Reaction center schematic view

Figure 1. Schematic representation of the artificial photosynthetic reaction center by a monolayer assembly by A-S-D triad and antenna molecules for light harvesting (H), lateral energy migration and energy transfer, and charge separation across the membrane via multistep electron transfer (a) Side view of mono-layer assembly, (b) top view of a triad surrounded by H molecules, and (c) energy diagram for photo-electric conversion in a monolayer assembly.

Figure 23-18 Schematic view of photosynthetic reaction centers and the cytochrome //complex embedded in a thylakoid membrane. Plastocyanin (or cytochrome c6 in some algae and cyanobacteria) carries electrons to the PSI core.

Figure 10.2 Schematic view of a nuclear reaction in the laboratory and center-of-mass systems. [From Weidner and Sells (1973).]...

All these ligands have extensive chemistry here we note only a few points that are of interest from the point of view of catalysis. The relatively easy formation of metal alkyls by two reactions—insertion of an alkene into a metal-hydrogen or an existing metal-carbon bond, and by addition of alkyl halides to unsaturated metal centers—are of special importance. The reactivity of metal alkyls, especially their kinetic instability towards conversion to metal hydrides and alkenes by the so-called /3-hydride elimination, plays a crucial role in catalytic alkene polymerization and isomerization reactions. These reactions are schematically shown in Fig. 2.5 and are discussed in greater detail in the next section. 

On the basis of the fact that (R)-BMPP coordinated to the metal center can induce asymmetric addition of methyldichlorosilane across the carbon-carbon double bond of 2-substituted propenes to afford an enantiomeric excess of (R)-2-substituted propylmethyldichlorosilanes, the following processes should be involved in these reactions (a) insertion of the metal center into the silicon-hydrogen bond (oxidative addition of the hydrosilane) (b) addition of the resulting hydridometal moiety to the coordinated olefin preferentially from its re face (in a cis manner) to convert the olefin into an alkyl-metal species and (c) transfer of the silyl group from the metal center to the alkyl carbon to form the product. Since process (b) most likely involves diastereomeric transition states or intermediates, the overall asymmetric bias onto the R configuration at the chiral carbon would have already been determined prior to process (c). A schematic view of such a process is given in Scheme 1. 

It is obvious that such equilibria would exist for all the other catalytic intermediates. The result of all this is coupled catalytic cycles and many simultaneous catalytic reactions. This is shown schematically in Fig. 5.5. The complicated rate expressions of hydroformylation reactions are due to the occurrence of many reactions at the same time. As indicated in Fig. 5.5, selectivity towards anti-Markovnikov product increases with more phosphinated intermediates, whereas more carbonylation shifts the selectivity towards Mar-kovnikov product. This is to be expected in view of the fact that a sterically crowded environment around the metal center favors anti-Markovnikov addition (see Section 5.2.2). 

Figure 8.3 A schematic two-dimensional view of the potential energy surface and wave-packet dynamics in the ultrafast photodissociation of Hgl2 [adapted from Voth and Hochstrasser (1996), Zewail (1996)]. The transition state for the I + Hgl reaction is along the bisector, dashed line, with the lowest barrier at the bottom of the potential along that line. The UV excitation creates a localized wave-packet along the bisector. The center of the packet is displaced from the saddle point to a compressed configuration along the symmetric stretch. During the dissociation the wave-packet bifurcates, as shown, and each component is followed in the figure. It shows the coherent vibrational motion in the Hg—I well. ...

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