Stepwise separations

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]. 

Fig. 32 shows a stepwise separation of broad zones of thiamine mono- and diphosphate on a CS-KU-2 cellosorbent. The dynamic criteria (A and A.) predict the zones limited by sharp boundaries with a complete yield of the components at pH 4.9 for thiamine diphosphate and at pH 6.5 for thiamine monophosphate. It is noteworthy that the criterion A predetermines the slow motion ot the thiamine... 

Chankvetadze et al. have demonstrated the potential of flow-counterbalanced capillary electrophoresis (FCCE) in chiral and achiral micropreparative separations [27], Unlimited increase of separation selectivity can be achieved for binary mixtures, such as (+ )-chlorpheniramine with carboxymethyl-(3-cyclodextrin chiral selector, or a- and (3-isomers of a asparatame dipeptide. The carrier of the chiral selector or pseudo-stationary phase, electroosmotic flow (EOF), pressure-driven flow, or hydrodynamic flow can be used as a counterbalancing flow to the electrophoretic mobility of the analyte or vice versa, resulting in dramatic changes of the effective mobilities of the sample mixture components [28], This approach can be used for micropreparative CE, stepwise separations, and fraction collection of multicomponent mixtures [27],... 

Ion-exchange chromatography (IEC) Electrostatic interaction Gradient Short column wide applicability stepwise separation depending on elution conditions high capacity... 

For Ru" and Os", there is no available parent labile oxidation states, and the self-assembly processes must include some poorly reversible elementary steps (Torelli et al., 2004). For the s)mthesis of HHH-[RRu(L36)3] ", the procedure implies an intricate strategy, in which undesired coproducts are stepwise separated from the target helicate (Canard and Piguet, 2007 Figure 115). 

One of the first observations of such stepwise, separate phase transitions of the paraffinic part of an amphiphilic molecule was made on tetra-n-amyl ammonium thiocyanate At 315 K the crystals undergo a major first-order transition [AS = 71 J/(K mol) with a 4.8 % volume change]. The final transition to the isotropic, molten salt at 323 K is entropically smaller [AS. = 59 J/(K mol) with a n gible volume change]. Since typical alkali thiocyanate entropies of fusion are 30-40 J/(K mol) it is reasonable to assume that the high-temperature crystal state is that of a condis crystals with considerable conformational freedom. 

Figure 2.19 Cleavage of montmorillonite shown for (a and b) potassium montmorillonite layers by stepwise separation in MD simulation, and for (c and d) octadecylammonium-modified Energy components (van derWaals, Coulomb, montmorillonite (CEC = 91 mequiv/1(X)g). total, internal) and representative structures are (After Ref. [84].)...

It must be pointed out that transformations of this type do not take place stepwise, as indicated above the ultimate change is equivalent to the simultaneous separation of the hydroxyl group and attachment of the migrating... 

Allylation under basic conditions. Allylation can be carried out under basic conditions with allylic acetates and phosphates, and under neutral conditions with carbonates and vinyloxiranes. The allylations under neutral conditions are treated separately in Section 2.2.2.1 from those under basic conditions. However, in some cases, allylations of the same substrates are carried out under both basic and neutral conditions to give similar results. These reactions are treated together in this section for convenience. Allylic acetates are widely used for Pd-catalyzed allylation in the presence of bases tertiary amines or NaH are commonly used[6,7,4l]. As a base, basic alumina or ICF on alumina is conveniently used, because it is easy to remove by filtration after the reaction[42]. Allyl phosphates are more reactive than acetates. The allylation with 40 proceeds stepwise. At first allylic phosphate reacts with malonate and then allylic acetate reacts with amine to give 41(43]. 

Fractionation. Kett-McGee developed the ROSE process for separating the heavy components of cmde oil, eg, asphaltenes, resins, and oils, in the 1950s. This process was commercialized in the late 1970s, when cmde oil and utility costs were no longer inexpensive. In the ROSE process (Fig. 11), residuum and pentane ate mixed and the soluble resins and oils recovered in the supetctitical phase. By stepwise isobatic temperature increases, which decrease solvent density, the resin and oil fractions ate precipitated sequentially. 

The bifurcational diagram (fig. 44) shows how the (Qo,li) plane breaks up into domains of different behavior of the instanton. In the Arrhenius region at T> classical transitions take place throughout both saddle points. When T < 7 2 the extremal trajectory is a one-dimensional instanton, which crosses the maximum barrier point, Q = q = 0. Domains (i) and (iii) are separated by domain (ii), where quantum two-dimensional motion occurs. The crossover temperatures, Tci and J c2> depend on AV. When AV Vq domain (ii) is narrow (Tci — 7 2), so that in the classical regime the transfer is stepwise, while the quantum motion is a two-proton concerted transfer. This is the case when the tunneling path differs from the classical one. The concerted transfer changes into the two-dimensional motion at the critical value of parameter That is, when... 

