CIDEP theory

5 CIDEP Studies. - 2.5.i CIDEP theory. The investigation of the interac- 

The kinetics of the generation of net CIDEP in triplet-radical quenching in liquids were theoretically analysed by Shushin. Analytical formula were derived for the CIDEP generation probability and rate, as well as for the triplet-radical quenching probability and rate. The theoretical expressions describe experimental results obtained for the 1-chloronaphthalene-TEMPO system quite well. 

The kinetics of multiplet CIDEP generation for radical pair recombination (F-pairs) in low-dimensional media have been studied theoretically by Shushin et al The results obtained show that the polarization of the multiplet CIDEP is sensitive to dimensionality only in the limit of weak spin-dependent interaction and large diffusion coefficient. The theoretical expressions have been discussed in relation to some experimental CIDEP results obtained for radical reactions in porous solids and on surfaces.  

CIDEP due to S-To mixing in radical pairs has been considered with a heuristic model by Adrian. The CIDEP mechanism is split into its sequential steps (1) singlet-triplet mixing (2) polarization in a weak spin exchange encounter and (3) possible depolarization by strong spin exchange. The repeated application of this model yields the polarization as a function of time after the formation of the pair, and the final polarization agrees well with the results of stochastic Liouville treatment.  

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The findings of such peculiar phase patterns attracted considerable attention because they could not explained by the ordinary CIDEP theories (the TM, STo mixing, and ST i mixing). Buckley et al.[22] and Closs et al.[23] independently interpreted the these peculiar patterns in terms of spin-correlated radical pairs. It is noteworthy that ESR signals due to radical pairs were found to be directly observable in solution even at room temperature. Since then, CIDEP due to this spin-correlated mechanism (SCM) has often been obtained not only in micellar solutions but also in other confined systems such as in viscous solutions [24], in linked systems [25], and in bacterial photosynthesis systems [26]. 

The first discovery of chemically induced dynamic electron polarization (CIDEP) was made by Fessenden and Schuler in 1963 (58). These authors observed the abnormal spectra of the H atoms produced during the irradiation of liquid methane. The low-field line in the esr spectrum was inverted compared to the corresponding high-field line. The related chemically induced dynamic nuclear polarization effect (CIDNP) was reported independently four years later by Bargon et al. (22) and by Ward and Lawler (134). Because of the wider application of nmr in chemistry, the CIDNP effect immediately attracted considerable theoretical and experimental attention, and an elegant theory based on a radical-pair model (RPM) was advanced to explain the effect. The remarkable development of the radical-pair theory has obviously brought cross-fertilization to the then-lesser-known CIDEP phenomenon. 

In this chapter we will present, mainly for the photochemist, an overview of the development of photochemically induced magnetic polarization by providing a brief summary of the background theory (Section II) and the various experimental techniques (Secion III). No attempts will be made to cover in detail all aspects of the enormous amount of work that has been published in this field. Instead, we shall aim at general illustrations of the utility and application of CIDEP/CIDNP to photochemistry. We shall draw heavily from our own experience in this field, which is most extensive in electron polarization studies. 

Based upon the current theories of CIDEP and CIDNP, we propose that in many photochemical systems the primary photochemical reaction of the excited triplet state contributes to magnetic polarization via the triplet mechanism. The secondary reaction of the polarized primary radicals may transfer their initial polarization to the "secondary radicals" provided that the radical reactions can compete with the radical spin-lattice relaxation process (59,97). On the other hand, secondary reactions of the primary radical pair or the uncorrelated F pair contribute to polarization by the radical-pair mechanism. A general scheme showing the possible and simultaneous operations of both the... 

The overall diagram of evolution of the excited states and reactive intermediates of a photoinitiating system working through its triplet state can be depicted in Scheme 10.2 [249]. Various time resolved laser techniques (absorption spectroscopy in the nanosecond and picosecond timescales), photothermal methods (thermal lens spectrometry and laser-induced photocalorimetry), photoconductivity, laser-induced step scan FTIR vibrational spectroscopy, CIDEP-ESR and CIDNP-NMR) as well as quantum mechanical calculations (performed at high level of theory) provide unique kinetic and thermodynamical data on the processes that govern the overall efficiency of PIS. 

The controversy over the mechanism of CIDEP in radical reactions extends also to CIDNP. An elaboration of a theory for CIDNP based upon the radical-pair approach has been offered recently,424 425 and a brief note has been published which questions the need for an alternative to the radical-pair approach to account for CIDNP, and further shows that the same assumptions are necessary in both the radical-pair and Overhauser mechanisms, namely, that nuclear relaxation can occur in the escaping radicals.426 427 It would seem that, whereas in CIDEP the experimental observations require some alternative to the radical-pair treatment for satisfactory explanation, so far, CIDNP observations can be explained on the basis of the radical-pair theory, although this does not preclude the possibility that other mechanisms are also operative. 

The Theory of the CIDEP Effect. — Applications to the Study of Chemical Reactions and Magnetic Properties. The Chemically Induced Dynamic Electron Polarization (CIDEP Effect). 

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