DeMore mechanism

As indicated by the involvement of water vapor and an inert third body, this reaction has several channels (see DeMore et al., 1997, for a review). There is both a bimolecular channel, which is pressure independent, and a termolecular channel, which is pressure dependent. In addition, the rate constant increases in the presence of gaseous water, suggesting that the reaction proceeds through a mechanism such as... 

Most of these conditions are chronic and debilitating, and their underlying mechanisms are not well understood. Postulated relationships include demoralization and depression resulting from the anorexia and malaise caused by profound systemic effects. These disorders may also affect the amine systems that are thought to mediate the depressive syndrome. 

Here, pathway 1 (reaction 1) is the coordinated addition of ozone (1) to ethylene (2), which proceeds through the formation of a weakly-boimd complex that transforms into primary ethylene ozonide (PO) or 1.2.3-trioxolene upon passing through the symmetrical transient state (TSl). Pathway 2 (reaction 2, the DeMore mechanism [15]) involves the collision during spontaneous orientation of the reagents (3) and the rotational transition to the biradical transient state (TS2) (4) followed by the formation of the same PO. Proceeding from the above-said, we supplement this pathway with the reaction of detachment of molecular oxygen and the formation of intermediate biradical (5) the latter may either decompose with the formation of formaldehyde (6) and carbene (7) or transform into acetaldehyde (8) or epoxide (9). Finally, pathway 3 involves the transition of ozone into the triplet state (10). This pathway is similar to reaction 2. Here, the same biradical (5) is formed it transforms into the... 

Raper and DeMore photolyzed solutions of O2 in CO at 77 °K with 2537 A radiation and found evidence for a reaction between electronically excited O2 and CO. They postulated the following mechanism... 

Uncertainties in initial conditions and reaction rate constants lead to uncertainties in the predictions of a chemical mechanism. Quantifying the effects of possible errors or uncertainties in initial concentrations c-o and rate constants kj can be viewed in two steps. First, the probability distributions of c,o and kj, P Cio) and P kj), can be estimated. Because the true values c/o and kj are not known, estimating the uncertainties in them (and thereby estimating P[c, )) and P(kj)) necessarily requires some subjective judgments. The uncertainties in the initial concentrations can generally be estimated based on knowledge of the experimental techniques and the reproducibility of the measurements. Estimates of uncertainties in rate constants are available in periodic reviews of kinetic data (DeMore et al., 1997). 

The calculated rate constant at 300 K is 7 x 10 cmVmoI. sec in good agreement with the experimental values (DeMore, 1970 Heidner et al., 1973) of 4-10 X 10"cmVmol. sec. Tully predicted that an average of roughly 30% of 1.98 eV exothermicity of this process goes into product vibration this suggests that the collision with 0(D) is a very efficient mechanism for vibrational excitation of Nj. 

The data obtained [8] for monochloroethylene and 1,1-dichloroethylene revealed, that it is another possibihty for reaction to occur not only mechanism Criegee but also DeMore mechanism is important and must to be taken into account. In present work we investigated transitions states, with give remarkable contribution in rate constant Analogous data were obtained for butane-2 in Ref. [9,10]. 

The more precise results are CCSD ones, where not only reliable activation energies are but a good correspondence to experimental rate constants exists. So there is a base to hope to relation of constants (and so ratio of rates) is reliable in terms of CCSD. The latter fact means that role of second charmel (nonconcerted mechanism) of reaction arises with the substitutiom This is trae as for fluoro- so as for chloro-substituents. While in case of ethylene and monosubstituents Criegee mechanism is prevailing one (more than 100 times). Role of DeMore mechanism in case of disub-... 

Saddle points TSl and TS2 were localized, IRC curves to regents valley were found. These last calculations resulted in geometries of complex with lengths C.O = 3.8 A in case of DeMore mechanism and C.O = 2.8A in case of Criegee mechanism. 

FIGURE 7.5 Change of rate constant of different mechanism (a) Criegee (b) DeMore. 

As for DeMore mechanism (Fig. 7.5b), there exists another, than in case of fluorine dependence on substitutor - rate constant increases with the substitution as all methods revealed. Such dependence is probably explained by a fact that conjugation stabiles the stracture of TS2, and this effect is more pronounced for 1,1-di-chloroethylene - conjugation is higher presented in 1,1-dichloroethylene than for monochloroethylene. 

Comparison of results of imphcation of three methods taking irrto acconnt their features allows one to conelude that in case of addition of ozone to 1,1-dichloethylene the competition of Criegee and DeMore mechanisms is presented, and also prevailing of latter. 

The primary step of acetylene ozonation has been studied by BILYP, PBEO, CASSCF, MRMP2, and CCSD methods using the 6-31-l-G, aug-cc-p VDZ, cc-pVTZ, and aug-cc-p VTZ basis sets. The study confirms that the intermediate complex, the transition states of the concerted addition (Criegee s mechanism) and unconcerted addition (DeMore s mechanism) are involved in this reaction. Although Criegee s mechanism is dominant, the DeMore mechanism also contributes. ... 

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