Photochemical reactions qualitative theories

An interesting and important qualitative theory of photochemical reactions has been developed by Peters and applied to the analysis of the... 

The oxidative deterioration of most commercial polymers when exposed to sunlight has restricted their use in outdoor applications. A novel approach to the problem of predicting 20-year performance for such materials in solar photovoltaic devices has been developed in our laboratories. The process of photooxidation has been described by a qualitative model, in terms of elementary reactions with corresponding rates. A numerical integration procedure on the computer provides the predicted values of all species concentration terms over time, without any further assumptions. In principle, once the model has been verified with experimental data from accelerated and/or outdoor exposures of appropriate materials, we can have some confidence in the necessary numerical extrapolation of the solutions to very extended time periods. Moreover, manipulation of this computer model affords a novel and relatively simple means of testing common theories related to photooxidation and stabilization. The computations are derived from a chosen input block based on the literature where data are available and on experience gained from other studies of polymer photochemical reactions. Despite the problems associated with a somewhat arbitrary choice of rate constants for certain reactions, it is hoped that the study can unravel some of the complexity of the process, resolve some of the contentious issues and point the way for further experimentation. 

To strike a balance, structural information will be presented only for those cases of direct relevance to the photoinitiated reactions, namely (i) a few examples of CO2-HX and N2O-HX spectra in which large, qualitative differences can be seen, (ii) a compilation of all experimentally derived distances and angles, and (iii) comparisons to calculated values, when a combination of experiment and theory is needed to draw conclusions. What has been rather absent so far in studies of weakly bonded complexes, is information about systems involving heavy atoms. It is true that species like iodine and bromine do not necessarily bring joy to the lives of electronic structure theorists, but these are the systems most amenable to the experimental photoinitiated reaction studies. Also, the possibility of marked differences for seemingly homologous series makes quantitative evaluations imperative. Our discovery of the qualitative structural difference between CO2-HCI and C02-HBr underscores this point [34]. As pragmatists, we were able to quickly exploit this difference, but more importantly, it may be possible in the future to systematically use such differences to examine different photochemical entrance channels. 

Charge transfer takes place not only in electrode processes but also in numerous homogeneous chemical reactions, and above all in redox reactions. The approach worked out by Gurney is applicable in this case too, since the transfer of an electron from the reducing agent A to the oxidizer B takes place at the moment when the energy levels of the initial (A + B ) and the final (A + B ) states are equalized. This equalization takes place as a result of fluctuations of the medium. These ideas in a qualitative form and without a direct relation to Gurney s theory were formulated by Libby[190], who proceeded from Franck s ideas on photochemical processes and absorption spectra in solutions. In the most complete form, these concepts were formulated approximately at the same time in the well-known paper by Platzmann and Franck[191] (see also [192]). Similar... 

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