Radicals short-lived transient species

Whereas decamethylferrocene—here regarded as the prototype of a metallocene—can be easily transferred to the corresponding radical cation or anion, this is not the case for decamethylsilicocene (1). CV measurements have already shown that the cation 1+ is only a very short-lived transient species.10 Consequently, chemical experiments to prepare salts containing 1+ have also failed so far. Similar observations have been made concerning... 

Recent technical developments in laser Raman spectroscopy have made it possible to measure the Raman spectra of short-lived transient species, such as electronically excited molecules, radicals and exciplexes, which have lifetimes on the order of nano- (10-9) and pico- (10-12) seconds. These shortlived species may be generated by electron pulse radiolysis, photo-excitation and rapid mixing. However, the application of electron pulse radiolysis is limited in its adaptability and selectivity, while rapid mixing is limited by mixing rates, normally to a resolution on the order of milliseconds. Thus, photoexcitation is most widely used. 

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. 

Using these characteristics of lasers, photochemical reactions that cannot be conducted with conventional Hght sources can be brought about, and a multiphoton reaction is one of these processes. Multiphoton reactions can be classified in two categories (1) stepwise reactions through short-lived transient species (Path 1 in Scheme 1) and (2) reactions by concerted multiphoton absorptions (Path 2 in Scheme 1). In many cases. Path 1 predominates when the substrates are complex organic compounds. The transient species can be either long-lived excited states, such as triplet states, or chemical species, such as radicals, carbenes, nitrenes, and thermally unstable molecules. 

Free radicals are short-lived, highly-reactive transient species that have one or more unpaired electrons. Free radicals are common in a wide range of reactive chemical environments, such as combustion, plasmas, atmosphere, and interstellar environment, and they play important roles in these chemistries. For example, complex atmospheric and combustion chemistries are composed of, and governed by, many elementary processes involving free radicals. Studies of these elementary processes are pivotal to assessing reaction mechanisms in atmospheric and combustion chemistry, and to probing potential energy surfaces (PESs) and chemical reactivity. 

The development of the two-color and laser jet approaches has also allowed the study of the photochemical behavior of excited states of reaction intermediates, i.e., transient species that are chemically distinct from the original ground or excited state, such as neutral and ion radicals, biradicals, carbenes, and ylides. In fact, the study of excited reaction intermediates has been more comprehensive than the study of upper states. Originally, the short-lived nature of the ground-state transient itself led to the incorrect assumption that the excited transient would be too short-lived to participate in any chemical or photophysical processes other than deactivation to the ground state. However, this is now known not to be the case and some surprising differences between the ground- and excited-state behavior of reaction intermediates have been observed. 

These short pulses induce a non-equilibrium situation in a very short time scale, such that a sufficiently high concentration of transient free radical species is formed. These short-lived free radical species are detected in their lifetimes, by following the changes in their characteristic properties such as optical absorption, electrical conductivity, spin density, Raman spectroscopy, etc. Pulse radiolysis has been found to be extremely useful in studying several of these free radical reactions. Although modern pulse radiolysis techniques are capable of producing much shorter pulses seconds), most of the relevant... 

Although a standard deviation of about 10% is now quite usual for rate constant measurements from fast-flow discharge and pulsed photolysis studies, it is still found that the same reaction studied in different laboratories by the same technique may give results of similar precision, but which differ by far more than would be expected on the basis of that precision. Potential sources of such discrepancies are many but, with experience, likely errors can sometimes be identified in particular cases. For example, reactions between short lived radical intermediates are common in combustion processes. The measurement of the rate constants of such reactions, where the reaction is second order with respect to the transient species, pose particular but well recognized difficulties stemming from the need to determine the absolute concentration of the reacting radical (Chapter 1). This is difficult to achieve and has been a common source of error in this type of determination as exemplified in the series of studies on the rate of the reaction CHO -I- CHO — CH2O -H CO. 

Although EPR signals related to hydrocarbon cations radicals generated by electrochemical oxidation or chemical oxidation can be readily detected, only a few examples have been reported for cation radicals that are produced by irradiation of solutions of electron donors and an acceptor. Because electron spin polarization offers the advantage of detecting transient species via their EPR signal intensities, chemically induced dynamic electron polarization (CIDEP) spectra can give information not only about short-lived radical intermediates... 

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