Cage formation from solvent molecules

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... 

The photochemistry of benzaldehyde (90% 13C=0), 519, deoxybenzoin (99%) 13C=0), 521, and / -chloro benzoin (99% 13C=0), 522, in cyclohexane-Dn solution has been studied633 by spectroscopic techniques, such as XH chemically induced dynamic nuclear634 or electron polarization635 (CIDNP/CIDEP) or dynamic nuclear polarization636 (DNP). In all these cases the formation of benzaldehyde-D with emissive 13C=0 polarization has been observed and the results rationalized by intermolecular hydrogen (deuterium) abstraction by the photoexcited ketones from the solvent molecules and by reactions of cage-escaped radicals (equations 303-308), Benzoin, 520, is formed also. 

One important discrepancy should be noted between photochemical and chemical ion radical reactions. In the photochemical mode, an oxidized donor and a reduced acceptor remain in the same cage of a solvent and can interact instantly. In the chemical mode, these initial products of electron transfer can come apart and react separately in the bulk solvent. For example, one-electron oxidation of phenylbenzyl sulfide results in formation of the cation radical both in the photoinduced reaction with nitromethane and during treatment with ammoniumyl species. Sulfide cation radicals undergo fragmentation in the chemical process, but they form phenylbenzyl sulfoxide molecules in the photochemical reaction. The sulfoxide is formed at the expense of the oxygen atom donor. The latter comes from the nitromethane anion radical and is directly present in the solvent cage. As for the am-... 

Alkali-metal-ion-O-donor-solvent cages exhibit low-frequency bands in the i.r. spectrum characteristic of the cation-O polyhedra. Similar bands are seen with N-donors. The bands can be used to establish the nature of cation co-ordination and serve as probes to examine ion-solvent interactions.87 An analysis of the i.r. spectra of ternary mixtures LiClCX-Sj-Sz showed a preferential solvation of the Li+ ion by NH3 and methylamines (St) in MeCN or THF (S2) and by MeCN (S2) in MeN02 (SO. The appearance of wide bands in the ion-cage vibration region is related to the formation of different species [Li(Si) -i(S2)i]+. The Li+-solvent molecule interaction energy decreases when the number of S2 molecules in the first solvation shell increases. The mean composition of the first solvation shell was obtained from intensity measurements of the molecules not bonded to the ions in favourable cases (S2 = ND3 or MeCN), the solvation number of the Li+ ion in the pure solvent... 

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