Solvent and Concentration

Theoretical calculations by statistical mechanical simulation, in agreement with experimental data, have shown that polar solvents affect the energy of E isomers of esters more than that of the Z rotamer. The energy difference was reduced from 5.2 kcal mol-1 in the gas phase (e = 1) to 1.6 kcal mol-1 in the acetonitrile (e = 35.9) and from 8.5 to 5.2 kcal mol-1 for methyl formate and methyl acetate, respectively. This may be explained by the increased polar character of the carbonyl group in the more polar solvent [39]. This was also observed in DMSO (e = 47.2) where the proportion of the minor E isomer tends to increase [1]. The keto-enol 

Proton NMR studies of N-methyl formamide (NMF) and NMA at high dilution in deuterated solvents have shown that the level of cis isomer of NMF is 8% in water, 10.3% in chloroform, 8.8% in benzene, and 9.2% in cyclohexane, while the level of cis-NMA (a model for the secondary peptide bond) is 1.5% in water and does not change very much in nonpolar solvents [18]. Ab initio molecular calculations suggest that the small difference in dipole moments in cis and trans forms explain the relative insensitivity of amides to solvent change, unlike esters [22,41], This may be explained by nearly identical free energies of solvation for the two isomers [18]. The energy difference between cis and trans isomers in aqueous solution (AG° = 2.5 kcal mol-1) accounts for the preferential trans conformation adopted by most peptide bonds. Similar results were obtained with nonproline tertiary amides [22]. 

As anticipated, temperature modulates the Z E ratio and increases the rates of CTI. In most cases, esters and thioesters cannot be differentiated at room temperature whereas both rotamers of the corresponding amides, carbamates (Table 8.2), and ureas are observed in these conditions, but are not affected to the same extent. 

In particular, for the secondary amides of nonproline dipeptides, A St may vary depending on the protonation state [21] and hence influence the temperature-de-pendent energy barrier in a way described by the equation AGt = A Hi - TASt, though amide and carbamate CTI are usually essentially enthalpy-driven. The solvent dielectric constant e also varies depending on temperature (i.e. 47.6 at -50 °C and 32.1 at 50 °C for acetonitrile) and strongly influences the charge distribution on the pseudo double bond. 

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Crystallization conditions such as temperature, solvent, and concentration can influence crystal form. One such modification is the truncation of the points at either end of the long diagonal of the diamond-shaped crystals seen in Fig. 4.11b. Twinning and dendritic growth are other examples of such changes of habit. 

The H NMR spectrum of pyridazine shows two symmetrical quartets of an A2X2 or A2B2 type dependent on the solvent and concentration. The satellites have been used to obtain all coupling constants. Spectra of C-substituted pyridazines, methylthio- and methylsulfonyl-pyridazines, both as neutral molecules and as cations, N-1 and N-2 quater-nized species, pyridazinones, hydroxypyridazinones, A-oxides and 1,2-dioxides have been reviewed (b-73NMR88> and are summarized in Tables 6, 7 and 8. 

Observed rates of disappearance for diacyl peroxides show marked dependence on solvent and concentration.146 In part, this is a reflection of their susceptibility to induced decomposition (see 3.3.2.1.4 and 3.3.2.1.5). However, the rate of disappearance is also a function of the viscosity of the reaction medium. This is evidence for cage return (see 3.3.2.1.3).143 The observation144 of slow scrambling of the label in be.x LQy -carbonyl- %0 peroxide between the carbonyl and the peroxidic linkage provides more direct evidence for this process. 

The initiator association number y depends upon temperature, solvent and concentration of the system and is reported to be between two and seven (2) The association of polystyryl anions results in species A. Ak which do not react with styrene monomer. The macromolecular association is analogous to a reversible termination by combination. 

The amount of rotation a is not a constant for a given enantiomer it depends on the length of the sample vessel, the temperature, the solvent and concentration (for... 

Table 10.3. Solvent and Concentration Effects on the Photodimerization of Acenaphthylene<41)...

