Reactant orientation

The generalized Woodward-Hoffmann rule suggests that a synchronous addition of disulfonium dications at the double C=C bond of alkenes would be a thermally forbidden process and so would be hardly probable. Simulation of the frontal attack by ethylene on l,4-dithioniabicyclo[2.2.0]hexane 115 gave no optimal structure of an intermediate complex. On the other hand in the lateral approach of the reactants, orbital factors favor attack of the double bond by one of the sulfonium sulfur atoms of the dication. This pattern corresponds to SN2-like substitution at sulfur atom as depicted in Figure 5. Using such a reactant orientation, the structure of intermediate jc-complex was successfully optimized. The distances between the reaction centers in the complex, that is, between the carbon atoms of the ethylene fragment and the nearest sulfur atom of the dication, are 2.74 and 2.96 A, respectively. 

Rotational diffusion of particles occurs in polymer much slowly than in liquids. Therefore, the observed difference in liquid (k ) and solid polymer (ks) rate constants can be explained by the different rates of reactant orientation in the liquid and polymer. The EPR spectra were obtained for the stable nitroxyl radical (2,2,6,6-tetramethyl-4-benzoyloxypiperidine-l-oxyl). The molecular mobility was calculated from the shape of the EPR spectrum of this radical [14,15], These values were used for the estimation of the orientation rate of reactants in the liquid and polymer cage. The frequency of orientation of the reactant pairs was calculated as vor = Pvrot> where P is the steric factor of the reaction, and vIol is the frequency of particle rotation to the angle equal to 4tt. The results of this comparison are given in Table 19.2. 

We see that the rate constant of the bimolecular reaction is two to three orders of magnitude lower than the rate of reactant orientation. 

Hence, the phenomena of the low reaction rate in the polymer matrix cannot be explained by the limiting rate of reactant orientation (rotational diffusion) in the cage. This result becomes the impetus to formulate the conception of the rigid cage of polymer matrix [16-20]. In addition to the experiments with comparison of the rate constants in the liquid phase and polymer matrix, experiments on the kinetic study of radical reactions in polymers with different amounts of introduced plasticizer were carried out [7,9,15,21], A correlation between the rate constant of the reaction k and the frequency of rotation vOT of the nitroxyl radical (2,2,6,6-tetramethyl-4-benzoyloxypiperidine-/Y-oxyI) was found. The values of the rate constants for the reaction... 

All reactions collected in Table 19.6 are slow. They occur with rate constants that are sufficiently lower than the rate constants of diffusion in polymer, as well as the frequency of reactant orientation in the cage (vor =vrx P). Hence, physical processes are not limited by the rates of these reactions. However, polymer media influences the kinetics of these reactions. 

Reactions described earlier were not limited by rotational diffusion of reactants. It is evident that such bimolecular reactions can occur that are limited not by translational diffusion but by the rate of reactant orientation before forming the TS. We discussed the reactions of sterically hindered phenoxyl recombination in viscous liquids (see Chapter 15). We studied the reaction of the type radical + molecule, which are not limited by translational diffusion in a solution but are limited by the rate of reactant orientation in the polymer matrix [28]. This is the reaction of stable nitroxyl radical addition to the double bond of methylenequinone. 

This reaction occurs with an extremely low stcric factor P = 1.6 x 10-11. Therefore, the rate of reactant orientation is very low, and the reaction is limited by reactant orientation in the polymer matrix. This reaction occurs according to the following kinetic scheme ... 

Stereodynamics is focused on the dependence of forces on reactant orientation (or alignment) in the course of chemical reactions [15], One of the ideas behind this is to use these forces to control chemical reactions [17], Continuous efforts have been made toward this goal in recent years, and the ultimate step at the moment is coherent control [215], We shall not consider these questions here. We prefer to follow a second idea behind stereodynamic studies, which is more simply to refine our understanding of how reactive systems access the transition state region of the reaction. 

Reactant orientation and alignment can play an important role in determining chemically distinct pathways as well as influencing dynamics, and the... 

Kim et al. [44] reported results from a similar study of the decompositions of Mg(OH)2 and MgCOj in the TEM. They were able to determine in detail the product/reactant orientation relationships, by careful preparation of specimens, using ion-beam milling where necessary. They detected no intermediate phase in either decomposition. The decomposition of Mg(OH)2 produces MgO with a single orientation relationship to the substrate, while the decomposition of MgCOj yields MgO in two principal orientations. These topotactic relationships are explained in terms of the orientations of oxygen octahedra in the reactants and product. 

Enzymes are in fact supramolecular catalysts these proteins translate supramolec-ular interactions between targets and active sites into substrate selection, reactant orientation, and promotion of selective covalent bond formation into selective... 

At a sufficiently high P where K/ s rP kp, A xp = kp, the reaction is limited by translatory diffusion. By contrast, at low values of the steric factor the process is limited by the rate of particle-reactant orientation and kexp Agxp = ATahV P. In both cases, we have diffusion-controlled reactions in the first case, translatory diffusion is lim-... 

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