The Second Wave

Another major invention that came only two years later was the ability to grow epitaxial SiC on Si substrates [24]. Research on SiC gained new speed, but it was a spark that never really caught on. However, there was generally a greater awareness of SiC and its potential, which was important for the years to come. 

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The condition that gives rise to multiple shock fronts (i.e., allows a shock wave to bifurcate as indicated in Fig. 4.10(b)) will occur when the second wave propagation velocity (with respect to the laboratory) is given by (4.39). How-... 

Typical stress-time profiles for the various materials (28.5-at. % Ni, fee and bcc) and various stress regions are shown in Fig. 5.12. The leading part of the profile results from the transition from elastic to plastic deformation. The extraordinarily sharp rise in stress for the second wave in Fig. 5.12(a) and the faster arrival time compared with that in Fig. 5.12(b) is that expected if the input stress is above the transition, whereas the slower rise in Fig. 5.12(b) is that expected if the stress input to the sample is below the transition. The profile in Fig. 5.12(c) for the bcc alloy was obtained for an input particle velocity approximately equal to that in Fig. 5.12(a) for the fee alloy. The bcc alloy shows a poorly defined precursor region, but, in any event, much faster arrival times are observed for all stress amplitudes, as is indicative of lower compressibility. 

The second wave is ascribed to the reduction of the hydrogen peroxide either to hydroxyl ions or to water ... 

More recently it has become apparent that proton equilibria and hence pH can be equally important in aprotic and other non-aqueous solvents. For example, the addition of a proton donor, such as phenol or water, to dimethylformamide has a marked effect on the i-E curve for the reduction of a polynuclear aromatic hydrocarbon (Peover, 1967). In the absence of a proton donor the curve shows two one-electron reduction waves. The first electron addition is reversible and leads to the formation of the anion radical while the second wave is irreversible owing to rapid abstraction of protons from the solvent by the dicarbanion. 

In earlier sections, we have seen that the energy of a molecular orbital can be expressed in terms of the coefficients, a and the Coulomb and exchange integrals. For the second wave function, this can be expressed as... 

CPE on the second wave yields (Ph3Sn)2Hg and Ph3SnH, which is taken as evidence for the formation of Ph3SnCl as an intermediate. A 4e reduction at the fourth wave is also assumed possible, involving the dimer formed in reaction 2 ... 

As noted in Section 2.2.5, the effect of dimerization may also be seen on the second wave, the wave that corresponds to the reduction of the radicals formed at the first wave. The example presented in Figure 2.35 shows the cyclic voltammetry of benzaldehyde in basic ethanol.26 The second wave represents the reduction of the benzaldehyde anion radicals formed at the first wave that have escaped dimerization. In other words, Scheme 2.29 should be completed by Scheme 2.30. 

FIGURE 6.3. Concentration profiles for two separated waves (Figure 1.25) at a potential located beyond the second wave, a with no disproportionation process, b In the presence of a fast disproportionation. —, A , B —, C. 

In a reaction scheme where dimerization of an intermediate and its reduction compete as in Scheme 6.1 (taking reductions as an example), the location and characteristics of the second wave in cyclic voltammetry at which the intermediate B is reduced are governed by the outcome of this competition. 

At the level of the second wave, the right-hand side of the second equation is practically nil, thus leading to //l = //2 = i///2. Equation (6.97) may be recast as... 

At a still more negative potential (for reductions, positive for oxidations), the plateau of the second wave is reached, where the substrate is reduced (or oxidized) directly at the electrode. Then... 

Leventis et al. (2002) stndied the electrochemical reduction of 4-(4-snbstitnted-benzoyl)-7V-methylpyridinium cations. The anthors demonstrated two chemically reversible, well-separated one-electron waves for all except the 4-(4-nitrobenzoyl)-7V-methylpyridininm cation. The latter underwent not two, bnt three one-electron rednctions and the first wave corresponded to NO2 transformation into NO2. Correlating the third-wave potential of the nitro representative to the second-wave potentials of the others, Leventis et al. determined ap(N02 ). The statistically weighed value of ap(N02 ) was found to be -0.97. For comparison, ap(S ) is equal to -1.21. It is worth noting that a (N02 ) = -0.17 and ap(N02 ) = -0.97 were established in the framework of the same experimental approach althongh with a time lag of 30 years. 

After damage or infection, monocytes and KCs in the area detect the damaged cells or infectious agent and respond with release of primary mediators such as TNFa, IL-1 and some IL-6. These cytokines activate the surrounding cells, that respond with a secondary, amplified release of cytokines. This second wave includes large amounts of IL-6, which induce the synthesis of acute phase proteins in hepatocytes and chemoattractants such as IL-8 and MCP-1. These events will then lead to the typical inflammatory reactions. Both IL-1 and TNFa activate the central regulatory protein of many reactions involved in immunity and inflammation, nuclear factor kappa B (NFkB). These cytokines cause dissociation of NFkB from its inhibitor IkB, which makes translocation of NFkB to the nucleus possible. In the nucleus active NFkB induces the transcription of the second wave cytokines (see also Chapter 7 for the molecular mechanisms of cytokine-mediated cell activation). 

The radical-cation reacts with the nucleophile to form a radical intermediate, which is then oxidised to the carbonium ion, usually at a potential less positive than that required for die first electron transfer process. However, enol ethers show two one-electron waves on linear sweep voltammetry. The firet wave is due to the formation of the radical-anion and the second wave to oxidation of carbon radical intermediates to the carbonium ion. 

Around pH 6-8, two polarographic waves are seen and die sum of the two wave heights corresponds to a one-electron process. The first wave is due to the two reactions above and decreases in height because protons are in low concentration and do not diffuse sufficiently fast to the electrode surface. The second wave is due to formation of the radical-anion followed by proton transfer from a general acid present as a component of the buffer. In alkaline solution, the concentration of acid component in the buffer decreases and this wave moves towards more negative potentials. Finally, E>/. becomes independent of pH in very alkaline solution where... 

For secondary alkyl iodides, the two one-electron polarographic waves are more separated. Reduction of 2-iodooctane at the potential of the first wave alfords the dialkylmercury and 7,8-dimethyl-tetradecane by reactions of the sec-octyl radical. At the potential of the second wave only octane and octenes are isolated [37]. 2-Bromooctane shows only one polarographic wave and yields octane and octene on reduction at any potential [37]. Radicals generated by reduction of primary and secondary iodoalkanes will react with other cathode materials including tin, lead and thallium to form metal alkyls [48,49],... 

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