Experimental techniques photolysis system

The systems that we investigated in collaboration with others involved intermolecular and intramolecular electron-transfer reactions between ruthenium complexes and cytochrome c. We also studied a series of intermolecular reactions between chelated cobalt complexes and cytochrome c. A variety of high-pressure experimental techniques, including stopped-flow, flash-photolysis, pulse-radiolysis, and voltammetry, were employed in these investigations. As the following presentation shows, a remarkably good agreement was found between the volume data obtained with the aid of these different techniques, which clearly demonstrates the complementarity of these methods for the study of electron-transfer processes. 

The vivid interest in hydrazine as a powerful propellant has stimulated many investigations both of its thermal decomposition and of its oxidation. Although hydrazine decomposes much more readily than ammonia, the study of its homogeneous decomposition by classical means using a static system is complicated considerably by wall catalysis. Thus, other experimental techniques have had to be applied, e.g. decomposition flames, flash photolysis, studies of explosion characteristics and the shock-tube technique. 

The last few years have seen the emergence of projects aiming at elucidating the photobehavior of monochlorophenols in heterogeneous systems. Two studies were concerned with the behavior of 4-chlorophenol in a surface-adsorbed state, the substrates being silicalite and solid /Tcyclodextrin [40], and cellulose and silica [41]. In both cases, nanosecond transient photolysis with diffuse reflectance detection and photoproduct analysis were the experimental techniques employed. The results of these investigations are instructive in demonstrating the influence of the solid support on the outcome of the photolytic reactions. 

Until recently, two major objectives of chemical kineticists were to explain overall chemical change in terms of elementary reactions and to determine the rates of these individual steps over a wide range of temperature. Within the last twenty years, the development of experimental techniques, such as flash photolysis, shock tubes, and gaseous fast-flow systems, have made it possible to observe the rapid changes that accompany many elementary reactions. The rate of reaction is defined in terms of the appearance or disappearance of a particular chemical entity. For example, for a bimolecular atom-exchange reaction like... 

The experimental technique that has provided the vast majority of results on E V transfer from excited halogens is that introduced in 1974 by Leone and Wodarczyk. In brief, X is produced by a photolysis source, and the concentrations of X and of the vibrationally excited collision partner are monitored by observing their time-dependent infrared fluorescence. In our laboratory the apparatus has taken the form depicted in Fig. 1. It consists basically of a source and sample cell, a detector system, and an electronic system for signal enhancement and analysis. 

The experimental techniques used to study the kinetics of OH radical reactions can be separated Into two distinct methods, namely, absolute and relative rate techniques. The absolute methods have employed discharge flow, flash photolysis, modulation-phase shift, and pulsed radlolysls systems, while a variety of differing chemical systems have been used to determine relative rate data. These techniques are briefly discussed below. 

In these two systems unresolved emission corresponding to the Au = 1 sequence of NO was observed. Based on its variation with experimental conditions it was assigned to the second reaction, above, rather than to an energy transfer process. This work has not been correlated with the flash photolysis experiments, examination of the 02 excitation not being possible with the technique used. 

The role of quinones in photobiological reactions involving chlorophyll has also been investigated (405,406). Despite the great effort and the multidisciplinary approach, progress in this field is slow because of the enormous complexity involved in photosynthetic systems. Hopefully the recent advances in experimental ESR technique, including the coupling of rapid scan ESR to flash photolysis, will help to elucidate the nature of the physical and chemical processes in photosynthesis. 

While the significance of radicals in biological systems has been appreciated for decades, there is relatively little definitive experimental infonnation on the identity of the radicals and even less on the mechanisms by which they affect the physiology of living systems. The paucity of detailed information is a direct consequence of the fact that most radicals are highly reactive and, therefore, short-lived transient species. Despite the tremendous advances in spectroscopic and laser photolysis techniques, much less is known about radicals than about closed-shell species. The treatment of radicals by theoretical methods is, however, only marginally more difficult than that of closed-shell molecules. It is for these reasons that the numerous applications of quantum chemical techniques to radicals have proven to be complementary to experimental studies. 

Since 1970, direct photolysis of molecules or ions in low-pressure, collisionless environments, has permitted molecules to be excited to well-defined energy levels, while the use of pulsed lasers or coincidence techniques has provided an accurate external time base with which to measure the dissociation rate constants over many orders of magnitude. It is often the case that more precise experimental results lead to fundamental changes in the theoretical models which describe the phenomena. This has not happened in the case of unimolecular reactions. The statistical theory has remained surprisingly robust. Most molecular systems that dissociate on a bound potential energy surface do so in a statistical fashion. What has changed in the past 25 years is our ability to apply the statistical theory. It is now possible to calculate Unimolecular rate constants with essentially no adjustable parameters and which are in quantitative agreement with experiments. 

---