Photochemical kinetics reactions, literature

Field studies on the transformation of endrin in the atmosphere were not located in the available literature. Photochemical isomerization of endrin, primarily to the pentacyclic ketone commonly called delta ketoendrin or endrin ketone, was observed after exposure of thin layers of solid endrin on glass to sunlight (Burton and Pollard 1974). Minor amounts of endrin aldehyde were also formed in this reaction. Results of seasonal studies indicated that this isomerization would proceed with a half-life (first-order kinetics) of 5-9 days in intense summer sunlight, with complete conversion to the pentacyclic ketone in 15-19 days. Knoevenagel and Himmelreich (1976) reported that photodegradation of solid endrin in the laboratory... 

Several investigations concerning the thermodynamic and kinetic aspects of the thermal reactions of flavylium-type compounds have long been in the literature,133-371 while photochemical and photophysical aspects have been systematically examined more recently.[17-19,38 31 As we shall see below, pH jump, temperature jump, and flash photolysis experiments permit measurement of the rate constants of some of the reactions involved, and steady state titration experiments (using UV/Vis and NMR techniques) allow the measurement of equilibrium constants. In order to illustrate the complex reaction network in which these systems operate, we will now focus on the behavior of the 4 -methoxyflavylium ion (Figure 2 R4 = R7=H, R4- = OCH3).[391... 

The energy unit of photochemical reaction kinetics is one mol of photons (i.e. Na photons). In the photochemical literature it is often designated as hne einstein (Bolton, 1999), but this is not an lUPAC sanctioned unit. Therefore, the use of the unit einstein is discouraged. 

The work done on the system H2 + Br2 2HBr constitutes as nearly consistent a body of kinetic data as we shall find in the entire literature. The thermal and photochemical reactions are in quantitative agreement, and a consistent mechanism for both has been checked from a number of independent points of view. Nevertheless it would be foolhardy indeed to assert that the last word had been spoken on this relatively simple chain system. 

A review of the literature shows that photochemical reactions generally follow first-order kinetics. However, pseudo first-order, zero-order, and even fractional-order kinetics have been reported. Drugs reported to follow first-order kinetics include nifedipine (8) doxepin (41), riboflavin (28), minoxidil (48), adriamycin (52), doxorubicin, daunorubicin and epirubicin (61), folic acid (66), menadione sodium bisulfite (67), tetracycline hydrochloride (71), decarbazine (73), furosemide (74), democlocycline (76) and hydrocortisone, and prednisolone (90). Apparent pseudo first-order rate has been reported for pyridoxine (84). Indomethacin (91), sodium nalidixate (65), methotrexate (92), and metronidazole (68) have been reported... 

Literature concerning the unimolecular reactions of oxygen containing compounds is very extensive. To cover all the kinetic studies in this field would be virtually impossible in a review of this kind. By necessity we have limited our coverage, in the main, to reactions for which Arrhenius or transition state parameters have been reported. Some relative rate and kinetic isotope studies judged to be reliable, and to contribute significantly to the elucidation of the kinetics, have also been included. Photochemical and irradiation induced reactions do not generally produce unimolecular reactions which can be studied quantitatively therefore, the vast majority of the reactions reviewed here are those induced thermally. 

Despite the fact that hydroxyl radicals are of critical importance in the chemistry of many combustion processes, and are involved in photochemical processes in the atmosphere, there has only been a very slow development of reliable kinetics for the reactions of tiiis radical species. The problem lies very largely in the difficulty of preparing and measuring the concentration of the OH radical. In most instances, the older literature records chemistry which does not actually involve hydroxyl radicals at all, or only as a relatively unimportant species among other reactive radicals. This brief summary will not be exhaustive, but will cite the more reliable studies. The review by Wilson [6] has been of major assistance. 

In thermal kinetics the rate is proportional to concentration in the most simplest mechanism according to eq. (1.1). The proportionality constant is the rate constant k. In photokinetics, the equivalent proportionality constant is, according to eq. (1.2), the so-called photochemical quantum yield. In the literature, some different definitions of quantum yields are discussed but not always clearly distinguished. Therefore the problems with three different definitions are discussed here. Two others related to the partial reactions and independent of the time of the reaction are given in Section 2.1.2. 

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