Fast spin relaxation

Use of DPS, WURST and HS pulses (for 41 testing methods). Sample with fast spin relaxation. 

Magnetic measurements, although useful for confirmation of spin state, have little diagnostic value, and ESR signals are usually not observed, reputedly because of fast spin relaxation. 

A final noteworthy feature of these spectra is the lack of RPM polarization, which for these radicals would appear as low-field emissive, high-field absorptive transitions. It is curious that such polarization never develops at any delay time, even out to 20 xs where we have observed only TM polarization. The creation of RPM polarization requires reencounters of radicals on a suitable timescale and modulation of the exchange interaction between the unpaired electrons. This is normally accomplished by diffusion of the radicals between weak and strong exchange regions. That it never develops indicates that either these radicals do not make a significant number of reencounters, or perhaps it is due to the fast spin relaxation in the oxo-acyl radical. It may also be that the TM is simply so dominant that the RPM intensity is always much weaker and is never observed. At lower temperatures (cf. Fig. 14.1, top spectrum at 25°C), there does appear to be a slight superposition of an EIA pattern on top of the emissive TM polarization, but it has a very small effect. 

Fig. 10-8 shows the observed CIDEP spectra for the reaction of triplet eosin Y (FlBr/ ) with duroquinone. In this figure, CIDEP spectra of the duroquinone radical anion were only observed. The spectra of Xn" were not observed because of its fast spin relaxation. As clearly shown in Fig. 10-8, the initial spectrum measured at 60 ns after the laser excitation showed an emissive polarization, which was due to the usual p-type TM. This polarization was found to change as the delay time was increased. The spectrum measured at 200 ns after the excitation showed a strong absorptive polarization, which was proposed to be due to the d-type TM. Similar polarization changes were also observed for such dyes as erthrosin B (FlLi ) and dibromofluorescein (FlBr2 ) which contain heavy atoms. On the other hand, an emissive polarization was only observed for the reaction of fluoresein (Fl ), which contain no heavy atom. From these results, Tero-Kubota et al. concluded that the strong absorptive... 

Figure 15.13 presents plots for the reaction between the hydrogen and hydroxyl radicals. For the radical-radical reactions involving OH, the spin factor equal to unity can be assumed because of an abnormally fast spin relaxation of OH, occurring within 1 ns. ° At room temperature the reaction H OH H2O, like most of the radical-radical reactions and some of the radical-molecule reactions, occurs at rates limited by diffusion. However, one cannot assume that these reactions remain diffusion-controlled at high temperatures. As illustrated in Figure 15.13, if diffusion coefficients of the reactants increase with temperature faster than k, the reaction rate can be limited by the chemical step. 

A high-spin Fe + signal, similar to an oxidized form of rubredoxin, was also detected. After reduction with NADPH and NADH, two iron-sulfur clusters, [4Fe-4S] and [2Fe-2S], were observed by EPR. The [4Fe-4S] cluster, but not the [2Fe-2S] cluster, could also be reduced by dithionite or 5-deaza-flavinyoxalate. A third, and as yet unidentified, iron-sulfur cluster with g = 2.057 was also observed. This could be reduced by dithonite but not by NADH or NADPH. The extremely fast spin-relaxation rates of semiquinone and flavin radicals suggested that they are in close proximity to the [4Fe-4S] cluster or the high-spin Fe " " centre. 

Spin effects for the hydroxyl radical can be neglected due to the fast spin relaxation time, which from this work is estimated to be <20 ps. 

H3C)C6H40 and p—OC6H4O , whilst for reactions between e + OH, N3, Br2 and 12 the spin factor is found to be close to unity. For the e + OH the spin factor is close to unity because of the unquenched orbital angular momentum in linear radicals, which through the spin-orbit coupling mechanism can lead to very fast spin relaxation. 

Determine how fast spin relaxation on the hydroxyl radical needs to be to reproduce the observed polarisation phase, assuming the dissociative route of H2O2 to proceed via the singlet state. 

Forbes further argues that because such a large polarisation can be created on the escaped (R -I- OH), even very fast spin relaxation may not completely quench the polarisation. In this mechanism, the nature of the precursor is irrelevant as well as the hydroxyl spin relaxation time. However the concentration of scavengers and the lifetime of both the symmetrical and non-symmetrical radical pairs will influence the magnitude of the E/A phase as well as spin-dependent reactivity. Whilst this mechanism offers the simplest explanation for the observed polarisation phase, it... 

Paramagnetic impurities in the cement (e.g. Fe ) provide FI relaxation centres at pore surfaces. These particles have strong dipole moments due to unpaired electrons, which causes very fast spin relaxation. Barberon et al. (2003), Godefroy et al. (2001), Korb et al. (1997) and McDonald et al. (2005) showed how the surface relaxivity of water on pore surfaces may be calculated given the surface density of paramagnetic Fe impurities. A measure of the pore volume then develops from the fast-exchange model of relaxation of Brownstein and Tarr (1979) and Zimmerman and Brittin (1957). 

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