Solution reactions solvent cages

Collisions in the gas phase, whether they result in a reaction or not, are timed somewhat uniformly. In solutions, however, solute pairs undergo multiple collisions within a solvent cage. Once two solute species are in one cage, they are likely to remain neighbors for some time, during which they experience repeated collisions. 

A particularly interesting question which remains unanswered is whether dinuclear photoproducts are produced directly from the photoexcited parent molecule or whether they are formed by reaction of free radicals within the solvent cage. In principle this question can be answered by making time-resolved IR measurements on the molecules in the gas phase, where no solvent cage can interfere. Thus, it may transpire that a full understanding of the photolysis of these dinuclear compounds will require complementary experiments in solution and in the gas phase. 

J.R. Bolton In solution most photochemical electron transfer reactions occur from the triplet state because in the collision complex there is a spin inhibition for back electron transfer to the ground state of the dye. Electron transfer from the singlet excited state probably occurs in such systems but the back electron transfer is too effective to allow separation of the electron transfer products from the solvent cage. In our linked compound, the quinone cannot get as close to the porphyrin as in a collision complex, yet it is still close enough for electron transfer to occur from the excited singlet state of the porphyrin Now the back electron transfer is inhibited by the distance and molecular structure between the two ends. Our future work will focus on how to design the linking structure to obtain the most favourable operation as a molecular "photodiode . 

Shape selectivity and orbital confinement effects are direct results of the physical dimensions of the available space in microscopic vessels and are independent of the chemical composition of nano-vessels. However, the chemical composition in many cases cannot be ignored because in contrast to traditional solution chemistry where reactions occur primarily in a dynamic solvent cage, the majority of reactions in nano-vessels occur in close proximity to a rigid surface of the container (vessel) and can be influenced by the chemical and physical properties of the vessel walls. Consequently, we begin this review with a brief examination of both the shape (structure) and chemical compositions of a unique set of nano-vessels, the zeolites, and then we will move on to examine how the outcome of photochemical reactions can be influenced and controlled in these nanospace environments. 

Figure 3.2 An enthalpy profile for a unimolecular reaction in solution, involving the formation of a radical pair inside a solvent cage. Adapted from [61],...

At high pressures, a non-covalent ionic complex can be regarded as a microsolvated ion. It represents the simplest model for ions generated in a dynamic environment, such as in a solvent cage in solution. The main difference is that the behavior of a microsolvated ion is not perturbed by those environmental factors (solvation, ion pairing, etc.) which normally affect the fate of intimate ion-dipole pairs in solution. Hence, a detailed study of the dynamics and the reactivity of microsolvated ions may provide valuable information on the intrinsic factors governing the reaction and how these factors may be influenced by the solvent cage in solution.4 493... 

In Scheme 4.26, the nonchain mechanism explains the failure of the radical traps to affect the yield of the reaction. According to this mechanism, the reactive radical intermediates are held within the solvent cage. Evidently, the radical traps cannot penetrate the solvent cage like an electron can. In other words, the radical traps present in the solution, even in great excess, cannot intercept the radicals in the framework of the nonchain radical mechanism. Galli (1988a) provided other similar examples of this phenomenon. 

Most treatments encountered in discussions of collision theory primarily are concerned with reactions in the gas phase. However, most of the reactions in chemistry and biochemistry occur in solutions. In solutions, the molecules are moving in the potential field of their neighbors rather than freely as in the gas phase. Thus, the potential energy varies and holes in the solvation shell permits displacement of the molecule from its original position. There are also rapid collisions with molecules that make up the solvent cage as the molecule makes its series of discontinuous displacements. Two molecules in solution that become neighbors will tend to collide a number of times (often referred to as an encounter) before they separate (or before they react). 

Let us first consider a very fast reaction between uncharged nonpolar reactants in solution. In this case, the rate is controlled by the number of encounters. Once A and B diffuse into the same solvent cage, they will react hence the rate of these diffusion-controlled reactions is determined by how fast A and B diffuse together in solution. 

The interconversion of aldoses and the respective 2-ketoses in alkaline solution may be somewhat more complex than originally supposed, as it has been reported that a partial transfer of hydrogen from C-2 of the aldose to C-l of the corresponding ketose occurs during the reaction.29 This observation is not inconsistent with isomerizations that involve 1,2-enediol intermediates. The transfer could occur as a result of a rapid conversion in which some of the protons originally at C-2 of the aldose molecules are retained by the solvent cage that surrounds the intermediate 1,2-enediol, and are, therefore, available for addition to C-l of the resulting ketose. It should be noted that other interpretations, such as hydride-transfer mechanisms, are also possible. 

If radicals diffuse from the solvent cage, fragmentation products are formed. Abstraction of hydrogen from the solvent by a phenoxy radical results in phenol, which can almost always be observed among the photoproducts of aryl esters in solution. Chemical evidence for the reaction of phenoxy radical with solvent is the formation of nearly stoichiometric amounts of 4-methyI-phenol and acetone from the irradiation of 4-methylphenyl benzoate (60) in isopropyl alcohol.34... 

The dimethyl ester of this acid in solution shows a quantum efficiency photochemical products. On the other hand, when the same acid is copolymerized with a glycol to form a polymeric compound with molecular weight 10,000 the quantum yield drops by about two orders of magnitude, 0.012. The reason for this behavior appears to be that when the chromophore is in the backbone of a long polymer chain the mobility of the two fragments formed in the photochemical process is severely restricted and as a result the photochemical reactions are much reduced. If radicals are formed the chances are very good that they will recombine within the solvent cage before they can escape and form further products. Presumably the Norrish type II process also is restricted by a mechanism which will be discussed below. 

The quantum yield for the primary photochemical process differs from that of the end product when secondary reactions occur. Transient species produced as intermediates can only be studied by special techniques such as flash photolysis, rotating sector devices, use of scavengers, etc. Suitable spectroscopic techniques can be utilized for their observations (UV, IR, NMR, ESR, etc.). A low quantum yield for reaction in solutions may sometimes be caused by recombination of the products due to solvent cage effect. 

It is convenient to label the relative slowness of encounter pair reaction as due to an activated process and to remark that the chemical reaction (proton, electron or energy transfer, bond fission or formation) can be activation-limited. This is an unsatisfactory nomenclature for several reasons. Diffusion of molecules in solution not only involves a random walk, but oscillations of the molecules in solvent cages. Between each solvent cage in which the molecule oscillates, a transformation from one state to another occurs by passage over an activation barrier. Indeed, diffusion is activated (see Sect. 6.9), with a typical activation energy 8—12 kJ mol-1. By contrast, the chemical reaction of a pair of radicals is often not activated (Pilling [35]), or rather the entropy of activation... 

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