Direct stripping mechanism

Chemical dynamics simulations of the gas phase 5 2 reactions of methyl halides have been carried out at many different levels of theory and compared with experimental measurements and predictions based on transition state theory and RRKM (Rice-Ramsperger-Kassel-Marcus) theory. Although many 5 2 reactions occur by the traditional pre-reaction complex, transition state, post-reaction complex mechanism, three additional non-statistical mechanisms were detected when the F -CH3-I reaction was analysed at an atomic level (i) a direct rebound mechanism where F attacks the backside of the carbon and CH3-F separates (bounces off) from the iodine ion, (ii) a direct stripping mechanism where F approaches CH3-I from the side and strips away the CH3 group, and (iii) an indirect reaction where the pre-reaction complex activates the C-I bond causing a CH3-I rotation and then the 5 2 reaction. The presence of these processes demonstrate that three non-statistical effects, (i) recrossing of the transition state is important, (ii) the transfer of the translational energy from the reactants into the rotational and vibrational modes of the substrate is inefficient, and (iii) there is... 

Values for rs p and T for the three different models considered above are given in Table III. Once again, the equation for the hydrogen atom stripping mechanism was directly integrated to give I to1al, but for the... 

Fig. 9.35 Direction of motion of reactants and products relative to centre of mass (stripping mechanism).

A and BC approach to the centre of mass, A strips off B and then A and C continue to move almost undisturbed in their original direction. These type of reactions are said to occur by a stripping mechanism when PES are attractive. 

Worsnop et al., in a study on the exchange of vdW bonds in Xe + At2 collisions, found that the product velocity vectors in the center-of-mass system fall on a narrow drcular band. This is a consequence of the weakness and similarity of the vdW bond strength (D 1 kJ/mol) compared with the collision energy ( = 6-15 kJ/mol). There is a hole in the XeAr distribution for 0 < 45°, with 6 measured from the initial Xe atom direction (Fig. 9). One could perhaps expect to a observe a stripping mechanism in this reaction. 

Most three atom ion-molecule reactions that exhibit direct mechanism behaviour do not proceed solely by a spectator stripping mechanism since product ions exist far beyond the critical energy. The migration mechanism is an attractive candidate for the direct mechanism since it contributes to the simultaneous occurrence of forward scattering, large momentum transfer to the product atom and stability to the product ion far beyond the critical energy of the spectator stripping model. It is, of course, clear that the actual mechanism is a mixture of a number of direct processes. 

So far, we have presented evidence for direct reaction mechanisms and shown that various dynamic aspects of reactions can be interpreted in terms of these mechanisms. From what has been described in the previous sections and other related experiments, we see that the stripping mechanism is rather widespread, if not universal. 

At high enough energies, the reaction will proceed by a direct-interaction mechanism and the many-body problem may be reduced to a consideration of pairwise interactions. Consider the reaction A BC,C)AB. On the one hand, A may interact exclusively with B this problem has been treated classically and it is implicit in the spectator-stripping model. On the other hand, sequential collisions, for example, of A with B and then C, may also yield AB here, both classical and quantum mechanical descriptions have been utilized. 

A fairly direct way of observing galvanic effects, which also permits changes in mechanical properties to be measured, involves the preparation of a composite specimen formed by attaching a strip, or strips, of one metal to a panel of another one. Tensile test specimens that include the areas of galvanic action can be cut from these panels after exposure, as shown in Fig. 19.30. 

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