V-Coupling

The relative configuration of the protons which is deduced from the coupling constant Jab confirms the conformation of this part of the structure of this molecule. On the other hand, the V//// coupling constant of the ethyl group attached to oxygen (7 Hz, Fig. 2.19) reflects equal populations of all stable conformers around the CC bond of this ethyl group. 

Since the vinyl-C//proton at (<5//= 6.33 shows no additional V////couplings apart from the doublet of doublets splitting (cis and trans coupling), the side-chain is a 1-isoprenyl chain E and not a 1-methylbuta-l,3-dienyl residue F. 

Fluorine-19 NMR has also been used to study annular tautomerism in pyra-zoles, both for fluoro- and for trifluoromethyl-pyrazoles (99H355). Comparison of data chemical shifts and V coupling constants of 64a and 64b]... 

Attempts of Sanko and Stefanovskii to make direct measurements of the oxidation potentials of couples involving these chromium species were not successful. An estimate of the potential of the Cr(VI)/Cr(V) couple was given by West-heimer, based on the equilibrium between chromium(VI) and iron(II), viz. 

Since the reaction is markedly displaced to the left, the standard potential of Fe(III)/Fe(ir) couple (0.77 V) must exceed that of the Cr(VI)/Cr(V) couple therefore Fft/s niust be < 0.6 V. This estimate seems reasonable. Regarding the oxidation potential of chromium(IV) there is an estimate, also by Westheimer, that the Cr(IV)/Cr(III) couple has a value of 4/3 1.5 V. This was based on the fact that... 

The N, C -coupling reactions of primary amines with BENAs are very sensitive to steric factors in BENAs. For example, the reactions with terminal BENAs are difficult to stop at the mono-alkylation step, whereas in internal BENAs, it is very difficult to isolate the bis-coupling product. A special procedure, based on this fact, enables one to synthesize bis-oximes (460) containing various oximinoalkyl substituents at the nitrogen atom. It should be emphasized that diastereoselectivity of /V,( -coupling reactions of amines with terminal BENA is very low. 

N,C-Coupling reactions of BENAs with derivatives of NH-acids. As mentioned above (see Scheme 3.226), NH-acids catalyze the rearrangement of BENAs (501, 502), which is the most general process that hinders both the synthesis of BENAs and their desired transformations. Hence, silyl derivatives of AH-acids should be used in /V,( -coupling reactions with BENAs. 

Figure 21 Typical cyclic voltammograms recorded at increasing scan rates (a < b < c) for a reversible electron transfer (having E° = 0.0 V) coupled to catalytic regeneration of the reagent...

Fox, P. Venkatasubbiah, V. Coupled anaerobic/aerobic treatment of high-sulfate wastewater with sulfate reduction and biological sulfide oxidation. Water Sci. Technol. 1996, 34 (5/6), 359-366. 

The redox potentials of trigonal prismatic complexes span a similar range as that observed for octahedral V(III) complexes. The complexes V(bpy)(Cl4Cat)2 (bpy = 2, 2 -bipyridine, CUCat = tetrachlorocatecholato) and V(bpy)(acac)(Cl4Cat) have V(III/II) couples at —1.74 and —1.75 V versus Cp2Fe/MeCN respectively. The V(III/IV) couples are —0.35 and —0.50 V, respectively, values which are comparable to the octahedral TAN complexes above. However, the V(IV/V) couples are at 0.92 and 0.06 V versus Cp2Fe/MeCN respectively, a significant difference when compared to the other two couples [28]. 

Research related to the use of vanadium phosphates or V2O5 as oxidants of gases such as CO and SO2 in commercial processes shows that solid vanadyl sulfate can serve as a gas-permeable solid-phase electrolyte [102]. Two reversible redox features are observable at slow scan rates (20-150 mV s ) by CV in a gas-tight two-electrode cell packed with powdered VOSO4 3H2O between a 10-mm carbon disk and a 3-mm glassy carbon electrode. The V(IV/V) couple was observed at 0.55 V versus C, and the V(IV/III) couple was observed at —0.97 V. Unlike in aqueous solution where vanadyl sulfate is reduced to [V(H20)6] ", the V=0 bond in the solid remains intact. The oxidation of CO(g) can be observed when it is introduced into this cell. 

The vast majority of electrochemical data on americium ions has heen obtained in aqueous solutions. Americium can exist in aqueous solutions in the oxidation states III, IV, V, and VI. The divalent state is difficult to attain in aqueous solutions because of the proximity of the standard potential of the Am(III)/Am(II) couple to the solvent/supporting electrolyte breakdown potential. Previous reviews have presented the formal and standard potentials for the various americium couples and these reviews should be consulted by the interested reader for more detailed discussion [133, 134]. Table 3 contains a summary of selected formal potentials Ef from these reviews in 1 M HCIO4 for convenience. AU values are calculated from various measurement techniques except for the Am(VI)/Am(V) couple (Am02 /Am02" "), which was determined directly. 

Q ITom Ihe data given in Table 7.12. calculate the values of the standard reduction potentials lor the V V couple. 

The processes occurring in the hypolimnion and in the epilimnion have to be considered separately. The appearance of As(III) in the anoxic hypolimnion is in qualitative agreement with the thermodynamic redox sequence, because it appears together with Fe(II) and S(-II). The pe calculated from the As(III)/As(V) couple at 30-31 m on October 19, 1989, is in agreement with pe calculated from Fe(II)/Fe(III) (pe 0), but is higher than indicated by the presence of S(-II). 

---