HFCM

In the past, this field has been dominated by ruthenium, rhodium and iridium catalysts with extraordinary activities and furthermore superior enantioselectivities however, some investigations were carried out with iron catalysts. Early efforts were reported on the successful use of hydridocarbonyliron complexes HFcm(CO) as reducing reagent for a, P-unsaturated carbonyl compounds, dienes and C=N double bonds, albeit complexes were used in stoichiometric amounts [7]. The first catalytic approach was presented by Marko et al. on the reduction of acetone in the presence of Fe3(CO)12 or Fe(CO)5 [8]. In this reaction, the hydrogen is delivered by water under more drastic reaction conditions (100 bar, 100 °C). Addition of NEt3 as co-catalyst was necessary to obtain reasonable yields. The authors assumed a reaction of Fe(CO)5 with hydroxide ions to yield H Fe(CO)4 with liberation of carbon dioxide since basic conditions are present and exclude the formation of molecular hydrogen via the water gas shift reaction. H Fe(CO)4 is believed to be the active catalyst, which transfers the hydride to the acceptor. The catalyst presented displayed activity in the reduction of several ketones and aldehydes (Scheme 4.1) [9]. 

Fig. 6-1. Magnetic field eflects on the singlet-triplet (S-T) conversion of radical pairs (a) The Ag mechanism (AgM) (h) The hyperfine coupling mechanism (HFCM) (c) The levelcrossing mechanism (LCM). (Reproduced Ifom Ref. [34] by permission from The Chinese Chemical Society)...

In Section 6.2, we saw how the S-T conversion rate of radical pairs is influenced by an external magnetic field for the AgM, HFCM, and LCM. These MFEs on the S-T conversion also affect the yield of cage and escape products (Tc and Ye), which are formed through radical pairs as follows ... 

Fig. 6-2. Theoretical prediction of the magnetic field dependence on the product yield (7(5)) in the reactions through radical pairs (a) the Ag mechanism (AgM), (b) the HFC mechanism (HFCM), (c) the mixed effect of the AgM and the HFCM, and (d) the LCM. The full curves indicate the magnetic field dependence of cage (escape) products produced from S-(T-) precursors. The broken curves indicate the dependence of escape (cage) products produced from S-(T-) precursors. In this figure, cage products mean those produced from singlet radical pairs. The curves for triplet states are omitted for simplicity, but they show similar dependence as those of escape products. (Reproduced from Ref. [34] by permission from The Chinese Chemical Society)...

Since the Bia value are less than 10 mT for most organic radical pairs, the magnetically induced changes due to the HFCM are usually saturated below 0.1 T. It is noteworthy that no analytical prediction of the magnetic field dependence of Yc (B) and Ye (B) is possible in the case of the HFCM. On the other hand, the quantitative Yc (B) and Ye (B) values can only be obtained by numerical calculations with the stochastic Liouville equation [27]. 

Here, M and M are the excited singlet or triplet states of M, respectively. As shown in reaction (6-15a), a singlet radical pair is initially produced in this reaction. The ion-radicals produced in very viscous nonpolar solvents cannot leave for the bulk but recombine with the probability nearly equal to unity because the initial intercharge distance (5 to 15 nm) is less than the Onsager radius for nonpolar solvents (30 nm). The S-T conversion (reaction (6-15d)) is expected to occur though the HFCM. Thus, the S-T conversion rate should be reduced by magnetic fields. 

Brocklehurst et al. employed squalane as S and fluorene as M. They measured the time profile of fluorene fluorescence during and after pulse radiolysis and found that the fluorescence intensity was increased by a 0.3 T magnetic field as shown in Fig. 6-3(a). They also measured the time dependence of the magnetic field enhancement of the fluorescence intensity as shown in Fig. 6-3(b). This figure shows that the MFE is very small or zero during the pulse, but that it rapidly reaches an apparent plateau (40 % increase) after about 100 ns. This is due to the fact that the MFE grows in several tens ns, which is the order of the S-T conversion due to the HFCM as shown in Chapter 3. 

Brocklehurst et al. found that the MFE on the fluorescence intensity at 200 ns after the pulse increase with increasing B from 0 T to 0.1 T, but that the MFE shows a saturated value (40 % increase) with increasing B from 0.1 T to 0.5 T as shown in Fig. 6-3(c). Such a MFE on the singlet yield can be explained by the HFC from an S-precursor as shown in Fig. 6-2(b). According to the HFCM, the triplet yield should be increased by the fields of 0.1 - 0.5 T, but such a MFE on the triplet yield was not clear in this reaction. Later, such MFEs on the triplet yield were found in photochemical reactions as shown in section 6.6. Similar results were also found in cyclohexane, but the observed MFEs were less than those observed in squalane. In benzene, there was no detectable MP on the fluorescence intensity. This solvent effect can be explained by the effect on the lifetime of the generated ion-radical pairs. This means that the more viscous the solvent is the longer the radical pair lifetime becomes. 

