Stability solvents

A difficult problem that prevented the use of nanofiltration in organic solvents for a long time was the limited solvent stability of polymeric nanofiltration membranes, and the lack of ceramic nanofiltration membranes. For polymeric membranes, different problems occurred zero flux due to membrane collapse [54], infinite nonselective flux due to membrane swelling [54], membrane deterioration [55], poor separation quality [ 5 6], etc. I n an early study of four membranes thought to be solvent stable (N30F, NF-PES-10, MPF 44 and MPF 50), it was observed that three of these showed visible defects after ten days exposure to one or more organic solvents, and the characteristics of all four membranes changed notably after exposure to the solvents [15]. This implies that these membranes should be denoted as semi-solvent-stable instead of solvent stable. 

Less information is available about the stability of ceramic membranes. It is generally thought that ceramic membranes have excellent solvent stability. Acid conditions may be more problematic it was shown [57] that an alumina nanofiltra-tion membrane was very sensitive to corrosion effects in dynamic experiments, whereas the performance of a similar titania membrane was stable in the pH range from 1.5 to 13. 

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Ding C F, Wang X B and Wang L S 1998 Photoelectron spectroscopy of doubly charged anions intramolecular Coulomb repulsion and solvent stabilization J. Phys. Chem. A 102 8633... 

FIGURE 8 9 A polar solvent stabilizes the transition state of an SnI reaction and in creases its rate... 

Recently a definitive study of several isoxazol-5-ones using infrared and ultraviolet spectroscopy (Table I) has shown that the balance between the various tautomers is a delicate one and that all three of the structural types can predominate depending upon the nature of the substituents and the conditions of the experiment. However, the hydroxy form is only found when it is stabilized by chelation (i.e., a carbonyl substituent in the 4-position). The other compounds exist in the CH form in nonpolar media increasing polarity of the solvent stabilizes increasing amounts of the more polar NH forms. 

Solvent Polar solvents stabilize the carbocation intermediate by... 

Certain solvents stabilize intermediates by strong solvation. The best known example is that of the electron itself which forms stable solutions in a number of solvents including liquid ammonia and hexamethyl-... 

Requirements of Standards. The general requirements for luminescence standards have been discussed extensively (3,7-9) and include stability, purity, no overlap between excitation and emission spectra, no oxygen quenching, and a high, constant qtiantum yield independent of excitation wavelength. Specific system parameters--such as the broad or narrow excitation and emission spectra, isotropic or anisotropic emission, solubility in a specific solvent, stability (standard relative to sample), and concentration--almost require the standard to be in the same chemical and physical environment as the sample. 

Solvents such as organic liquids can act as stabilizers [204] for metal colloids, and in case of gold it was even reported that the donor properties of the medium determine the sign and the strength of the induced charge [205]. Also, in case of colloidal metal suspensions even in less polar solvents electrostatic stabilization effects have been assumed to arise from the donor properties of the respective liquid. Most common solvent stabilizations have been achieved with THF or propylenecarbonate. For example, smallsized clusters of zerovalent early transition metals Ti, Zr, V, Nb, and Mn have been stabilized by THF after [BEt3H ] reduction of the pre-formed THF adducts (Equation (6)) [54,55,59,206]. Table 1 summarizes the results. 

Table 1. THF-stabilized organosols of early transition metals. (Reprinted from Ref. [53], 2007, with permission from Wiley-VCH.) Solvent Stabilized Early Transition Metal Colloids...

The solvent-free controlled thermolysis of metal complexes in the absence or presence of amines is the simple one-pot synthesis of the metal nanoparticles such as gold, silver, platinum, and palladium nanoparticles and Au-Ag, Au-Pt, and Ag-Pd alloy nanoparticles. In spite of no use of solvent, stabilizer, and reducing agent, the nanoparticles produced by this method can be well size regulated. The controlled thermolysis in the presence of amines achieved to produce narrow size dispersed small metal nanoparticles under milder condition. This synthetic method may be highly promising as a facile new route to prepare size-regulated metal nanoparticles. Finally, solvent-free controlled thermolysis is widely applicable to other metal nanoparticles such as copper and nickel... 

