Energy transfer chain reactions

The reactivity shown in Scheme 3 results from the low bond dissociation energy (BDE) of the P-H bond [11] k=l.2 10 M s for the H-transfer from R02P(0)H to a primary C-centered radical) and the fast halogen-atom transfer from a C-halogen bond to a phosphonyl radical [9,12] (fc=4 10 M s for f-Bu-Br and k=83 10 M s for Cl3C-Br). Piettre et al. [13] pointed out that these chain reactions were even more efficient when dialkylthiophosphites and the corresponding dialkylphosphinothioyl radicals were involved. 

A carbonyl chromophore in a macromolecule can participate in a variety of photochemical processes that can have as end result the degradation of the polymer via processes like the Norrish Type I or Type II reaction, the triggering of a chain reaction leading to peroxidation, the transfer of energy to another chromophore or, it can also behave as an energy sink if a suitable, non-degradative path, is available to the triplet state. 

Chain reactions can lead to thermal explosions when the energy liberated by the reaction cannot be transferred to the surroundings at a sufficiently fast rate. An explosion may also occur when chain branching processes cause a rapid increase in the number of chains being propagated. This section treats the branched chain reactions that can lead to nonthermal explosions and the physical phenomena that are responsible for both branched chain and thermal explosions. 

There is a third explosion limit indicated in Figure 4.1 at still higher pressures. This limit is a thermal limit. At these pressures the reaction rate becomes so fast that conditions can no longer remain isothermal. At these pressures the energy liberated by the exothermic chain reaction cannot be transferred to the surroundings at a sufficiently fast rate, so the reaction mixture heats up. This increases the rate of the process and the rate at which energy is liberated so one has a snowballing effect until an explosion occurs. 

The rather stable diacyl peroxides such as dibenzoyl or phthaloyl peroxide have attracted special interest as some of their reactions, mostly not chain reactions, are chemiluminescent. Triplet-singlet energy transfer is very often involved, and emission generally occurs only when a fluorescer is present since the primary excited products cannot emit in the visible range of the spectrum. 

Combustion processes are fast and exothermic reactions that proceed by free-radical chain reactions. Combustion processes release large amounts of energy, and they have many applications in the production of power and heat and in incineration. These processes combine many of the complexities of the previous chapters complex kinetics, mass transfer control, and large temperature variations. They also frequently involve multiple phases because the oxidant is usually air while fuels are frequently liquids or solids such as coal, wood, and oil drops. 

A wide variety of chemical reactions can occur following ionization or excitation of a molecule in both gaseous and condensed phases. These may be of uni-molecular or bi-molecular nature, initiated by electrons, ions or by the transformations of excited or ionized molecules. These reactions include, but are not limited to, dissociation, elimination of atoms and smaller molecules (H, H2, etc.), transfer of H, H2, H, and H2, fragmentation, ion-molecule reaction, luminescence and energy transfer, neutralization, chain reaction, condensation, and polymerization, etc. These reactions will not be reviewed in this chapter but may be found elsewhere in this book. A brief summary is also found in Chapters 4 and 5 of Ref. 2. In the next section, some features of yields and mechanisms following excitation and/ or ionization in the liquid phase are discussed with special reference to water. 

The existence of differential reactivity for various sites suggests the possibility that energy absorbed at one site on the chain may be transferred down the chain until it localizes in a site with an unusually high cross section for reaction. Shulman, Gueron, and Eisinger154 claim that energy absorbed in poly dAT at the excited singlet level is transferred to a common excimer between A and T, whence it crosses to a triplet triplet excitons have been observed in poly A with a jump time of 10"8 to 10 10 sec. 

Energy transfer along the polynucleotide chain may be one factor in the variable cross section for reaction of the sites on the chain. A poly... 

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