Radical recombination dynamics

Chemically induced dynamic nuclear polarization is a spectroscopic technique that takes advantage of the coupling between electron and nuclear spins to detect products of radical recombinations by nuclear resonance. It is suited to investigation of the dynamics of radical processes, particularly the events just preceding radical recombinations. First observed in 1967 by Bargon and Fischer32 and independently by Ward and Lawler,33 the phenomenon consists of... 

Additionally, in order to examine the charge-recombination dynamics we turned to complementary nanosecond transient absorption measurements. Once more, the spectral fingerprints of the radical ion pair state emerged immediately after the laser pulse and their decays yielded charge-recombination lifetimes in the order of 4.0 ps (Fig. 9.38). 

Ogrodnik, A., Keupp, W., Volk, M., Auermeier, G., and Michel-Beyerle, M. E., 1994, Inhomogeneity of radical pair energies in photosynthetic reaction centers revelaed by differences in recombination dynamics of P when detected in delayed emission and in absorption. J. Phys. Chem., 98 343293439. 

Volk, M., Ogro(hiik, A., and Michel-Beyerle, M. E., 1995, The recombination dynamics of die radical pair P H- in external magnetic and electric fields. In Anoxygenic Photosynthetic Bacteria, (R. E. Blankenship, M. T. Madigan, and C. E. Bauer, eds.) pp. 595n626, Kluwer Academic Publishers, Dordrecht, The Nedierlands. 

Irradiation of solutions of concentration higher than 3% results in the formation of an insoluble gel because of intermolecular radical recombination similar to (4). In dilute solutions, however, intramolecular radical recombination occurs leading to cyclization. Decrease in the hydro-dynamic volume of the molecule is responsible for the decrease in the intrinsic viscosity of the solution. 

We shall consider in this review mainly the information to be obtained from ka on unimolecular dynamics. We shall in this context first review the ethane dissociation-methyl radical recombination system, to show what can be done for such a well-studied model reaction and what could eventually be done for other reactions as well. We shall then give a short summary for the most recent results on various classes of unimolecular reactions. We are afraid that this enumeration is not complete, and we apologize for any important omissions. Mechanistic and physical organic aspects, which have previously been extensively reviewed, will not be discussed. 

The spin-dependent decay of P+H" is complex. It comprises two decay pathways as shown in the kinetics scheme of Fig.3. We will briefly explain how spin-dependent recombination dynamics are responsible for inherent deviations from monoexponentiality in Fig.2 and can fully account for them, as shown by rigorous simulations. On the basis of a brief summary of radical pair dynamics, the effect will be outlined in the following section. 

The rate of diffusion controlled reaction is typically given by the Smoluchowski/Stokes-Einstein (S/SE) expression (see Brownian Dynamics), in which the effect of the solvent on the rate constant k appears as an inverse dependence on the bulk viscosity r), i.e., k oc (1// ). A number of experimental studies of radical recombination reactions in SCFs have found that these reactions exhibit no unusual behavior in SCFs. That is, if the variation in the bulk viscosity of the SCF solvent with temperature and pressure is taken into accounL the observed reaction rates are well described by S/SE theory. However, these studies were conducted at densities greater than the critical density, and, in fact, the data is inconclusive very near to the critical density. Additionally, Randolph and Carlier have examined a case in which the observed diffusion controlled, free radical spin exchange rates are up to three times faster than predicted by S/SE theory, with the deviations becoming most pronounced near the critical point. This deviation was attributed to some sort of solvent-solute clustering effect. It is presently unclear why this system is observed to behave differently from those which were observed to follow S/SE behavior. Possible candidates are differences in thermodynamic conditions or molecular interactions, or even misinterpretation of the data arising from other possible processes not considered. 

The dynamics of photoinduced charge separation, kcs, and charge recombination, kcr (Fig. 2a), have been studied in several families of hairpins containing an Sa linker and a single G C base pair by means of femtosecond time-resolved transient absorption spectroscopy [27, 28]. Both the singlet state and anion radical of Sa have strong transient absorption centered at 575 nm. The difference in the independently determined band shapes for Sa ... 

Whereas other experimental methods have been used to obtain values of kti no other method provides values of k-t or equilibrium data. There are, however, several important limitations of our method. First, the method is restricted to relatively fast hole transport processes that can compete with charge recombination of the Sa -G+ radical ion pair (Fig. 6). This precludes the use of strong acceptors which can oxidize A as well as G (Fig. 2a). We find that hole transport cannot compete with charge recombination in such systems, even when a charge gradient is constructed which should favor hole transport [35]. Second, the method is unable to resolve the dynamics of systems in which return hole transport, k t, is very slow (<104 s-1) or systems in which multiple hole transport processes occur. Third, since the guanine cation radical cannot be detected by transient spectroscopy, the method is dependent upon the analysis of the behavior of Sa-. In section 3.4 we de-... 

Advances in pulse radiolysis studies in the gas phase have been summarized in several review papers. In a comprehensive review by Sauer [4], a review presented by Firestone and Dorfman [5] in 1971 was referred to as the first review on gas-phase pulse radiolysis. Experimental techniques and results obtained were summarized by one of the present authors [6], with emphasis on an important contribution of pulse radiolysis to gas-phase reaction dynamics studies. Examples were chosen by Sauer [7] from the literature prior to 1981 to show the types of species that were investigated in the gas phase using pulse radiolysis technique. Armstrong [8] reviewed experimental data obtained from gas-phase pulse radiolysis together with those from ordinary steady-state radiolysis. Advances in gas-phase pulse radiolysis studies since 1981 were also briefly reviewed by Jonah et al. [9], with emphasis on an important contribution of this technique to free radical reaction studies. One of the present authors reviewed comprehensively the gas-phase collision dynamics studies of low-energy electrons, ions, excited atoms and molecules, and free radicals by means of pulse radiolysis method [1-3]. An important contribution of pulse radiolysis to electron attachment, recombination, and Penning collision studies was also reviewed in Refs. 10-15. 

Finally, solute radical ions can be generated by light-induced, one-photon or multiphoton ionization of their parent compounds (Chaps. 5 and 16). This approach is particularly useful in the ultrafast studies of short-lived, unstable radical ions that aim to unravel their solvation, recombination, reaction, and vibrational relaxation dynamics of the primary charges (see, e.g., Chap. 10). Whereas the time scale of radiolytic production of secondary ions is always limited by the rate with which the primary species reacts with the dispersed parent molecules, light-induced charge separation can occur in <100 fsec. There are many studies on photoionization of solute molecules in liquid solutions we do not intend to review these works. 

The potential of mean force due to the solvent structure around the reactants and equilibrium electrolyte screening can also be included (Chap. 2). Chapter 9, Sect. 4 details the theory of (dynamic) hydro-dynamic repulsion and its application to dilute electrolyte solutions. Not only can coulomb interactions be considered, but also the multipolar interactions, charge-dipole and charge-induced dipole, but these are reserved until Chap. 6—8, and in Chaps. 6 and 7 the problems of germinate radical or ion pair recombination (of species formed by photolysis or high-energy radiolysis) are considered. 

With so many uncertainties, it is hardly surprising that the difficulties inherent in a successful application of the diffusion equation (or molecular pair analysis) to recombination probability experiments are very considerable. Chemically induced dynamic polarisation (Sect. 4) is a fairly new technique which may assist in the study of recombination of radicals following their diffusive separation from the solvent cage. 

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