Photochemical elementary reactions

Light energy interacts with matter in quantum units called photons which contain energy E = hv (Section 6.2.1.2). The frequency v is related to the wavelength A by 

There is a cross-section for absorption, a, which characterizes the size of the target a photon has to hit to be absorbed. The rate of absorption is given a little differently, since the photons travel much faster than the A molecules (which can be treated as stationary). If the flux of photons (number traversing a given area per unit time) is I, then the rate of absorption per unit volume is 

The attenuation of a light beam as it traverses a volume of light-absorbing material of thickness dl can be expressed as 

The integration of equation 6.6-11 with the boundary condition that I = I0 at l = 0 gives the Beer-Lambert law (with cA/mol L-1 = c A/NAv)  

An electronically excited molecule can undergo several subsequent reaction steps. In addition to dissociation and rearrangements, there are processes involving light. These are  

Photochemistry is the contribution of the energy needed, in the form of electromagnetic radiation, for a reaction system to react. 

In general, as for thermally-activated reactions, photochemical reactions ate complex and distinct from the basic steps. In general, only one is photochemical and is called the primary act, the rest being conventional thermal steps. 

In an experiment, the primary act is almost never separable from the other elementary steps. 

Photochemical reactions ate governed by two very old laws a qualitative and a qrtantitative law. 

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With chain and radical reactions (including photochemical ones) the intermediate steps are elementary reactions of atoms and radicals with molecules. The lifetimes of atoms and radicals are relatively short. 

Addition of chlorine atoms to ethylene and chloroethylenes has been extensively investigated. Some work has also been done on termal chlorination of propylene and butene at higher temperatures. The references and the elementary reactions postulated have been listed by Steacie (97). The mechanism generally assumed is the well known chain chlorination mechanism. When the initiation is photochemical, this comprises the following reactions... 

The forward and reverse directions of an elementary reaction represent a particular case of a reversible process. The principle of microscopic reversibility states that, for such a reaction, there is a single common pathway, i.e. a single transition structure for forward and reverse directions [ 14]. For the thermal interconversion of cis- and trans-stilbenes, the single transition structure comprises two PhCH moieties, singly bonded with a dihedral angle of 90°. (In contrast, the forward and reverse paths in the more complicated photochemical interconversion of cis- and trans-stilbenes are different, and the principle of microscopic reversibility does not apply.)... 

After a historical introduction of PET and its fundamentals, in the first two chapters, the following contributions cover some elementary reactions, Le. bond cleavage, as well as oxygenations and PET of charged species. The last two chapters turn to applications which are currently being extensively studied in research as well as in the chemical industry. Photoimaging technologies are discussed in the final contribution and may be taken as an example for photochemical methods which are essential in industrial applications. 

In equation (1) K y is referred to as the Stern-Volmer constant Equation (1) applies when a quencher inhibits either a photochemical reaction or a photophysical process by a single reaction. <1>° and M° are the quantum yield and emission intensity (radiant exitance), respectively, in the absence of the quencher Q, while <1> and M are the same quantities in the presence of the different concentrations of Q. In the case of dynamic quenching the constant K y is the product of the true quenching constant kq and the excited state lifetime, t°, in the absence of quencher, kq is the bimolecular reaction rate constant for the elementary reaction of the excited state with the particular quencher Q. Equation (1) can therefore be replaced by the expression (2)... 

Tn recent years, a number of reaction models have been proposed to account for the chemical features of photochemical smog observed in atmospheric and laboratory studies (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11). Because of the complexity of smog chemistry and a lack of detailed knowledge of many relevant elementary reactions, numerous assumptions and simplifications are made in these mechanistic interpretations. A model for the chemistry of smog is presented here with a critical evaluation of the factors that control the major course of the reactions. The photooxidation of propylene (CsHq) in the presence of nitric oxide and nitrogen dioxide (NO + NO2 = NO, ) is used as a prototype for this study. 

Chapter 2 covers kinetics, which provides useful information about reaction mechanisms, and allows us to distinguish between possible mechanisms in many cases. Elementary reactions do not involve intermediates, but go through a transition state. Although this transition state cannot be isolated, it can be studied in various ways which provide insights into the reaction mechanism, and this forms the subject matter of Chapter 3. This is followed by three chapters on the most important intermediates in organic chemistry anions, radicals and cations. A final chapter on molecular reactions concerns thermal and photochemical processes. The concepts of frontier orbitals and the aromatic transition state allow us to predict which reactions are allowed and which are forbidden , and provide insights into why most reactions of practical interest involve multi-step processes. 

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. 

Kurylo MJ. 1978a. Elementary reactions of atmospheric sulfides. J Photochem 9 124-126. 

The description of complex photochemical reactions is facilitated by subdividing the total reaction into a series of elemental steps, of which the primary (or initiation) process follows the absorption of a photon, and the subsequent processes are thermal (or dark) reactions. The interaction of solar radiation with the atmosphere and its constituents, and the ensuing primary photochemical processes are examined in this chapter. Dark reactions will be discussed whenever appropriate. The identification of the individual reaction steps and their characterization by rate laws are the subject of chemical kinetics. Some familiarity with reaction kinetics and the behavior of elementary reactions is essential to the discussion of atmo-... 

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