Reaction mechanism rebound

We focus our attention on the DIPR (direct interaction with product repulsion) model and its variant, the DIPR-DIP model, mainly because it can be used to predict an entire range of dynamic observables in chemical reactions angular and recoil velocity distributions, rotational energy and orientation and vibrational energy of the reaction products. It is also able to account for the switch from the rebound to the stripping reaction mechanism for a given system when the collision energy is increased. The beauty of the model is its ability to include semiempirical parameters, each of which is related to a different physical phenomenon. 

Passing to direct reactions, the stripping reactions are more readily treated theoretically than rebound reactions. In particular, it has been found that in many stripping reactions of the type A + BC AB + C the transfer of atom B from molecule BC to AB does not involve recoil. Por such spectator-stripping reactions (C is the spectator), angular and energy distributions of products can be readily calculated, thus permitting a simple verification of the assumption on the reaction mechanism. 

The reaction mechanism has tJso been investigated by DFT calculations, lending support for the stepwise mechanism [111, 112]. The first key intermediate is the diruthenium nitrenoid species (S = 1/2). In the transition state of the C-H bond scission, the coordinated nitrenoid interacts with the allylic H atom, polarizing the C-H bond. Subsequent H atom transfer to the N atom generates a diradical species comprising a diruthenium amide tethered to an allylic radical (S = 3/2). Of interest, the unpaired electrons are primarily localized at the two Ru centers (and the allylic fragment), with litde spin density (0.14) at the N atom. Finally, radical rebound forms the cyclized amine. 

These results are inconsistent with a radical rebound mechanism because this mechanism is a two-step process that requires the involvement of intermediates. Instead the results suggest that the hydroxylation is a concerted process, much like a singlet carbene reaction, which does not involve intermediates. However, this conclusion is in conflict with the properties of singlet carbene reactions discussed above. Subsequent studies on a number of substituted methylcyclopropanes and other stained hydrocarbon systems established that these findings were not anomalous. 

Boundar y conditions need to be assessed based on the type of connections to be used for the member supports. The engineer must keep in mind that support details must provide sufficient strength, ductility and stability to enable the member to develop full collapse mechanism. Support capability to resist reaction forces for both the loading and rebound phases of the response must be considered when assessing boundary conditions. 

A and BC approach to centre of mass, A strips off B and then AB and C return roughly in the direction from which they came. These reactions are said to occur by a rebound mechanism and generally occur when the surface are repulsive. In such reactions the life-time of activated complex, i.e. (ABC) must be short and reaction is said to be direct or impulsive. If life-time is much, rotation may occur and the products may separate in random directions. For many such reactions, the life-time of complexes has been observed less then 5 x 10 13sec. J.C. Polanyi discussed the relationship of these reactions with shapes of PES with special attention to mass effects. 

F. Ogliaro, N. Harris, S. Cohen, M. Filatov, S. R de Visser, and S. Shaik, A Model Rebound Mechanism of Hydroxylation by Cytochrome P450. Stepwise and Effectively Concerted Pathways, and Their Reactivity Patterns, J. Am. Chem. Soc. 2000,122, 8977. Calculations explain puzzling aspects of cytochrome P450 hydroxylation reactions in terms of two, different, reactive spin states of the enzyme. 

The reaction follows the consensus mechanism for aliphatic —H activation by oxyl-ferryl compounds (35) in which the first step is H-atom abstraction via TS1 to give a hydroxo-Fe(III) complex with a C-centered alkyl radical, labeled IN. This is followed by a rebound step via TS2 to give the final product, ethanol and the ferrous active site. Overall, this is a two-electron oxidation process where the bonding orbital serves as the electron donor and the H-atom abstraction is rate limiting. 

This is evident when the mechanism of a heterogeneous reaction is considered in more detail. Molecules from the gas phase strike the surface. They may rebound or they may become attached to the surface and sojourn there for a period. 

Whereas several transient species have been observed for dioxygen activation by MMOH, no intermediates were found by rapid-mixing spectroscopic methods for the actual methane hydroxylation step. Mechanistic probes, i.e. certain non-natural substrates that are transformed into rearranged products only if the reaction proceeds via a specific intermediate such as a radical or a cation, give ambivalent results Some studies show that products according to a pathway via cationic intermediates are obtained in sMMO hydroxylations and at least one study suggests the presence of a radical intermediate [40]. Computational analyses of the reaction of MMOHq with methane suggest a so-called radical recoil/rebound mechanism in which MMOHq... 

Reactions showing backward scattering have small impact parameters and cross sections. The mechanism is called rebound because P hits MN head on, pulls M away from N and, under the influence of the repulsive forces between M and N, PM moves almost totally backwards. N also rebounds backwards from PM under the same repulsive forces, and returns only slightly deflected from its original path (Figure 4.12). 

The back reaction will have the exact reverse characteristics. The activated complex will lie in the exit valley, and reaction will be enhanced by high vibrational energy. There will be high translational energy in the products, the cross section will be small, and the molecular beam contour diagram will show predominantly backward scattering, typical of a rebound mechanism. 

This means that vibrational energy favours reaction, from which it can be inferred that the activated complex lies in the exit valley, and that there is a late barrier. This is a rebound mechanism where the reactants have to get very close together before reaction can occur. 

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