Hydrocarbon pairs

Where nucleic acids are concerned, the enhanced hydrophobicity of abiotic polyfluorinated aromatic bases (e.g., tetrafluorobenzene or tetrafluoroindole deoxyribose derivatives) was exploited as an alternative to natural hydrogen bonding to achieve selective and stable nucleic acid base pairing in duplex DNA [85], The DNA replication was examined using polyfluorinated-nucleotide analogs as substrates. A DNA polymerase active site was able to process the polyfluorinated base pairs more effectively than the analogous hydrocarbon pairs, demonstrating hydrophobic selectivity of polyfluorinated bases for other polyfluorinated bases [86]. 

For hydrocarbon pairs in different solvents and over moderate temperature ranges (to 100°C), a linear dependency of log S°12 on (1/T) can be assumed (12, 14, 26). An example is shown in Figure 5, where log S° for the hexane-benzene pair in five different solvents is plotted against the reciprocal absolute temperature. The relationship can be considered linear for engineering applications. Selectivity decreases with increasing temperature, and this explains the unusual maximum in the variation of selectivity with solvent concentration shown by the system ethylbenzene-ethyl cyclohexane with hexyleneglycol as solvent (Figure 3). 

Adsorption and separation processes involve also the active sites existing on the external surface of (CH3)4N - montmorillonite, their role being more important in the case of adsorption of isoparaffins and cyclohexane molecules. This is indicated by a significantly smaller differences between the specific retention volumes of iso- and cycloparaffins on natural and tetramethylammonium samples than the difference in Vm values characteristic for n-paraffines on the same adsorbents. Thus, the tetramethylammonium montmorillonite adsorbs n-alkanes selectively from the mixtures containing iso- and cycloparaffines, which is confirmed by the values of relative retention volumes for such hydrocarbon pairs as, for instance, n-heptane / 2,4- dimethylpentane. These can be easily calculated from the data presented in Table 5. 

The interaction parameters for the hydrocarbon-hydrocarbon pairs are considerably less than those for the nonhydrocarbon—hydrocarbon pairs. To evaluate the need for the interaction parameter for hydrocarbon-hydrocarbon pairs, phase equilibria has been predicted both with and without interaction parameters for ternary systems for which experimental data are available. The ternary systems used and the results of the study are given in Table III. These results indicate that there is no significant advantage for using hydrocarbon-hydrocarbon interaction parameters in predictions for ternary systems. Therefore, for general calculations, hydrocarbon-hydrocarbon interaction parameters are set at zero, but nonzero interaction parameters are used for nonhydrocarbon-hydrocarbon pairs. 

For nonpolar hydrocarbon pairs known to follow regular solution theory, (5-32) can be used to estimate A , and Aj,. 

This semi-empirical rule tries to account for the influence of differences in size and in ionization potential on the intermolecular potential of the unlike interactions although, as Douslin et aL point out, CF4 and CH4 possess similar ionization potentials and so this is not a significant factor for this particular fluorocarbon -f hydrocarbon pair. The use of equation (6) nevertheless leads to a considerable improvement between the theoretical and experimental values of rPa but there is still an unaccounted weakness in the unlike interactions. The reality of this weakness is further emphasized by the fact that the experimental value of F 2 is 2% larger than the value predicted by the Lorentz rule. 

The PR and SRK packages contain enhanced binary interaction parameters for all library hydrocarbon-hydrocarbon pairs (a combination of fitted and generated interaction parameters), as well as for most hydrocarbon-nonhydrocarbon binaries. 

Gordina et al. [30] observed a new absorption band peaking around 450 nm in p-chloranil-doped PMDA-ODA film. They concluded that this new band is attributed to intermolecular CTC between p-chloranil (F = 2.45 eV) as a typical electron acceptor and the ODA moiety in the PI. Fig. 3 illustrates a good linear relationship between A and 7p in the chloranil-hydrocarbons pairs in non-polar solvents as... 

J. Spanget-Larsen, Theor. Chem. Acc., 98, 137 (1997). The Alternant Hydrocarbon Pairing... 

This expression can be used for other hydrocarbon pairs, with less confidence however. 

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