Direct interaction with product repulsion

A widely-used model in this class is the direct-interaction with product repulsion (DIPR) model [173—175], which assumes that a generalised force produces a known total impulse between B and C. The final translational energy of the products is determined by the initial orientation of BC, the repulsive energy released into BC and the form of the repulsive force as the products separate. This latter can be obtained from experiment or may be assumed to take some simple form such as an exponential decay with distance. Another method is to calculate this distribution from the quasi-diatomic reflection approximation often used for photodissociation [176]. This is called the DIPR—DIP model ( distributed as in photodissociation ) and has given good agreement for the product translational and rotational energy distributions from the reactions of alkali atoms with methyl iodide. 

The mechanism of the electron transfer and the subsequent dynamics of the system have both been oversimplified at the moment. A more refined model of the dynamics is discussed in the next section the direct interaction with product repulsion (DIPR) model and its variant called DIPR-DIP(DIP-distributed as in photodissociation). The purpose of sections 2.4 and 2.5 is to explicit the multi-dimensional and multi-PES character of the electron jump step. 

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. 

The modified spectator stripping model (polarization model) thus appears to be a satisfactory one which explains the experimental velocity distribution from very low to moderately high energies. The model emphasizes that the long-range polarization force has the dominant effect on the dynamics of some ion—molecule reactions. However, a quite different direct mechanism based on short-range chemical forces has been shown to explain the experimental results equally satisfactorily [107, 108]. This model is named direct interaction with product repulsion model (DIPR model) and was originally introduced by Kuntz et al. [109] in the classical mechanical trajectory study of the neutral reaction of the type... 

A DIET process involves tliree steps (1) an initial electronic excitation, (2) an electronic rearrangement to fonn a repulsive state and (3) emission of a particle from the surface. The first step can be a direct excitation to an antibondmg state, but more frequently it is simply the removal of a bound electron. In the second step, the surface electronic structure rearranges itself to fonn a repulsive state. This rearrangement could be, for example, the decay of a valence band electron to fill a hole created in step (1). The repulsive state must have a sufficiently long lifetime that the products can desorb from the surface before the state decays. Finally, during the emission step, the particle can interact with the surface in ways that perturb its trajectory. 

If the substituents are nonpolar, such as an alkyl or aryl group, the control is exerted mainly by steric effects. In particular, for a-substituted aldehydes, the Felkin TS model can be taken as the starting point for analysis, in combination with the cyclic TS. (See Section 2.4.1.3, Part A to review the Felkin model.) The analysis and prediction of the direction of the preferred reaction depends on the same principles as for simple diastereoselectivity and are done by consideration of the attractive and repulsive interactions in the presumed TS. In the Felkin model for nucleophilic addition to carbonyl centers the larger a-substituent is aligned anti to the approaching enolate and yields the 3,4-syn product. If reaction occurs by an alternative approach, the stereochemistry is reversed, and this is called an anti-Felkin approach. 

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