Radical ions laser flash photolysis method

Photolytic methods are used to generate atoms, radicals, or other highly reactive molecules and ions for the purpose of studying their chemical reactivity. Along with pulse radiolysis, described in the next section, laser flash photolysis is capable of generating electronically excited molecules in an instant, although there are of course a few chemical reactions that do so at ordinary rates. To illustrate but a fraction of the capabilities, consider the following photochemical processes ... 

One principally different method of photoexcitation consists in the so-called two-color, two-laser flash photolysis. This method allows exciting ion-radicals already generated during the first laser (first color) flash photolysis. With a short-time delay after the first laser flash, the ion-radicals are selectively excited by the second laser (second color) flash. This flash is of the wavelength tuned to the absorption of the ion-radical initially formed. As a result, the ion-radical transforms into... 

It is useful to briefly discuss some of the common and, perhaps, less common experimental approaches to determine the kinetics and thermodynamics of radical anion reactions. While electrochemical methods tend to be most often employed, other complementary techniques are increasingly valuable. In particular, laser flash photolysis and photoacoustic calorimetry provide independent measures of kinetics and thermodynamics of molecules and ion radicals. As most readers will not be familiar with all of these techniques, they will be briefly reviewed. In addition, the use of convolution voltammetry for the determination of electrode kinetics is discussed in more detail as this technique is not routinely used even by most electrochemists. Throughout this chapter we will reference all electrode potentials to the saturated calomel electrode and energies are reported in kcal mol. ... 

Flash photolysis and laser flash photolysis are probably the most versatile of the methods in the above list they have been particularly useful in identifying very short-lived intermediates such as radicals, radical cations and anions, triplet states, carbenium ions and carbanions. They provide a wealth of structural, kinetic and thermodynamic information, and a simplified generic experimental arrangement of a system suitable for studying very fast and ultrafast processes is shown in Fig. 3.8. Examples of applications include the keton-isation of acetophenone enol in aqueous buffer solutions [35], kinetic and thermodynamic characterisation of the aminium radical cation and aminyl radical derived from N-phenyl-glycine [36] and phenylureas [37], and the first direct observation of a radical cation derived from an enol ether [38],... 

A number of alkene radical cations have been generated in matrices at low temperature and have also been studied by ESR, CIDNP, and electrochemical methods. However, until recently very little absolute kinetic data have been available for the reactions of these important reactive intermediates in solution under conditions comparable to those used in mechanistic or synthetic studies. In a few cases, competitive kinetic techniques have been used to estimate rates for nucleophilic additions or radical cation/alkene cycloaddition reactions. In addition, pulse radiolysis has been used to provide rate constants for some of the radical cation chemistry relevant to the pho-topolymerization of styrenes. More recently, wc and others have used laser flash photolysis to generate and characterize a variety of alkene radical cations. This method has been extensively applied to the study of other reactive intermediates such as radicals, carbenes, and carbenium ions and is particularly well-suited for kinetic measurements of species that have lifetimes in the tens of nanoseconds range and up and that have at least moderate extinction coeffleients in the UV-visible region. 

---