Radical combination reactions, solvent effects

For irradiation at a higher dose rate, the radical-radical combination reactions (R6) wiU efficiently occur and compete with the addition reactions of radicals and solute molecules to initiate the polymerization (R2), while the addition reactions (R2) effectively occur during irradiation at a lower dose rate, because of the reduction of radical loss (Nakagawa 2010). This will lead to an increased polymer yield with a decreasing dose rate. As solvent radicals work not only as an initiator (R2) but also the terminator (R4) of polymerization, the probability for polymer radicals to terminate with solvent radicals (R4) will be less by irradiation at a lower dose rate. This will make it easy for polymer radicals (R5) to produce a polymer with a higher molecular weight. 

The cage effect described above is also referred to as the Franck-Rabinowitch effect (5). It has one other major influence on reaction rates that is particularly noteworthy. In many photochemical reactions there is often an initiatioh step in which the absorption of a photon leads to homolytic cleavage of a reactant molecule with concomitant production of two free radicals. In gas phase systems these radicals are readily able to diffuse away from one another. In liquid solutions, however, the pair of radicals formed initially are caged in by surrounding solvent molecules and often will recombine before they can diffuse away from one another. This phenomenon is referred to as primary recombination, as opposed to secondary recombination, which occurs when free radicals combine after having previously been separated from one another. The net effect of primary recombination processes is to reduce the photochemical yield of radicals formed in the initiation step for the reaction. 

In comparison, the level of detail in the understanding of radical ion reaction mechanisms is much lower for a number of reasons. Due to the inherently complex nature of the electron transfer-chemical reaction-electron transfer (ECE) mechanism, measurement of substituent, solvent and isotope effects will usually provide a combination of effects on all the steps involved. Introducing a donor substituent on a substrate will, for example, not only change the relative stability of the transition structures and intermediates with localized charges, but will also affect the rate constant of electron transfer and self-exchange between two substrates as well as the rate of back electron transfer. 

The values of k ]k for cyclohexyl radicals in the gas phase and in liquid cyclohexane (23°C) have been found to be 0-5 and respectively, (Becke aZ., 1954 Cramer, 1967). Comparison of these values with those determined in the present work show that there is little effect of temperature, phase or solvent on kf k for the cyclohexyl radical. This behaviour is in accord with the conclusions drawn from the photolysis studies but is in marked contrast to those from the hydrogen atom-olefin experiments. The absence of large effects of temperature and solvent suggest that the disproportionation and combination reactions of the cyclohexyl radical proceed through activated complexes in which the interactions between the radicals are very similar, if not identical. 

The nature of the transition state of nucleophilic reactions with LL [low lowest unoccupied molecular orbital (LUMO)] substrates is analyzed and reviewed. In cation-anion combination reactions, a partial radical character is developed on both the nucleophile and the substrate. Examination of a simple state diagram shows that this diradicaloid character is increased as the LUMO of the substrate is lowered. The model is further extended to other LL substrates such as carbonyl functions and activated olefins. Three empirical manifestations of the diradicaloid character of the transition state are discussed (1) the correlation between the ionization potentials of the nucleophiles and their nucleophilicity toward LL substrates (2) the a-effect phenomenon and (3) the variations in the positional selectivity of 9-nitromethylenefluorene in nucleophilic reactions as a function of the solvent. 

---