Energy transfer, important elementary processes

Pulse radiolysis experiments on solid polymers have provided new insight into the mechanism of radiation damage of polymers. Recent studies on some practically important polymers clarified the pathways of transfer of radiation-induced excitation energy from polymer matrix to additives thus the roles of additives in the radiation resistance or sensitivities of polymers are understood in terms of elementary energy transfer processes. The usefulness of this method is verified not only in the basic science but also in the field of application. 

Primary process" has been used in accordance with this recently suggested definition (2) "Any continuous sequence of one or more primary steps which starts with the light absorption step." In this sense a primary step is "any one of the elementary transformations of an excited state molecule of the species which absorbs light. The absorption step Itself is also a primary step" (2). Important primary processes of OTM compounds which are described here include (1) absorption, (ii) dissociative reactions, (iii) intramolecular "twisting" isomerizations, (iv) intermolecular energy transfer, (v) inter-molecular electron transfer, (vi) luminescence. Reactions involving OTM compounds as quenchers have also been included. 

In order to appreciate the use of transition-state rate expressions, it is important to be reminded of the different time scales of the processes that imderpin the chemistry we wish to describe. The electronic processes that define the potential-energy surface on which atoms move have characteristic times that are of the order of femtoseconds, 10 sec, whereas the vibrational motion of the atoms is on the order of picoseconds, 10" sec. The overall time scale for bond activation and formation processes that control catalysis vary between 10 and 10 sec. This implies that on the time scale of the elementary reaction in a catalytic process, many vibrational motions occur. If energy transfer is efficient, then the assumption that all vibrational modes except the reaction coordinate of the chemical reaction are equilibrated is satisfied. Kramersl l defined this condition as Eb > 5kT. Under this condition the transition state reaction-rate expression applies ... 

The conservation of energy and momentum is the fundamental requirement which determines the behavior of the SE s in metals, semiconductors, and ionic compounds irradiated by particles. Although we shall not deal with the basic physics of elementary collision processes in our context of chemical kinetics, let us briefly summarize some important results of collision dynamics which we need for the further discussion. If a particle of mass mP and (kinetic) energy EP collides with a SE of mass ms in a crystal, the fraction of EP which is transferred in this collision process to the SE is given by... 

The interest in proton transfer to and from carbon arises partly because this process occurs as an elementary step in the mechanisms of a number of important reactions. Acid and base catalysed reactions often occur through intermediate carbonium ions or carbanions which are produced by reactions (1) and (2). A knowledge of the acid—base properties of carbonium ions or carbanions may also help in understanding reactions in which these species are present as reactive intermediates, even when they are generated by processes other than proton transfer. Kinetic studies of simple reactions such as proton transfer are also important in the development of theories of kinetics. Since both rates and equilibrium constants can often be measured for (1) and (2) these reactions have been useful in the investigation of correlations between rate coefficients and equilibrium constants (linear free energy relations). 

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