Field-dependent CIDNP

CIDNP of the enol product showed a net emissive spectrum. Despite the complex mechanism, a field-dependent CIDNP study resolved that QH2 and the enol were the main in-cage products while the acetone was formed largely in the scavenged reaction. This conclusion was consistent with similar kinetic and mechanistic systems studied by Laroff and Fischer (87). 

Much experimental and theoretical work has been done on flexible biradicals obtained by the photolysis of cyclododecanones. Field-dependent CIDNP... 

Field-dependent CIDNP experiments in low magnetic fields reveal that CIDNP is of the comparatively rare S-T+i-type. The curves exhibit the bell-shaped behaviour t /pical for intersystem crossing at a level intersection, which has long been known for biradicals with a polymethylene chain. However, the rigidity of the dyads removes a complication present in flexible biradicals, namely, that the CIDNP experiment weighs certain conformations and distances between the radical termini more heavily than others.Hence, in the dyad experiments the value Bq at which the extremum of the field-dependence occurs corresponds directly to the condition y/lBo=2 /. An exponential dependence of / on the spacer length n was found with a damping factor of about 0.75A, which is comparable to that in other biradical systems. 

Finally, it should be noted that the effects are field-dependent. In one case ) no effects were seen when the reactions were run within the spectrometer fields, but large effects were observed for samples which were allowed to react in the earth s field or in a low field of a separate magnet and which were transferred to the probe of the spectrometer shortly after the end of the reaction. These field effects on CIDNP will deserve much future investigation. So far, only a limited amount of information is available. w. 48,4 ) and no thor-... 

The chemistry underlying the polarization processes of tryptophan, tyrosine, and histidine (electron transfer for the first two amino acids, hydrogen transfer for the third) has long been known. For the frequently used sensitizer 2, 2 -dipyridyl 16, the kinetics of the quenching by tryptophan, tyrosine, and histidine have recently been studied in detail by time-resolved CIDNP experiments the effects of added surfactants on the polarizations of tyrosine with flavins 17 or other sensitizers have been investigated. Several low-field CIDNP investigations of the field-dependence have been carried out, with a view to optimizing... 

After the classical studies by Turro and co-workers, who determined the exit rates of radicals from micelles from the changes in steady-state [62a] or time-resolved [62b] CIDNP spectra when an external scavenger was added, and analyzed the magnetic field dependence of the CIDNP signals [62c], activity in this field seems to have quieted down a little. 

In a study of phenacylphenylsulfone photolysis, CIDNP data were taken as evidence that the primary radical pairs cannot recombine to regenerate the starting material because the micelle forces a certain orientation of the radicals [63], From low-field 13C CIDNP and SNP measurements on cleavage of benzylic ketones in sodium dodecyl sulfate micelles, it was inferred [64] that the exchange interaction in these systems is several orders of magnitude smaller ( 10lorads 1 at a reduction distance of 6 A cf. the values in Section IV.B) and the distance dependence is much weaker (a x 0.5 A" cf. the discussion of Eq. 10) than generally assumed for radical pairs. By numerical solutions of the stochastic Liouville equation for a model of the micelle where one of the radicals is kept fixed at the center of the micelle while the other radical is allowed to diffuse, the results of MARY experiments, 13C CIDNP experiments at variable fields, and SNP experiments could be reproduced with the same set of parameters [65],... 

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... 

To unravel the mechanisms of the elementary stages of radical reactions by means of the CIDNP method in low magnetic fields, one usually analyses the dependencies of CIDNP efficiencies on the magnetic field strength (field dependence of CIDNP). In this case, the data on radical pair composition are obtained on the basis of comparison of experimentally measured and theoretically calculated CIDNP field dependencies. 

As a rule, the field dependence of CIDNP shows up as curves with extremum, which are located in the magnetic field with the strength of about... 

At present, the investigation of CIDNP field dependencies for products of transformation of biradicals is the most reliable indirect method of demonstrating the involvement of biradicals in chemical reactions. The structure of the biradical can be inferred by comparison of observed with calculated field dependences. For this purpose, one might employ various approximations of the radical pair described in a number of specialized monographs. 

Polyakov NE and Leshina TV (1990) Study of CIDNP field dependencies in the photoreduction of quinones by amines. Journal of Photochemistry and Photobiology A Chemistry 55 43-51. 

There are various quantitative descriptions of the pair mechanism -34) which are based on different treatments of the time dependences. For high-field polarizations, at least, they all lead to similar agreements of observed with calculated CIDNP patterns. 

With Equations (5), (25), and (30) one can also deduce approximate rules for the dependences of the CIDNP intensities on the strength of the magnetic interactions and on the field Bq. For the simple model system of a radical pair with one proton in the first radical, the population difference of the nuclear spin states a) and S) is seen to be... 

Azumi et al. carried out variable-field CIDNP experiments on the biradicals resulting from intramolecular hydrogen abstractions in polymethylene-linked xanthone and xanthene moieties [61a, 61b]. They also studied the temperature dependence of the mean exchange interaction for... 

CIDNP and CIDEP Studies.—Two mechanisms have been proposed to account for the observation of electron spin polarization in radical reactions (CIDEP), the first being termed the radical pair mechanism, in which the polarization results from the mixing of the singlet and triplet states of the radical pair by the magnetic interactions within the radicals, and the second the triplet mechanism, in which the polarization originates in the triplet as a result of spin-selective intersystem crossing from the photoexcited singlet state, as is known to occur in several systems from ODMR measurements (see above). It has recently been pointed out 480 that, for the latter mechanism, unequal population of the triplet sub-levels will depend upon zero-field D and E terms and Zeeman terms, but also... 

---