Fig. 8. Schematic illustration of the tunnelling in a CNT-based device (a) under no bias voltage, there are no orbitals available for conduction, (b) with small bias voltage, only one molecular orbital of a CNT contributes to the carrier transport and (c) when the next molecular orbital enters the bias window, current increases stepwise. Gate voltage can shift all the orbitals upward or downward. AE indicates the energy separation of molecular orbitals.

The homolog of epinephrine in this series is a potent vasoconstrictor. Reaction of 3,4-dimethoxypropiophenone with butyl nitrite leads to nitrosation at the a position (36). Stepwise reduction of the nitrosoketone leads to the amino alcohol (37). Removal of the methyl ether affords racemic 38. Resolution of this last followed by separation of the (-) isomer gives levonor-defrine (38). ... 

Figure 8.13 Schematic diagram of 2-D planar cliromatographic separation using "D. In the first dimension (a and b) the total solvent strength was reduced stepwise at constant selectivity (Sx, Svi Sx Syi) to acliieve differences in polarity. In the second dimension (c and d), the...

W. Markowski and K. L. Czapiriska, Computer simulation of the separation in one- and two-dimensional tliin-layer chromatography hy isocratic and stepwise gradient development ,/ Liq. Chromatogr. 18 1405-1427 (1995). 

Our strategy consisted of the following steps A mixture of potential chiral selectors is immobilized on a solid support and packed to afford a complete-library column , which is tested in the resolution of targeted racemic compounds. If some separation is achieved, the column should be deconvoluted to identify the selector possessing the highest selectivity. The deconvolution consisted in the stepwise preparation of a series of sublibrary columns of lower diversity, each of which constitute a CSP with a reduced number of library members. 

According to Eigen and Tamm [87,88], ion-pair formation proceeds stepwise, starting from separated solvated ions which form a solvent-separated ion pair [C+SSA ]°, followed by a solvent-shared ion pair [C+SA ]° and finally a contact ion pair, [C+A ]° [Eqs. (4)-(6)]. All these species are solvated. The types of ion pair formed depend on the relative strength of the interaction of the involved species. 

An affinity sorbent based on WPA-PG carrying immobilized human IgG was applied to the isolation of the first component of the complement (Cl) from human serum and for its separation into subcomponents Clr, Cls and Clq by the one-step procedure [126,127]. Cl was quantitatively bound to the sorbent at 0 °C. The activities of subcomponents Clq and Clr2r2 in the unbound part of the serum were found to be 0.8% and 3.3% of the initial activities in serum. This fraction, therefore, could be used as a R1 reagent for determining the hemolytic activity of Cl. Apparently, the neighboring macromolecules of immobilized IgG resemble to some extent an immune complex, whereas Cl formation is facilitated due to the mobility of polymer chains with the attached IgG macromolecules (Cl is usually dissociated in serum by 30%). After activation of bound Cl by heating (30 °C, 40 min) the activated subcomponent Clr is eluted from the sorbent. Stepwise elution with 0.05 mol/1 EDTA at pH 7.4 or with 0.05 mol/1 EDTA + 1 mol/1 NaCl at pH 8.5 results in a selective and quantitative elution of the activated subcomponent Cls and subcomponent Clq. 

The Swedish-Norwegian continuous process and the Hercules process are discussed below Swedish-Norwegian Chematur and Norsk Spraengstoffindustri Continuous Process This process, patented in Sweden in 1953 (Ref 42), employes stepwise nitration of mono- and/or di-nitrotoluene to trinitrotoluene in a series of counter-current units, each unit consisting of a nitrator and a separator... 

As has been mentioned in an earlier section, aqueous chlorination of sulphoxides leads to sulphones. If excess reagents are used, sulphonyl chlorides may be formed directly from sulphoxides in good yields (equation 75)89,90,193. In order for this reaction to be synthetically useful, the sulphoxide used should be symmetrical. The product is presumably formed in a stepwise manner via the sulphinyl chloride [RS(0)C1] and the sulphinic acid [RS(0)0H]. In the case of chloromethyl dichloromethyl sulphoxide, the only sulphonyl chloride formed is chloromethanesulphonyl chloride (equation 76) and this may be readily separated from the other products by distillation90,193. Similarly, oxidation of dichloromethyl methyl sulphoxide and methyl trichloromethyl sulphoxide with chlorine in aqueous acetic acid leads to the formation of methanesulphonyl chloride in 75% and 86% yields respectively. Other species are also produced but these are much more volatile and thus easily removed (equations 77 and 78). In the absence of acetic acid the yields are somewhat reduced. 

Conjugated cations, anions and radicals can give the Diels-Alder reaction. In such a case, the two cr bonds are formed in two separate steps (stepwise... 

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