Dioxans solvent shifts

The phase transitions for the weakly charged PAA gels which are swollen in mixtures of water with solvents differing in the dielectric constant, (methanol (e = 33.7), ethanol (e = 25.8) and dioxane (e = 2)) were studied in Ref. [29]. It was shown that the decrease in the dielectric constant of the solvent shifts the point of the phase transition to the region of lower concentration of a precipitant. The amplitude of the collapse also decreases when the dielectric constant is lowered [29]. 

Several conclusions can be drawn from Table 3. First, in accordance with the two-state model, /So and jSj all increase with decreasing HOMO-LUMO gap. Second, the intrinsic second-order polarizability of p-nitroaniline is increased by two-thirds when the solvent is changed from p-dioxane to methanol or A-methylpyrrolidone, even when the values are corrected for the differences in (A ). As we have adopted the value for p-nitroaniline in dioxane as a standard, it should therefore be noted that molecules that truly surpass the best performance of p-nitroaniline should have a second-order polarizability of l. p-nitroaniline (dioxane). As a third conclusion, there is a poor correlation between and the static reaction field as predicted by (91). This is in part due to the fact that the bulk static dielectric constant, E° in (89), differs from the microscopic dielectric constant. For example, p-dioxane has long been known for its anomalous solvent shift properties (Ledger and Suppan, 1967). Empirical microscopic dielectric constants can be derived from solvatochromism experiments, e.g. e = 6.0 for p-dioxane, and have been suggested to improve the estimation of the reaction field (Baumann, 1987). However, continuum models can only provide a crude estimate of the solute-solvent interactions. As an illustration we try to correlate in Fig. 7 the transition energies of p-nitroaniline with those of a popular solvent polarity indicator with negative solvatochromism. 

In contrast to this early empirical solvent scale, one of the more recent, introduced by Eliel and Hofer in 1973 [25] and based on the solvent-dependent conformational equilibrium of 2-z-propyl-5-methoxy-l,3-dioxane, should be mentioned cf. Table 4-9 in Section 4.4.3). In general, polar solvents shift this conformational equilibrium towards the more dipolar axial cis isomer. The authors proposed calling the standard molar Gibbs energy changes associated with this equilibrium, he D scale D for... 

It would seem energetically improbable that all benzene-solute collision complexes should be planar, since it is reasonable to suppose that steric requirements would play an important role in the determination of the geometrical preferences of the interacting molecules. Anderson has found, for example, that the solvent shifts of some 1,3-dioxans are highly dependent on the size of the axial substituent in the... 

NINE KINDS OF CDX-INDUCED C CHEMICAL SHIFTS AND THE DIOXANE-INDUCED SOLVENT SHIFTS OF MO... 

From the comparison between the inclusion shifts and the solvent shifts, it may be possible to deduce roughly the situation of the included molecule. The lengths of the skeleton carbons on MO and orange II are 9A. The torus heists of the monomers and DMyS are SA [9] and IlA [2b], respectively, a- and jS-cdx-Ep have a longer torus. Thus, nine kinds of cdx having different inner diameters and torus heights were used to include MO. The inclusion shifts were compared with each other and with the dioxane-induced solvent shifts (Figure 6). 

Fig. 6. Nine kinds of cyclodextrin-induced C chemical shifts and the dioxane-induced solvent shifts of methyl orange.

Solution Properties. Lignin in wood behaves as an insoluble, three-dimensional network. Isolated lignins (milled wood, kraft, or organosolv lignins) exhibit maximum solubiUty in solvents having a Hildebrand s solubiUty parameter, 5, of 20.5 — 22.5(J/cm ) (10 — ll(cal/cm ) > and A// in excess of 0.14 micrometer where A]1 is the infrared shift in the O—D bond when the solvents are mixed with CH OD. Solvents meeting these requirements include dioxane, acetone, methyl ceUosolve, pyridine, and dimethyl sulfoxide. 

The teirperature dependencies of the chemical shift values for both Cl and C4 were determined in four different solvents (water, dimethyl sulfoxide, methanol and dioxane) and are shown in Figures 8 and 9. The resonance for Cl at 298 C varied from 101.6 ppm in D2O to 104.0 ppm in methanol. The resonance for C4 at the same temperature varied from 75.3 ppm in dimethyl sulfoxide to 78.3 ppm in methanol. The most pronounced tenperature dependence is observed in water and dioxane, where Cl and C4 signals varied from 101.4 ppm to 101.9 ppm (Cl, water, 278-358 K) and from 75.7 ppm to 76.5 ppm (C4, dioxane, 288-360 K), respectively. Thus, both tenperature and solvent dependence of C shifts indicate different conformational behavior of the molecule at various physico-chemico conditions. This feature is manifested even more clearly by the dependencies of the three-bond proton-carbon J and J coupling constants (< ) - Hl -Cl -04-C4 and f = H4-C4-04-C1 ) which are plotted against tenperature in Figures 10 and 11. 

On the basis of kinetic data, it was suggested that appreciable charge separation in the activated complex (equation 13) could be avoided by means of such proton transfers, where HA is a general acid (H2O, ROH, RO—OH). Upon change from a polar protic solvent to the nonpolar solvent dioxane, the reaction was observed to be second-order in hydrogen peroxide and the second molecule of H2O2 obviously played the role of HA in the 1,4-proton shift. The rate of oxidation was shown to increase linearly with the pfsTa of solvent HA. In general, it was concluded that solvent interactions provide a... 

Any heteronuclear signal of a solvent or an added reference substance can be used for referencing I3C shifts. For example, 13C shifts can be directly measured relative to a deuterium signal of the deuterated solvent usually required for field/frequency stabilization. However, homonuclear shift references such as the l3C signals of tetramethylsilane (TMS), carbon disulfide, benzene, cyclohexane, 1,4-dioxane or the easily localizable mul-tiplet signals of deuterated solvents (Fig. 2.22) are predominantly applied in 13C NMR. 

Since medium effects may be expected for any resonance, the solvent must be mentioned when 13C shift values are tabulated. In this work, all 13C shifts are given relative to TMS. Those shifts which were originally reported relative to other references (e.g. carbon disulfide, <5CS2, or 1,4-dioxane, <5C4hho2)> have been converted into shifts relative to TMS (t>TMs) using the known shift difference between common reference substances, as... 

Figure 3.55 Examples of solvatochromic plots in solvent mixtures, (a) The fluorescence of 4-aminophthalimide in ether/dimethylformamide shows a much steeper non-linearity than its absorption band because of the large dipole moment of the emitting state, (b) The absorption spectrum of an aminobenzene in dioxan/water displays a red shift followed by a steep blue shift at high water concentrations...

As mentioned above in connection with the acetic acid synthesis, iridium complexes catalyze the water-gas shift reaction (equation 70). From IrCl3-3H20 and sulfonated derivatives of bipy and phen, water-soluble catalysts were obtained.444 Using dioxane as solvent, complexes of the type [Ir(cod)L2]+ (L= PMePh2, PPh3), [Ir(cod)L ]+ (L = diphos, phen, 4,7-Me2-phen, 4,7-Ph2-phen, 3,4,7,8-Me4-phen) and [Ir(cod)X] (X = 4,7-diphenylphenanthroline disulfonate) also catalyzed the reaction, with the anionic species being most active.470 The mechanism was thought... 

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