The ratio of cis (27) to trans (26) dimers produced in this reaction has been found to be solvent and concentration dependent, as can be seen in Table 10.3. The data in Table 10.3 show that the cis dimer predominates in all three solvents. It is also seen that as the acenaphthylene concentration is increased, proportionally more cis dimer is formed. This effect is slightly more pronounced in cyclohexane than in benzene (see Figure 10.2) Increases in the yield of dimer with increasing concentration would of course be expected... 

The 977-pyrido[3,4- ]pyrrolizin-9-one 145 has been prepared for its photochemical properties. The preparation involves an intramolecular Friedel-Crafts acylation of the acid chloride formed from 3-(l-pyrrolyl)pyridine-4-carboxy-late (Scheme 40). The product is a photosensitizer, which absorbs visible light its absorption spectra are pH, solvent, and concentration dependent <1994SAA57>. 

The solubilities of 2,4-DIF-PMI and 2,4-DIF-PMTAI were fairly good in N,N-dimethyl acetamide and N,N-dimethyl foramide. The polybenzoxazole could not be dissolved in any organic solvent and concentrated sulfuric acid could dissolve it. 

For the same initiating system, solvent, and concentrations at -15 °C and +25 °C, Higashimura et al. [44], give rate and DP data for 4-tert-butoxystyrene (4-t-BuOSt) from which we calculated... 

The specific optical rotation of a solid substance is usually determined by measuring the angle of rotation at the wavelength of the sodium D-line at a temperature of 20°C, and calculating the result with reference to a layer 1 dm thick of a solution containing 1 g of the substance per ml. It is pertinent to mention here that the specific optical rotation of a solid is always expressed to a given solvent and concentration. 

Watts and Goldstein reported the solvent and concentration dependence of 2/H-f °f vinyl fluoride in cyclohexane and DMF. Smith and Ihrig 61 62> extended those results to other solvents and also measured the solvent dependence of 2/H-f the isomeric difluoroethylenes and trifluoroethylene (Table 23). The... 

Watts, V. S., Loemker, J., Goldstein, J. H. Solvent and Concentration Effects on 13C—H Coupling Constants and Chemical Shifts in some Dihaloethylenes. J. Mol. Spectry 17, 348(1965). 

Schmidt, R. L., Butler, R. S., Goldstein, J. H. The Role of Polar Factors in Collision Complex Models for the Solvent and Concentration Dependence of Nuclear Magnetic Resonance Parameters. J. Phys. Chem. 73, 1117 (1969). 

A vacuum crystalliser operates on a slightly different principle from the reduced-pressure unit since supersaturation is achieved by simultaneous evaporation and adiabatic cooling of the feedstock. A hot, saturated solution is fed into an insulated vessel maintained under reduced pressure. If the feed liquor temperature is higher than the boiling point of the solution under the low pressure existing in the vessel, the liquor cools adiabatically to this temperature and the sensible heat and any heat of crystallisation liberated by the solution evaporate solvent and concentrate the solution. 

PPEs are handicapped by an inherent sensitivity to fire and to common chemicals, such as hydrocarbons, chlorinated solvents and concentrated mineral acids. They have rather high coefficients of friction and are slightly attacked by UV and weathering, requiring UV protection. The number of producers is limited. 

MgO- and Ag-modified membranes were obtained by homogeneous precipitation of the hydroxide from a typical solution consisting of 0.75 M urea and 0.2-0.5 M AgN03 or Mg(NOj)2 in water. The solution is introduced into the pores of the support and/or the y-alumina top layer by impregnation. An increase in temperature results in (I) evaporation of the solvent and concentration of the solution and (2) the decomposition of urea (at T > 90°C) resulting in the formation of NH3 and a decrease in the pH followed by precipitation of the metal hydroxide. The hydroxide is next converted to the oxide form at 350-450°C. 

Solvent and concentration effects in nitrogen NMR studies can be very significant. The N NMR chemical shifts for 1,2,4-thiadiazole (1) in ether solution (1 3 v/v) are - -106 for N-2, and -1-70 for N-4. These values are shielded with respect to nitromethane. A similar degree of shielding is observed in the N NMR spectra of 1,2,4-oxadiazole and in 1,2,5-thiadiazole <840MR(22)215>. 

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