Medium polar solvents

The TICT model was put forward to account for the observation that the dual fluorescence of DMABN with its normal band (B band) at around 350 nm and its anomalous one (A band, around 450 nm in medium polar solvents) depends on the... 

Figure 5.2. Grabowski s model of TICT formation in DMABN the locally excited (LE) state with near-planar conformation is a precursor for the TICT state with near perpendicular geometry. The reaction coordinate involves charge transfer from donor D to acceptor A. intramolecular twisting between these subunits, and solvent relaxation around the newly created strong dipole. Decay kinetics of LE and rise kinetics of the TICT state can be followed separately by observing the two bands of the dual fluorescence. For medium polar solvents, well-behaved first-order kinetics are observed, with the rise-time of the product equal to the decay time of the precursor, but for the more complex alcohol solvents, kinetics can strongly deviate from exponentiality, interpretable by time-dependent rate constants. 52 ...

In Table 2.5 a comparison between equilibrium (eq) and nonequilibrium (neq) IR intensity shifts (solvent-gas) is reported for some methylketones in a medium polarity solvent (1,2-dichloroethane) and in a polar solvent (acetonitrile). Data are taken from ref. [42], Nonequilibrium shifts are in very good agreement with experimental measurements, whereas a pure equilibrium model fails in reproducing the solvent-induced shifts. 

The PhytoLogix Discovery Process relies upon multiple standardized extraction and fractionation protocols that allow the generation of diversified extracts and fractionation libraries in a high throughput format at a reasonable cost. Each biomass was extracted with a medium polarity solvent combination, such as methylene chloride/methanol in a ratio of 1 1 followed... 

In medium polarity solvents, recent spectroscopic measurements have been able to distinguish between SSIP and CIP in studies on [S-naphtholate [22]. This work confirms the general picture expounded by Winstein on the existence of various charge-transfer intermediates ranging from tight CIP s to loose SSIP s [23]. 

Cm (30 D), the shift between the vapour state and a nonpolar solvent (fir = 2) will be ca. 4000 cm , and the shift from a nonpolar to a medium polar solvent (fij = 30) will be ca. 12000 cm [90]. The last term in Eq. (6-2) will be important only if //g or (//g — //g) is also large. In this case, the two preceding terms are usually much larger and will essentially determine the solvent dependence. Therefore, in an approximation, the last term may usually be neglected [33, 90]. Finally, it should be mentioned that because of the approximations made in deriving Eq. (6-2), one cannot expect that this equation for the solvent dependence of UV/Vis absorptions will be fully accurate [33, 90]. 

The Twisted Intramolecular Charge Transfer (TICT) model was put forward by Grabowski and coworkers [2,3,15] to account for the observation that the dual fluorescence of DMABN with its normal band (B band) at around 350 nm and its anomalous one (A band, around 450 nm in medium polar solvents) depends on the conformational freedom of the dimethylamino (DMA) group For compounds like MIN, where the DMA group is more or less fixed to a coplanar conformation with the benzonitrile sceleton, and where the lone pair orbital on the amino nitrogen is nearly parallel to the carbon p-orbitals constituting the benzonitrile 7t-system, only the B band is observed. For the... 

Fig. 1. Potential diagram of the TICT forming reaction for DMABN and derivatives in a medium polar solvent [24]. The reaction coordinate comprises intramolecular twist, relaxation of the surrounding solvent molecules and a further relaxation coordinate, possibly connected with the pyramidalization at the amino group [22] and is therefore multidimensional...

Findinfan eluent- a medium-polarity solvent such as dichloromethane (DCM) -j g erally a.goqd.jtarting.point. ... 

For organic phase SEC, medium-polarity solvents such as tetra-hydrofuran, toluene, chloroform and dichloromethane are used. However, more polar solvents such as dimethylformamide, N-methyl pyrrolidone or dimethylsulphoxide can be used, and for more specialised applications orthodichlorobenzene, trichlorobenzene and phenolic solvents at... 

Layers of silica gel and alumina have been employed for the separation and identification of 126 monocyclic phenols eluted with 3 solvents of increasing polarity (benzene, diisopropylether, and ethanol) [2]. Alumina is more basic than silica gel and strongly retains phenolic compounds, particularly those with more acidic properties such as chlorophenols and nitrophenols, even when eluting with a medium-polarity solvent. 

A is a polar, protic solvent B is an aprotic, medium polarity solvent Scheme 21... 

Szepesi et al. reported an ion-pair separation of eburnane alkaloids on a chemically bonded cyanopropyl stationary phase. As counter-ion, di-(2-ethyl hexyl)phosphoric acid or (+)-10-camphorsulfonic acid were used in a mobile phase consisting of hexane - chloroform -acetonitrile mixtures (Table 8.8, 8.9). Because of the poor solubility of the latter pairing ion, diethylamine (Table 8.9) was added to the mobile phase. Addition of diethylamine considerably reduced the k1 of the alkaloids, due to suppression of the ionization of the alkaloids. However, due to the strong acidic character of the pairing ion, ion-pairs were still formed under these conditions. The camphorsulfonic acid containing mobile phases were found to be very useful for the separation of optical isomers (Table 8.10, 8.11, Fig.8.8) 6. It was also found that the selectivity of the system could be altered by choosing different medium-polarity solvents (moderator solvents) as dioxane, chloroform or tetrahydrofuran. The polar component of the solvent system affected peak shape. Based on these observations, a method was developed to analyze the optical purity of vincamine and vinpocetine. For the ana-... 

