Methanol solutions solvation, separation

For reduction of resin 8, concerns about the possibility of traces of tin by-products contaminating subsequent assays led to the adoption of a different reduction procedure for library production.15 The tagged resins 8 are combined into one pool in a large peptide synthesis vessel, washed with methanol, filtered, and the resin left solvated. Separately, an aqueous solution of 0.5 M aqueous sodium hydrosulfite/0.5 M potassium carbonate is prepared and added to the resin (40ml/g of resin) and then the resin/solu-tion is bubbled with nitrogen at room temperature for 16 h. The resulting resin is washed with water, water/MeOH (1 1), MeOH, MeOH/NMP (1 1), NMP, DCM, MeOH, and ether, then filtered and dried overnight in vacuo prior to the next step. 

The tagged resins are pooled into one large batch in a 2-liter peptide vessel, washed with methanol, and then left solvated. Separately, solid Ox-one is dissolved in water (to a final concentration of 0.4 M), sonicating for 5 min to aid in solvation. The aqueous Oxone solution (10 equivalents with respect to the nitro group loading of the resin) is added to the methanol-solvated resin and stirred/bubbled for 16 h at room temperature. The resulting resin 10 was then washed with water, MeOH/water (1 1), MeOH, MeOH/NMP, NMP, DCM, and ether, then dried in vacuo overnight prior to the next step. 

Figure 4. Separation in methanol solutions with surface excess free energy e>f solvation at 0.005m and 1725 kPa...

The analogous diruthenium(II, II) compounds can be made starting from the blue solution of reduced RuC13 in ethanol (section 1.3.5) on extended refluxing with sodium acetate, whereupon the solvate Ru2(OAc)4(MeOH)2 separates. This loses methanol readily on drying the resulting anhydrous acetate will form weak adducts Ru2(OAc)4L2 (L, e.g. H20, thf). These are all paramagnetic with two unpaired electrons [100]. 

The ion pair extraction by flow injection analysis (FIA) has been used to analyze sodium dodecyl sulfate and sodium dodecyl ether (3 EO) sulfate among other anionic surfactants. The solvating agent was methanol and the phase-separating system was designed with a PTFE porous membrane permeable to chloroform but impermeable to the aqueous solution. The method is applicable to concentrations up to 1.25 mM with a detection limit of 15 pM [304]. 

In accordance with the increased charge separation in the transition state and products, the gas-phase EA of 22.6 kcal mol-1 for the reaction reduces to just 4.4 kcal mol-1 with inclusion of semiempirical solvation energies in water while the overall reaction, which is very endothermic in the gas phase becomes exothermic by 5.1 kcal mol-1 with solvation.179 The calculated EA is lower than the experimental values for substitution by A-methylaniline in methanol which fall in the range of 6-15 kcalmol-1 (Table 5).42,43 However, in aqueous solution these barriers would be lower than in methanol. 

B3LYP/6-31G //HF/6-31G energies, including aqueous solvation effects, predicted this reaction to be exothermic by 5.1 kcalmoG with an a of just 4.4 kcalmoG, which is lower than the experimental values for substitution by iV-methylanilme in methanol With the likely degree of charge separation in the transition state, it is reasonable to suppose that a in aqueous solution (s = 80) would be lower than the in methanol (8 = 33). 

The solvation dynamics following charge-transfer electronic excitation of diatomic solutes immersed in a methanol-water mixture provides a direct window on the molecular changes occurring upon solvent substitution. The solvation response of the mixtures is separated into methanol and water contributions in order to elucidate the role played by each molecular species on the solvation dynamics. Significandy different responses for the two solutes are found and related to the fact that the solute with the smaller site diameters is a much better hydrogen (H)-bond acceptor than the larger diameter solute (Skaf and Ladanyi, 1996). 

Carboxylates as the phenylacetate anion also eject electrons in methanol [75] giving benzyl anion after recombination between solvated electron and benzyl radical [76]. In phenyl substituted carboxylate anions (from benzoate to phenylbutyrate) in water the quantum yield of photoejected electron was found between 0.002 and 0.03 these values increase with increasing excitation energy and with the number of CH2 separating the phenyl and carboxylate group [77], In the case of phenylalanine and tryptophan in water, the mechanism seems to differ according to the conditions biphotonic and from a triplet state in neutral solution or monophotonic in basic medium [78, 79, 80]. In certain cases, the quantum yield for electron ejection is found to increase with pH [79], The anion of bromouracil also gives hydrated electron [81]. 

Another concept to consider in reversed-phase elution is selective elution. Selective elution consists of using sequential elution solvents to selectively remove several classes of solutes, or using wash solvents to remove impurities that will interfere with the analysis. An example of selective elution is the separation and isolation of herbicides and their metabolites by a reversed-phase C-18 mechanism. Figure 3.5 shows the separation of alachlor, a herbicide, and its sulfonic-acid metabolite. In this method, both compounds are sorbed to the C-18 resin by a reversed-phase mechanism. Even the ionic sulfonic acid is bound to the C-18 bonded phase. The metabolite, whose structure is shown in Figure 3.5, is a surface-active compound and is bound by reversed phase with its ionic functional group solvated by the aqueous phase. The parent compound is eluted with ethyl acetate while the ionic metabolite stays bound to the C-18 resin. Apparently the solubility of the ionic metabolite in ethyl acetate is too low for dissolution. When methanol is applied to the column, the sulfonic acid metabolite elutes from the column. Thus, a fractionation is obtained by selective elution (Aga et al., 1994). 

The effect of methanol and ethanol on the separations could possibly be explained by stronger solvation of the sample solutes in the mobile phase. However, this explanation becomes unlikely for an eluent containing only 5 % 1-butanol, the remainder being water. 

The physicochemical criteria approach to reverse osmosis separations Involving the surface excess free energy of solvation for ionized and nonlonized solutes has been demonstrated by this work to include nonaqueous solutions. The parameters and correlations presented in this work permit the prediction of reverse osmosis separations and permeation rates for different alkali metal halides for cellulose acetate OEastman E-398) membranes of different surface porosities from only a single set of experimental data for a sodium chloride-methanol reference feed solution system. 

In summary, SFE acts on four parameters that can be modified to obtain a good selectivity pressure, temperature, extraction period and choice of modifier. It is also possible to change the solvation character of the fluid by the introduction of an organic modifier (e.g. methanol, acetone). Nonetheless, to separate the analyte from the matrix requires a knowledge of the solubility and transfer rate of the solute in the solvent as well as of the physical and chemical interactions between solvent and matrix (Figure 21.8). 

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