After co-deposition the frozen matrix is allowed to melt (Scheme 1(B)). This generates solvent-stabilized metal microclusters (Solvated Metal Atoms, SMA), which are soluble in the excess of ligand and can be recovered and handled at low temperature (—50 to -30 °C) under an inert... 

The synthetic procedures applied utilize the photochemical generation of the solvent stabilized 16e -species [(ij5 -H3CC5H4)Mn(CO)2 x THF] 6 by photolysis of (7j5-H3CC5H4)Mn(CO)3 5. [(t/5-... 

The solvated H3N. . . HBr complex was studied by Ruiz-Lopez et al.m using the ellipsoidal cavity in the DFT(B88/P86)/SCRF calculations which demonstrated that the solvent stabilizes the ionic pair structure. The authors found only one conformational minimum of the complex, which electronic structure corresponded to that of anionic pair. 

In a study looking at solvent stability [424], various aqueous-miscible solvents (tetrahydrofuran, acetonitrile, isopropanol, methanol, and A,A-dimethylformamide) were used with pinacyanol chloride as substrate. Although a PAH was not used as a substrate, the results may be extrapolated to PAH reactions. The greater impact of peroxide as compared to the solvent on biocatalyst stability was reported in this study and the need to control peroxide concentration was noted. 

The apparent lowering of the rotational barrier in triafulvenes is open to interpretation either by substituent or solvent stabilization of ground-state polarity leading to a decrease of C3/C4 double bond character or by stabilization of a more polar - probably perpendicularly orientated184 — transition state by substituent or solvent effects. 

Dispersion during addition Use of alcohol solvents Stability and shelf life (gel and precipitate formation)... 

To test the validity of this mechanism, chromium carbonyl (1.0 g) was photolyzed under Ar at ambient temperature in a solution of methanol and hexamethylphosphoramide in the apparatus shown in Figure 5. The lamp was turned off periodically to check for the disappearance of slightly soluble Cr(C0) . Several photolyzing cycles were necessary to effect nearly complete conversion to the solvent-stabilized coordinately unsaturated species (equivalent to in Figure 4),... 

In the more polar solvent, chloroform, linear Arrhenius correlations were observed for the rearrangements of both 10a and 10b from —55 to 60°C. Arrhenius parameters were a = 3.6 kcal/mol and log A = 10.4 s-1 for 10a, and Ea = 4.05 kcal/mol and log A = 10.3 s 1 for 10b.66 These results accord with the idea that polar solvents stabilize the polar 1,2-H shift, hydride-like transition state (51), accelerating this reaction at the expense of potential competitors.4,22... 

Convincing evidence was found that the majority of acyclic aldo-nitrones exist in the Z-form, by investigating the ASIS-effect (aromatic solvent induced shift effect) (399). However, in some cases, specified by structural factors and solvent, the presence of both isomers has been revealed. Thus, in C -acyl-nitrones the existence of Z -and -isomers was detected. Their ratio appears to be heavily dependant on the solvent polar solvents stabilize Z-isomers and nonpolar, E-isomers (399). A similar situation was observed in a- methoxy-A-tert-butylnitrones. In acetone, the more polar Z-isomer was observed, whereas in chloroform, the less polar E-isomer prevailed. The isomer assignments were made on the basis of the Nuclear Overhauser Effect (NOE) (398). /Z-Isomerization of acylnitrones can occur upon treatment with Lewis acids, such as, MgBr2 (397). Another reason for isomerization is free rotation with respect to the C-N bond in adduct (218) resulting from the reversible addition of MeOH to the C=N bond (Scheme 2.74). The increase of the electron acceptor character of the substituent contributes to the process (135). 

However transient and elusive cationic zirconocenes may be, they are real and some of their salts have been isolated and characterized, both spectroscopically and by X-ray crystallography. None of these cations is ever completely free they are either coordinated by an additional ligand (typically a solvent molecule) or they interact with their counterions, no matter how non-coordinating these may be [16], Scheme 8.1 shows the typical reactivity pattern of a solvent-stabilized cation [Cp 2ZrR(solvent)]+ (A Cp = q5-C5H4Me)... 

In the gas phase, BuCl would undergo acid elimination[10], but in polar solution the Bu+ and Cl" ions are produced in a heterolytic fashion. This reflects the strong solvent stabilization of the ionic state ] 1). 

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