From the list in Table 3.9, cellulose and amylose-based phases are by far the most often used in preparative and, especially, SMB applications. These adsorbents offer good productivities because of their high loadabilities (Fig. 3.22). In addition, the four most commonly used CSP of this type separate a broad range of different race-mates. The major problem of these adsorbents is their limited solvent stability, especially towards medium-polar solvents such as acetone, ethyl acetate or dioxane. In the past their use in conjunction with aqueous mobile phases was not recommended by the manufacturer as well. However, this limitation was successfully overcome by recent studies, in which amylose- and cellulose-based CSPs are transferred to the reversed phase (RP) mode with aqueous mobile phases. The first results for the use of polysaccharide-type phases with aqueous solvents were reported by Ishikawa and Shibata (1993) and McCarthy (1994). The stability of the adsorbent after switching to RP conditions has been reported by Kummer et al. (1996) to be at least 11 months and by Ning (1998) to be 3 years. No peak deviation is observed after switching to RP mode. Novel developments have led to polysaccharide-based adsorbents dedicated to use with nearly all organic solvents (Cox and Amoss, 2004). 

In contrast, the Scott-Kucera model considers a solvent system composed of an apolar solvent A and a polar solvent B (Scott and Kucera, 1975). When this mixture is pumped through a column, a monolayer of the most polar solvent B is formed by adsorption of B on the adsorbent. Sample molecules are adsorbed on this monolayer instead of on the adsorbent surface. In other words, there is no displacement of adsorbed solvent molecules, and interaction between the molecules of the monolayer and the sample molecules determines the retention of the component. This theory has been adapted by saying that the model is only valid for medium polar mobile phases and solutes with a polarity lower than the most polar solvent in the eluent. These medium polar solvents are called hydrogen-bonding solvents (esters, ethers, ketones). A monolayer of these solvents behaves as a hydrogen-bonding phase. Inter-... 

The principle of adsorption chromatography (normal-phase chromatography) is known from classical column and thin-layer chromatography. A relatively polar material with a high specific surface area is used as the stationary phase, silica being the most popular, but alumina and magnesium oxide are also often used. The mobile phase is relatively nonpolar (heptane to tetrahydrofuran). The different extents to which the various types of molecules in the mixture are adsorbed on the stationary phase provide the separation effect. A nonpolar solvent such as hexane elutes more slowly than a medium-polar solvent such as ether. 

The new HPTLC Premium Purity Plate (Merck) is especially designed for demanding pharmacopoeia applications. It is carefully wrapped in a special plastic coated aluminum foil to prevent any plastic deposits from the wrapping material that could appear as an imknown extra zone when using medium-polar solvent systems such as toluene/ ethylacetate (95/5). These plates have no GLP-laser code. 

A distinction should be made between nano- and microliter capillaries. Pt-Ir capillaries with a capacity of 200 nl are usually used for application to HPTLC layers. With these, polar and medium-polarity solvents can be applied satisfactorily, but purely aqueous solutions cannot be applied reproducibly. Moreover, as they become blocked very eas-... 

The stmcture, stereochemistry, and potential conformational equilibria (Scheme 1) of 1,3,2-dioxathian 12 have not yet been studied theoretically but ab initio molecular orbital (MO) calculations at the MP2 level of its 2-oxide 13, both in the gas phase and in various solvents, have been reported <2000JP0187>. In the gas phase and in low-polarity solvents, 13-ax proved to be the only conformer present. In medium-polarity solvents such as acetonitrile or dimethyl sulfoxide, the equatorial counterpart 13-eq is of importance and participates up to 12% in the conformational equilibrium. In addition, both the dipole moments (/ii3.ax = 3.87 D /ti3 eq = 6.57 D) and the S=0 stretching vibrations (13-ax 1177-1180 cm ... 

Polarity of the solvent determines quantum yields in polyimides. The most effieient photocleavage was in medium-polar solvents. The selection of solvent may change the chemical mechanism of degradation and die associated products of such reactions. 

The influence of the solvent is noteworthy. In less polar solvents, most of the acid and amine is used to form complex A Cat and, to a smaller extent, complexes such as Aj Cat and Aj Cat. Low or medium polar solvents favor mechanism I however, a small fraction of amine remains and is able to attack the ot site of oxirane. Acid-base equilibrium [reaction (57)] controls the free amine concentration, i.e., the contribution of mechanism I. A similar contribution of the two kinetics, which have close activation energies, was observed. 

Sex hormones, corticosteroids, and sapogenins are the most important medium polar steroids and numerous solvent systems have been used to separate them. Based on the work of Hara and Mebe (18), the following conclusions can be drawn. Medium polar steroids can be separated using a medium polar system most of the applicable solvent systems contain one or two nonpolar solvents such as hexane, benzene or toluene and one medium polar solvent, e.g., chloroform, acetone, etc. 

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