Stepwise scanning —

Temperature-risiag elution fractionation (tref) is a technique for obtaining fractions based on short-chain branch content versus molecular weight (96). On account of the more than four days of sample preparation required, stepwise isothermal segregation (97) and solvated thermal analysis fractionation (98) techniques usiag variatioas of differeatial scanning calorimetry (dsc) techniques have been developed. 

The SCB distribution (SCBD) has been extensively studied by fractionation based on compositional difference as well as molecular size. The analysis by cross fractionation, which involves stepwise separation of the molecules on the basis of composition and molecular size, has provided information of inter- and intramolecular SCBD in much detail. The temperature-rising elution fractionation (TREE) method, which separates polymer molecules according to their composition, has been used for HP LDPE it has been found that SCB composition is more or less uniform [24,25]. It can be observed from the appearance of only one melt endotherm peak in the analysis by differential scanning calorimetry (DSC) (Fig. 1) [26]. Wild et al. [27] reported that HP LDPE prepared by tubular reactor exhibits broader SCBD than that prepared by an autoclave reactor. The SCBD can also be varied by changing the polymerization conditions. From the cross fractionation of commercial HP LDPE samples, it has been found that low-MW species generally have more SCBs [13,24]. 

The time factor in stepwise potentiostatic or potentiodynamic polarisation experiments is very important, because large differences can be caused by changes in the scanning rate. Since the steady state depends on the particular system and conditions of exposure, no set rule exists for the magnitude or frequency of potential changes. Chatfield etal. have studied the Ni/H2S04 system and have shown how becomes more passive with increase in sweep rate. 

FIGURE 6.17 Densitograms of Azulan extract scanned at 410 nm divided into fractions a to i (a) isocratic development, ethyl acetate in chloroform (1 5), (b) stepwise gradient development, 10 to 40% v/v of ethyl acetate in chloroform. (From Matysik. G., Soczewinski, E., and Polak, B., Chromatographia, 39, 497-504, 1994. With permission.)... 

The same family of compounds shows another interesting feature, namely, the existence of borderline cases exhibiting an intermediate behavior between the concerted and stepwise mechanisms. More precisely, the width of the cyclic voltammetric peak and the variation of its location with scan rate change from a concerted to a stepwise behavior as the scan rate is raised (Fig. 4 and Scheme 6). 

Fig. 4 Electrochemical reduction of a sulfonium cation (Scheme 6) showing the transition from the concerted to the stepwise mechanism as driving force increases upon raising the scan rate.33 The apparent transfer coefficient, a, is derived from the peak width according to equation (17).

Another striking example, recently discovered, is the reduction of iodoben-zene in DMF.61 The variation of the apparent transfer coefficient with the scan rate indicates that the mechanism passes from concerted to stepwise as the driving force increases (Fig. 5). As expected, the zone where the concerted mechanism prevails enlarges as one raises the temperature. In contrast, bromobenzene and 1-iodonaphthalene exhibit the characteristics of a stepwise mechanism over the whole range of scan rate. 

The example chosen in Fig. 8 corresponds to the passage from a concerted to a stepwise mechanism as observed by means of cyclic voltammetry upon increasing the scan rate and/ or decreasing temperature. At the peak, the free energy of activation is given by equation (43)30 32... 

How can these photochemical and electrochemical data be reconciled With the benzylic molecules under discussion, electron transfer may involve the n or the cr orbital, giving rise to stepwise and concerted mechanisms, respectively. This is a typical case where the mechanism is a function of the driving force of the reaction, as evoked earlier. Since the photochemical reactions are strongly down-hill whereas the electrochemical reaction is slightly up-hill at low scan rate, the mechanism may change from stepwise in the first case to concerted in the second. However, regardless of the validity of this interpretation, it is important to address a more fundamental question, namely, whether it is true, from first principles, that a purely dissociative photoinduced electron transfer is necessarily endowed with a unity quantum yield and, more generally, to establish what are the expressions of the quantum yields for concerted and stepwise reactions. 

This technique was used by Delmas et al. [404] to separate lipid extracts in seawater into various classes. Lipid classes that have been eluted away from the point of application may be burnt off the rod in a partial scan, allowing those lipids remaining near the origin to be developed into the place that has just been simultaneously scanned and reactivated. By analysis of complex mixtures of neutral lipids in this stepwise manner it is possible to be more selective about lipid class separations as well as to be more confident about assigning identities to peaks obtained from a seawater sample. In addition, this approach also reduces the possibility of peak contamination by impurities which would normally coelute with marine lipid classes (e.g., phthalate esters [403]). 

Equation (3.5) also shows that the activation free energy at the peak, AGj, is an increasing function of temperature, taking into account the explicit presence of T and also the variation of k, [equation (1.34)] and Dh. Thus, increasing scan rate and decreasing temperature favor the transition between concerted and stepwise mechanisms, and vice versa. 

The apparent transfer coefficient, as derived from the peak width and the variation of the peak potential with the scan rate, is small (between 0.2 and 0.3) in all cases. This rules out the occurrence of a stepwise mechanism (46, 47), in which the follow-up, bond-breaking step would have been... 

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