Low-field CIDNP

The formal treatment of polarization at low fields has been given by Kaptein and Hollander (77), Morris et al. (101), Hutchinson et al. (72,138) and Vyas and Wan (127). Qualitative features of both the scavenged and the in-cage products low-field CIDNP can be predicted by the nonvanishing matrix elements ahfs-For a triplet precursor,... 

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... 

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. 

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-... 

In 1963, Fessenden and Schuler [1] found during irradiation of liquid methane (CFLt and CD4) at 98 K with 2.8 MeV electron that the low-field line for both hydrogen and deuterium atoms appeared inverted (emissive signals) and that the central deuterium atom line was very weak. Although the cause of such anomalous ESR spectra was not clear at that time, similar anomalous signals have been observed in many reactions and have been called " Chemically Induced Dynamic Electron Polarization (CIDEP). CIDEP should be due to non-equilibrium electron spin state population in radicals and could also be explained later by the radical pair mechanism as CIDNP. 

As will be explained in Section 3.2, multiplet and higher multiplet effects are much more prominent at low fields. For that reason, CIDNP net effects prevail on today s high-field spectrometers. 

S-To-type, and reports of S-T i-type CIDNP imder these conditions are very rare, even in low fields. All recent examples involve radical pairs with very large hyperfine coupling constants caused by an inorganic nucleus ( P or Sn). Even then, diffusion must favour longer dwell times around the level crossing, as is seen by the observation that low-field net CIDNP of P from the radical-pair trimethylbenzoyl/dimethoxyphosphonyl still arises to 60-70% through S-To-type CIDNP in the less viscous solvent acetonitrile, while in the more viscous solvent dioxane the contribution of this intersystem crossing pathway decreases to 20-50%. ... 

It can safely be stated that flow or transfer devices are the most frequent add-ons to the conventional hardware a routine CIDNP experiment needs, for two reasons. First, CIDNP relies on chemical turnover, so a depletion of the reactants or a buildup of unwanted products are commonly encoimtered problems a flow system can help avoid. Second, CIDNP in low and variable magnetic fields is very attractive because valuable information can be obtained however, spectral resolution would be totally lost if the low field inducing the polarizations were also used for acquiring the free induction decay, so a means of transferring the sample between different magnets, or at least different field regions, is indispensable. 

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... 

Morozova O B, Tsentalovich Y P, Yurkovskaya A V and Sagdeev R Z 1998 Consecutive biradicals during the photolysis of 2,12-dihydroxy-2,12-dimethylcyclododecanone low- and high-field chemically induced dynamic nuclear polarizations (CIDNP) study J. Rhys. Chem. A 102 3492-7... 

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. 

It is necessary to note that CIDNP formed in high and low magnetic fields, such as the geomagnetic field, are two essentially different physical phenomena. 

A low magnetic field is defined as one in which the energy of the Zeeman interaction is of the same order of magnitude as the hyperfine interaction (see Figure 2). Therefore, a nonequilibrium population of nuclear states may arise not only via S-Tq transitions, but also through S-T and S-T+ transitions between energy levels of a radical pair. The observed sign of CIDNP will predominantly depend on the difference in efficiencies of S-T,. transitions. 

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