Free energy 596 INDEX

Solvents exert their influence on organic reactions through a complicated mixture of all possible types of noncovalent interactions. Chemists have tried to unravel this entanglement and, ideally, want to assess the relative importance of all interactions separately. In a typical approach, a property of a reaction (e.g. its rate or selectivity) is measured in a laige number of different solvents. All these solvents have unique characteristics, quantified by their physical properties (i.e. refractive index, dielectric constant) or empirical parameters (e.g. ET(30)-value, AN). Linear correlations between a reaction property and one or more of these solvent properties (Linear Free Energy Relationships - LFER) reveal which noncovalent interactions are of major importance. The major drawback of this approach lies in the fact that the solvent parameters are often not independent. Alternatively, theoretical models and computer simulations can provide valuable information. Both methods have been applied successfully in studies of the solvent effects on Diels-Alder reactions. 

Free valences and localization energies have been calculated for a series of pyrazoles (neutral molecules and conjugate acids) for homolytic substitution. In all the compounds the site with the lowest localization energy has the Wghest free valence index. This parallel between the two indices of reactivity is maintained in pyrazole, 1-methylpyrazole and their conjugate acids, but not in 1-phenylpyrazole and its conjugate acid. For the three compounds examined experimentally, (32), (33) and (35) (Section 4.04.2.1.8(ii)), only the predictions for (33) are in agreement with the experimental results. 

Here A Gx is the free energy of chain break and formation of new bonds Gm is the free energy of chain surface bond formation Gs is the free energy of the surface formation Gex.s is the excessive combinatorial free energy stipulated by different disposition of chain molecules on the surface ziGcom.s is the combinatorial free energy stipulated by different disposition of intermolecular chain surface bonds on chain molecule. The rest of the G terms possess the abovementioned physical sense. Index ( ) relates to the end state of the system. 

Valko et al. [37] developed a fast-gradient RP-HPLC method for the determination of a chromatographic hydrophobicity index (CHI). An octadecylsilane (ODS) column and 50 mM aqueous ammonium acetate (pH 7.4) mobile phase with acetonitrile as an organic modifier (0-100%) were used. The system calibration and quality control were performed periodically by measuring retention for 10 standards unionized at pH 7.4. The CHI could then be used as an independent measure of hydrophobicity. In addition, its correlation with linear free-energy parameters explained some molecular descriptors, including H-bond basicity/ acidity and dipolarity/polarizability. It is noted [27] that there are significant differences between CHI values and octanol-water log D values. 

Our extension of the LIE approach to calculate free energies of hydration (AGhyd) incorporated a third term proportional to the solute s solvent-accessible surface area (SASA), as an index for cavity formation within the solvent.19,27 The latter term is needed for cases with positive AGhyd such as alkanes and additional improvement occurred when both a and P were allowed to vary. Equation 8 gives the corresponding LIE/SA equation for... 

The cmc is a key property, because it is related to the free energy difference between monomer and micelles. The onset of micellization is detected by marked changes in such properties as surface tension, refractive index and... 

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