Evidently, this assumption is crude i.e., in the initial stages of reactions, most graft chains might be formed in the vicinities of the sur-... 

Kropf and co-workers200-207b carried out detailed investigations of the autoxidation of alkyl aromatic hydrocarbons, such as cumene, catalyzed by metal phthalocyanines. They concluded that in the initial stages of reaction, in which the concentration of alkyl hydroperoxides is quite low, initiation occurred by an oxygen activation mechanism, e.g.,... 

The initial reaction step involves the deposition of carbon and silica, as shown experimentally by Lin et al.14 In their work, they examined the reaction front separating the reaction product from the unreacted composite using transmission electron microscopy (TEM). During the initial stages of reaction, graphitic carbon and silica glass were formed at the SiC-alumina interface, as... 

At higher seed concentrations (about 230 mg HAP 1 ) a more basic calcium phosphate with Ca P = 1.52 + 0.04 crystallizes on the growth sites of the HAP seed material and no evidence is found for the presence of DCPD. The dependence of the growth phase on solid/solution ratio is of particular importance not only for the interpretation of the results of biological precipitation studies but also for the formation of calciiam phosphate in environmental systems. It may be e2q>lained by the competition between heterogeneous nucleation of DCPD and the growth of the active sites already present on the seed si >strate surface. The latter process occurs more extensively in the initial stages of reaction when the seed concentration is... 

Induction periods or abnormally slow initial stages of reaction characterize the oxidation in the presence of amines [65 69] and some alkenes [69, 71]. Present understanding of inhibitor action is even less clear than is that of retardation, although a number of comments can be made. It is unlikely that these additives function simply by removal of RCO3 radicals since, unless the rate coefficient for the process were abnormally high compared with that of (3), only retardation would result. Removal of RCO radical by a process competitive with (2) is also unlikely for similar reasons. 

Using the same approach and interpretation, values of — jq-ii.io o.44 jjj3 molecule s and Eub = 161.2 6.4 kJ mol were obtained [45] from studies of isobutene oxidation, as predicted by the similar thermochemistry and inert nature of methylallyl radicals due to electron delocalization. The agreement is good, and moreover the Arrhenius parameters are entirety consistent with Aif= 10 " cm molecule s and Elf = 163 kJ mot , which were obtained from studies of HCHO oxidation under conditions where the chain length was reduced virtually to zero. In the initial stages of reaction, the mechanism in KCl-coated vessels, where HO2 and H2O2 are efficiently destroyed at the vessel surface, is very simple. 

The unique behaviour of CH3 radicals is brought out starkly by comparison of the products of the separate oxidations of propanal and ethanal at 813 K using a mixture containing 2, 30, and 28Torr of aldehyde, O2 and N2, respectively [41,46]. Mechanisms for the initial stages of reaction may be written as follows. 

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