Thermal abstraction absolute rate

An earlier experimental study involved the generation of benzylchlorocarbene, phenylchlorocarbene, methoxy-phenylcarbene, dimethylcarbene, cyclobutylidene, adamantylidene, and fluorenylidene by means of laser flash photolysis and their reactions with oxiranes and thiiranes <1998JA8681>. Absolute rate constants ranging from 10 to 10 ° M s at 22 °C were measured in acetonitrile and cyclohexane. Thermally generated dimethoxycarbene was found to be inefficient at abstracting oxygen or sulfur from an oxirane or a thiirane. 

Table VII. Absolute Rate Constants for Selected Thermal Hydrogen Abstraction and Olefinic Addition Reactions by Atomic Fluorine"...

The competition with the thermal back-decomposition of the adduct, kdecomp was studied by lowering the O2 concentration in chamber experiments on benzene, toluene, and /7-xylene (with photolysis of H2O2 as NOx-free source of OH) [5]. At low levels of O2, the removal of the aromatics (observed by cryo-focusing/ GC/FID) corresponds to the abstraction channel, and increases to the sum of abstraction and addition in the limit of high O2. In Table 2, ratios decomp obtained from such measurements are compared to those calculated from our absolute rate constants. That an earlier chamber study on toluene and p-xylene [24] shows much less O2 dependence, has been attributed [19] to rather high concentrations of hydrocarbons, H2O2, and thus radicals in their experiments leading to further loss processes of the adduct. 

Tatle Absolute Rate Constants for 300 K Thermal Hydrogen Abstraction Reactions hy Atomic Fluorine. 

The accurate determination of rate constants for the reactions of 19F atoms is often hampered by the presence of reactive F2 and by the occurrence of side reactions. The measurement of the absolute concentration of F atoms is sometimes a further problem. The use of thermal-ized 18F atoms is not subject to these handicaps, and reliable and accurate results for abstraction and addition reactions are obtained. The studies of the reactions of 18F atoms with organometallic compounds are unique, inasmuch as such experiments have not been performed with 19F atoms. In the case of addition reactions, the fate of the excited intermediate radical can be studied by pressure-dependent measurements. The non-RRKM behavior of tetraallyltin and -germanium compounds is very interesting inasmuch as not many other examples are known. The next phase in the 18F experiment should be the determination of Arrhenius parameters for selected reactions, i.e., those occurring in the earth s atmosphere, since it is expected that the results will be more precise than those obtained with 19F atoms. 

A comprehensive listing of absolute thermal rate coefficients for hydrogen abstraction and olefinic addition reactions by atomic fluorine has been given in Table X. 

K, or reaction in about 1 of every 5 collisions with CH4. Since the ratio of H atom abstraction from CH4 versus addition to HC=CH was measured to be 0.41, (26.27) the absolute value for the rate of thermal F atom addition to HCSCH is about 1.7... 

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