Radical pair theory of CIDNP

II. The Radical Pair Theory of CIDNP A. Basic Concepts... 

From the radical pair theory of CIDNP [39, 40] it can be established that CIDNP phenomena in reaction products are proof for the occurrence of radical pair intermediates. 

This report outlines the development and present status of CIDNP. Sect. 2 gives a brief account of the experiments so far reported and the generalizations of reactions and effects. Theoretical formulations of the current radical pair explanation of CIDNP are presented in Sect. 3, in particular for the so-called high-field case. Sect. 4 applies this theory to the interpretation of CIDNP phenomena in several illustrative examples, and, finally. Sect. 5 is devoted to the discussion of a few pertinent questions. Most of the work described in the literature is mentioned. However, we are not aiming at a broad and complete survey of all aspects and prefer to emphazise the basic facts and applications. 

The quantitative theory of CIDNP " is developed to a state where the intensity ratios of CIDNP spectra can be computed on the basis of reaction and relaxation rates and the characteristic parameters of the radical pair (initial spin multiplicity, T) the individual radicals (electron g factors, hfcs, a) and the products (spin-spin coupling constants, J). On the other hand, the patterns of signal directions and intensities observed for different nuclei of a reaction product can be interpreted in terms of hfcs of the same nuclei in the radical cation intermediate. 

The theory of CIDNP depends on the nuclear spin dependence of intersystem crossing in a radical (ion) pair, and the electron spin dependence of radical pair reaction rates. These principles cause a sorting of nuclear spin states into different products, resulting in characteristic nonequilibrium populations in the nuclear spin levels of geminate (in cage) reaction products, and complementary populations in free radical (escape) products. The effects are optimal for radical parrs with nanosecond lifetimes. 

Fig. 13. CIDNP effects observed for the cyclobutane signals of the dimethylindene dimer during the photoinduced electron transfer reaction with chloranil (a), and simulated spectra based on the radical pair theory and assuming a ring-opened (extended) dimer radical cation (b), a ring-closed (localized) dimer radical cation (c) and the consecutive ( cooperative ) involvement of open and closed radical cations (d) [256]...

According to the radical pair theory [54], the CIDNP effect results from the competition between a geminate spin dependent radical (ion) pair reaction and the separation of the radicals (radical ions) by diffusion [55]. The free radicals (radical ions) may react with diamagnetic species (e.g. solvent) and accordingly, the escape products have certain nuclear spin states. overpopulated. An enhanced absorption or an emission line in the NMR spectrum is observed. 

If we are to use the radical pair theory to explain the effects of micellization on the cage reaction probability as well as the magnetic field effect, it is mandatory that we be able to observe CIDNP in these systems. In addition, since CIDNP is sensitive to events on the time scale of the radical pair lifetime, detailed analysis of the CIDNP can often lead to mechanistic insight to the dynamics of the radical pair. Below we describe one such result. 

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. 

Anomalous (in the sense of radical pair theory) polarization phases and magnetic field dependence were reported for substrates containing 19F nuclei, and were explained by cross-relaxation [67], Azumi and co-workers [68] investigated the photolysis of benzaldehyde. The polarizations could be accounted for by S-T0 mixing at high fields and by S-T mixing at low fields. At a field of 325 mT, however, the authors could not reconcile the CIDNP phase with the predictions of the radical pair mechanism from additional DNP experiments, they concluded that cross-relaxation with... 

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. 

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... 

On the basis of the idea that CIDNP effects are caused by interactions within radical pairs, quantitative formalisms capable of explaining many features of CIDNP were given by Closs and by Kaptein and Oosterhoff >. The theory was later refined and modified by various authors, but the new developments did not change the basic concepts. 

Various comparisons of experimental and predicted CIDNP spectra have been reported, and an example is given in Fig. 5. Similar agreement is obtained for any of the above-mentioned theories of Qoss-Kaptein-Oosterhoff > Adrian Kaptein and Fischer ). This may be taken as support for the basic radical pair concept which is inherent in all the formulations. 

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