Solvent and temperature dependence

An example of non-aromatic tautomerism has already been quoted (Table 13, Section 4.04.1.3.3(ii)) the equilibrium between the two enamines (152a) and (152b) is solvent and temperature dependent (70BSF3147). 

The selectivity and product composition is different from that obtained for direct chlorination. The selectivity of the r-butoxy radical is intermediate between that of chlorine and bromine atoms. The selectivity is also solvent- and temperature-dependent. 

Table 3 Solvent and Temperature Dependence of the 1 1 Formation Constants of Tetraisobutylethylene-Br2 CTC. ...

Mark-Houwink values confirm that for these conditions gelatin is behaves rod-hke conformation. Such of empirical equations can be relating the parameters of Mark-Houwink with T, which ultimately describe this type of thermodynamic parameters are relations between properties the solute with the solvent and temperature dependence. 

Table 3.2 Solvent and temperature dependence in the reaction of DDQ with a silylenol ether.

Iwata K, Ozawa R, Hamaguchi H (2002) Analysis of the solvent- and temperature-dependent Raman spectral changes of S1 trans-stilbene and the mechanism of the trans to cis isomerization dynamic polarization model of vibrational dephasing and the C=C double-bond rotation. J Phys Chem A 106 3614—3620... 

Although phosphorus trisulfide (P4S6—but see below) and phosphorus pentasulfide (P4S10) have been widely used in the preparation of heterocyclic compounds there is no recent review. Weintraub has given an account of solvent- and temperature-dependent reactions of the pentasulfide, the compound employed in almost all recently reported work.118... 

The stable tetracoordinated diorganolead(ll) compound 64 is obtained by the reaction of [FcN]Li with PbCl2 in 80% yield. The deep red crystals of 64 exist in the solid state only as the w ro-diastereomer 64a. The ratio of meso-j rar-diastereomers in solution is solvent and temperature dependent, consistent with an intermolecular exchange between diastereomers (Equation (25)).81... 

Few other studies are available on one-bond 13C-H or 13C-19F coupling constants as a function of solvent. Rahkamaa and Jokisaar 27> reported the solvent and temperature dependence of XJXZ in ethyl formate in three solvents CC14,... 

Solvent Effects and NMR Coupling Constants Table 13. Solvent and temperature dependence ofJJp H in phosphine... 

From this information the absolute molecular weight distribution and the intrinsic viscosity-molecular weight plot can be constructed. From this plot the solvent and temperature dependent Mark-Houwink coefficients for linear polymers and information for polymer chain-branching of non-linear polymers can be obtained. 

MULTIPLE BONDS 1,2-Dimethylcyclohexene HC1 solvent- and temperature-dependent a... 

Dynamic NMR shows the presence of two conformations in 1,3,5,7-tetroxocane a crown and a boat-chair. The equilibrium constant is strongly solvent and temperature dependent, with the highly symmetrical crown, which has a high dipole moment, having a lower entropy than the boat chair by about 6 J K-1 mol-1 (72JA1390,1389). Hie interconversion barrier is 50 kJ mol-1, and no ring inversion nor pseudorotation process has been detected in the boat-chair. 

Table 3. Solvent and temperature dependence of the absorption (241 nm) and CD spectra of 19...

Other aromatic ketones undergo photocyclization to form oxygen heterocycles. Methyl o-benzyloxyphenylglyoxylate (400) is converted in high yield into a mixture of the isomers (401 and 402).415 The stereochemistry of the cyclization is both solvent- and temperature-dependent, the former isomer (401) being formed almost exclusively in nonpolar solvents such as heptane or benzene, and in each case an excited triplet appears to be involved. The related aldehyde, O-benzylsalicylaldehyde (403), is similarly converted into cis-2-phenyl-3-hydroxy-2,3-dihydrobenzofuran (404), but in much lower yield a second product may possibly be the tram isomer.410... 

Comparison of the time resolved data in several solvents for 28 with that for a similar molecule in which one of the quinones was replaced by a dimethoxyphenyl group showed an increase in the lifetime of the fluorescence decay, as would be expected from the removal of a second acceptor, as discussed above. However, the data are complicated by the fact that the decays of the dimethoxyphenyl analog were decidedly biexponential. This result was interpreted in terms of an orbital symmetry effect on the electron transfer reaction. Interesting solvent and temperature dependencies were also observed with 28. 

Solvent and Temperature Dependence of 29Si Chemical Shifts for 66b... 

Thus, four types of products (often involving stereoisomers) may be formed in dione-olefin reactions. The situation is further complicated by the fact that product compositions show solvent and temperature-dependence 125) (effects of other variables have not been investigated) so that results from different laboratories have sometimes been at variance. Prediction of results is hazardous. 

Laurence et al. [237, 238] have shown that some of the absorption bands of the solvatochromic indicator dyes previously used by Kamlet and Taft to fix the reference lines exhibit a solvent- and temperature-dependent vibrational fine structure, which makes the determination of the band maxima, v ax, difficult. Furthermore, the indicator dyes used are thermo-solvatochromic, i.e. v ax is temperature-dependent. No allowance for this temperature-dependence of the a, / , and n values has been made previously. 

Figure 11.2 Chemical structures of some well-studied LCPs. The top four structures are usually used as lyotropic polymers, and the bottom two are thermotropes. Estimates of the persistence lengths (Xp) and effective diameters (d) of these molecules are (Xp, d) = (12,1.04) nm for HPC, (90, 1.5) nm for PBLG. 129.0.6) nm for PPTA. (20.0.6) nm for PBZT. (12.0.6) nm for Vectra A. and (6.0.5) nm for OQO(phenylsulfonyl)10. These values, listed in Baek et al. (1994), Brelsford and Krigbaum (1991), and Farmer et al. (1993), are both solvent- and temperature-dependent, and thus should be used with caution. (From Donald and Windle 1992, with permission from Cambridge University Press.)...

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