Werner et al. measured solvent, isotope, and magnetic field effects in the geminate recombination of radical ion pairs [23. 26]. They found similar MFEs in reaction (6-21) in acetonitrile (ACN), dimethylformamide (DMF), ethanol, and 2-propanol. Their typical results on the magnetic field dependence of the pyrene triplet yield in acetonitrile are shown in Fig. 6-9. This figure shows that the yield of each reaction decreases with increasing B from 0 T, but that the increase is saturated at 65 mT. Such MFEs can be explained by the HFCM, where the S-T conversion rate in (6-21c) is reduced by magnetic fields. The Bm value of the HFCM can be expressed by Eqs (6-11) and (6-12). 

The magnetic isotope effect (MIE) is one of the most important techniques which have been developed in the course of studies of MFEs on chemical reactions. It is noteworthy that the MIE is a new type of isotope effect This effect comes from the difference in nuclear spin, but not in nuclear mass. According to the HFCM, the S-T conversion of radical pairs depends on the HF interaction between nuclear and electron spins in the component radicals, even in the absence of an external magnetic field. Therefore, it is possible for MIEs to appear in most reactions which show MFEs. 

These increases in the MFEs seem to approach their saturated values A B =. 34 T. It is noteworthy that these increases do not show saturation at 5 0.1 T, where the MFEs due to the HFCM should be saturated as shown in Chapter 6. 

These MIEs are very small at B = 0 T, but they increase with increasing B fi-om 0 T to 0.2 T. With increasing B fi om 0.2 T to 1.34 T, however, these MIEs decrease again. It is noteworthy that these MIEs can not be explained by the HFCM, because the MIEs due to the HFCM should decrease with increasing B fi-om 0 T as shown in Chapter 6. 

Because the observed MFEs and MIEs on Trp and Te at such high fields (0-1.34 T) could not be explained by the ordinary AgM and HFCM, the author s group proposed the relaxation mechanism in 1984 and succeeded in interpreting such novel MFEs and MIEs [2]. In 1997, however, Tanimoto et al. reported that they had found no MIE on trp for the same reactions of the benzophenone isotopes [3]. They have recently realized their mistakes and confirmed our results on the existence of the MIEs in these reaetions [4]. 

S)//(0T)) was found to only increase with increasing B from 0 mT. This MFE was similar to that observed for the intensity (Ej(By) of the transition absorption due to A and could be explained by the HFCM. Tanimoto et al. also observed similar MFEs on the exciplex fluorescence intensity for the polymethylene-linked compounds with A=phenanthrene and D=dimethylaniline [4]. Their Blc values are also listed in Table 8-1. 

In this reaction, two p-aminophenylthiyl radicals (ArS- ) is produced, but its HF coupling constant has not yet been obtained. Because the author s group had already observed the MFE in this reaction [11], this was another candidate for detecting the MIE of S. Indeed, we found the a values for the Si-enrichment observed for this reaction in the absence and presence of external magnetic fields as shown in Fig. 9-4(a) and Table 9-2. The fi-dependence of this MIE of S can be explained by the HFCM. 

If ES involves a radical pair, the recombination rate of ES fe) is possible to be influenced by an external magnetic field. On the other hand, ki and k should be independent of the field. Harkins and Grisssom [4] studied MFEs on the conversion of unlabeled and deuterated ethanolamine to acetaldehyde and ammonia in bacteria by ethanolamine ammonia lyase. In this reaction, AdoCbP acts as a coenzyme and a radical pair is easily generated through the enzyme-induced homolysis of the C-Co bond. The escape 5 -deoxyadenosyl radical from the pair initiates the conversion reaction. They measured MFEs on the Vmax and Vmax/Km valucs at 25°C and obtained the results as shown in Fig. 15-4. The Vmax value was independent of B up to 0.25 T. This is reasonable because kj should be independent of B. On the other hand, the Vmax/ m values of the unlabeled and deuterated systems exhibited decreases of 25 % (at 0.1 T) and 60 % (at 0.15 T), respectively. These magnetically induced deceases can be explained by the HFCM, where k2 should be increased by such low fields as 0.1-0.15 T. At higer fields, the values were found to increase from their minimum... 

Bent NNO-HF was first detected by Klemperer and co-workers using a Rabi-type molecular beam electric resonance spectrometer [161]. They obtained rotational constants and projections of the dipole moment on the a-and fi-axes of the complex. It was found that the N2O axis makes an angle of 47° with the separation between the N2O and HFCMs, which was found to be 3.5 A. Hydrogen appears to be near the O atom with an NOH angle of 116°. Subsequent molecular beam experiments by Dayton and Miller also detected the bent isomer, but in this work an infrared laser was used to excite the HF chromophore [162]. Matrix isolation techniques, in... 

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