Low-viscosity solvents

In reversed-phase TLC, mobile phases with high eontent of water do not wet alkyl-bonded siliea layers unless partially modified silica plates are used. It is well known that high-viscosity solvents do not generate the same plate number as low-viscosity solvents because of the solute diffusion coefficient and slow mass transfer [21]. 

Mobile phase selection (1) Choose a low viscosity solvent which separates the mixture and moves the desired component to an Rf of ca. 0.35 (2) If several cospounds are to be separated which run close together on TLC, adjust the solvent strength to put their midpoint at an R value of ca. 0.35 (3) If compounds are well separated, choose a mobile phase which provides an R, value of ca. 0.35 for the least retained component. 

Coating formulations (lacquers) are clear, transparent, low-viscosity, solvent-free liquids that are stable with a shelf life of several months and are suitable for optical applications ranging from the ophthalmic to the optical fibre market. As an example of the former, Figure 4.15 shows a plate half coated with a layer of ABRASIL coating material only a few micrometres thick with the scratch traces from steel wool being observed on the uncoated half.18... 

Viscosity-dependent fluorescence (VDF) typically occurs in molecules which, following absorption of excitation light, undergo nonradiative decay by intramolecular twisting or torsional motions (12-19). In ordinary low-viscosity solvents, VDF compounds exhibit... 

The phases Pi..Pj in the system may or may not be entirely uniform in temperature due to the need for heating/cooling, the presence of reaction, the degree of insulation etc. Also, the total thermodynamic pressure P in the system may not be entirely constant throughout the system even if low viscosity solvents are used in the syntheses. ... 

The observation that the yield of orrfto-rearranged product depends on the bulk of para-substituent is unique. It would be interesting to find out whether this also holds true with photorearrangement in a low viscosity solvent, e.g., in an experiment where only the substituent size would be changed preserving the electronic effect of the substituents. One can argue that the presented photorearrangement in polyethylene film is almost a photoreaction in the solid state. Such reactions occur with the minimum amount of atomic or... 

Solvents were initially selected primarily on the basis of the conductivity of their salt solutions, the classical example being propylene carbonate (PC). However, solutions based on PC on its own were soon found to cause poor cyclability of the lithium electrode, due to uncontrolled passivation phenomena. Solvent mixtures or blends were therefore developed and selection currently focuses on a combination of high dielectric solvents (e.g. ethylene carbonate, EC) with an alkyl carbonate (e.g. dimethy(carbonate, DMC), to stabilize the protective passivation film on the lithium electrode, and/or with a low viscosity solvent [e.g. 1,2-dimethoxyethane (DME) or methyl formate (MF)], to ensure adequate conductivity. 

A number of analogous compounds to BA have been reported, including 5,5 -dibenzo-[a]-pyrenyl (BBPY) [116]. These compounds exhibit emission spectra similar to BA. It would be interesting to explore the ultrafast dynamics of BBPY in order to test the generality of the GLE model. It would also be interesting to study the femtosecond dynamics of BA as a function of applied pressure. Static experiments on the emission of BA, reported by Hara et al. [123], demonstrate that in low viscosity solvents an increase of pressure affects the emission similarly to an increase of solvent polarity. As the pressure is increased, however, the LE/CT interconversion is slowed down. It would be interesting to measure C(r) in these environments and compare the solvation dynamics with LE/CT dynamics, in order to test the generality of the GLE dielectric friction model. 

The rotational relaxation of low-viscosity solvents takes place in times of ps. This can be observed through time-resolved spectroscopy when the dipole moment of the excited molecule differs substantially from that of the ground state species. Table 8.2 gives a few values of these relaxation times the alcohols show rather slow relaxation because of the hydrogen bonds which associate the solvent molecules. 

Solvents and electrolytes should also be inexpensive, nontoxic, and nonflammable. The latter two characteristics are not well satisfied by most organic solvents, but with reasonable safety precautions and reasonable ventilation they can be used routinely without incident. Another solvent property, viscosity, may be of importance on occasion. High viscosities are useful when one wishes to extend the time interval over which mass transport occurs purely by diffusion, such as for potential-step experiments, but a low-viscosity solvent is preferred when efficient mass transport is required, as in preparative electrolyses. 

Due, in the main, to quadrupolar line-broadening effects, together with the poor sensitivity, there has been relatively little general interest in 14N magnetic resonance studies and, although the quadrupolar broadening has been shown to be minimized by the use of low viscosity solvents,465 the 14N chemical shifts of only four pyrroles have been reported.465,466 The 14N chemical shift of pyrrole has been reported as either 230465 or 227466 ppm relative to the N02 ion 14N resonance, and values of 227, 230, and 225 ppm have been recorded for 1-methylpyrrole, 2,5-dimethylpyrrole, and l-methyl-2-methoxy-carbonylpyrrole, respectively.465 The associated effect of electric... 

The general procedure is as follows.63 First a low viscosity solvent system (e.g. ethyl acetate/light petroleum b.p. 30-60 °C) (1) is found which separates the mixture and moves the desired components on analytical t.l.c. to an RF of 0.35 (2). If several compounds are to be separated which run very close on t.l.c., adjust the solvent to put the midpoint between the components at RF 0.35. If the compounds are widely separated, adjust the RF of the less mobile component to 0.35. 

The low temperature luminescence spectra of the title compounds in poly(methyl methacrylate) (PMMA) are shown in Fig. 4. In addition to the solvatochromic effect the MLCT luminescence bands show a pronounced rigidochromic effect. However, not only the emission energies, but also the band shapes change when the low viscosity solvents are replaced by a rigid glassy matrix. 

Carbon dioxide is an appealing solvent in which to conduct biocatalysis, mostly because of the desirable physical properties of the solvent. Two of the most influential solvent properties are its high diffusivity and low viscosity. Solvent properties and product solubility are easily varied, especially near the critical temperature and pressure. 

In many brighteners deactivation from the Si state depends on the local environment. When fixed in (highly viscous) solid substrates, brighteners fluoresce with high quantum yields (0 ca. 0.9). In (low-viscosity) solvents, such as are used before or during application of the brightener, the quantum yield is often low and the probability of formation of photoproducts is higher. 

Although the contact approximation is a good alternative to EM for low-viscosity solvents, at higher viscosities and especially in the static limit (D = 0), CA becomes inappropriate. In this limit both cp0 and cpc turn to zero, but cp(r(l) does not, if ro / cr. Particles that are separated by the distance ro > ct survive because there is no diffusion to bring them in contact where the reaction takes place. Of course, this is nonsense if electron transfer is remote. Sooner or later it involves in the reaction all immobile partners wherever they are, turning cp(/ o) to zero. 

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