Solvent property scales representative parameters

For a two-parameter treatment of solvent effects (with two independent solvent vectors), only two critical subsidiary conditions must be defined in order to force the two solvent parameters to represent physically significant solvent properties. Four other trivial arbitrary conditions have to be defined in order to fix zero reference points and scale-unit sizes. However, for a three-parameter treatment (with three independent solvent vectors), already six critical subsidiary conditions must be defined, in addition to the six trivial reference or scale-factor conditions. On the contrary, singleparameter treatments require no definition of critical subsidiary conditions, but only one reference (zero) condition and one standard (unit) condition, whose arbitrary assignment changes only the reference solvent and the scale-unit size (265, 276). 

If the nonreaction solvents (the majority of the Swain solvents) exhibit linearity between two solvent parameters, the solvent properties can be represented by only one parameter. For the nonreaction solvents, the converted N values are independent of the converted Y values only to the extent that small deviations from linearity exist in the plot of N versus Y. The scaling... 

Correlations with Reactivity Results. In addition to the correlations described previously, we have used the scale, alone or in combination with the 7T scale (in which case all correlations have been by the method of multiple linear regression analysis) to rationalize solvent effects on many additional types of properties and reactivity parameters. Representative examples are as follows. 

Parameters describing and correlating the solvent abilities of liquids have been based on a great variety of chemical and physical properties. Some are measures of solvent basicity, and others are obtained from direct determinations of the solubility of a representative solute in a range of liquids. For example, the solubility of hydrogen chloride in liquids at 10 °C was used in this way by Gerrard and co-workers [69,70] and the resulting solvent basicity scale was compared with other scales by Arnett [71] and by Dack [72]. 

Since these different scales do not represent the same kind of solvent properties they are not always comparable. However, the different solvent parameters are not actually physically independent quantities. 

The basic premise of Kamlet and Taft is that attractive solute—solvent interactions can be represented as a linear combination of a nonspecific dipolarity/polarizability effect and a specific H-bond formation effect, this latter being divisible into solute H-bond donor (HBD)-solvent H-bond acceptor (HB A) interactions and the converse possibility. To establish the dipolarity/polarizability scale, a solvent set was chosen with neither HBD nor HBA properties, and the spectral shifts of numerous solvatochromic dyes in these solvents were measured. These shifts, Av, were related to a dipolarity/polarizability parameter ir by Av = stt. The quantity ir was... 

Two of the hydrophihcity scales in Table 2 were derived from experimental measures of the behavior of amino acids in various solvents, namely partitioning coefficients [K-D index of Kyte and Doolittle (30)] or mobility in paper chromatography [Rf index of Zimmerman et al. (31)]. By contrast, the Hp index was obtained from quantum mechanics (QM) calculations of electron densities of side chain atoms in comparison with water (32). The Hp index is correlated highly with these two established hydrophobicity scales (Table 4). Therefore, like the polarizability index, it is possible to represent fundamental chemical properties of amino acids (hydrophUicity, Hp) with parameters derived from ab initio calculations of electronic properties. However, in contrast to polarizabihty (steric effects), hydrophihcity shows significant correlation with preference for secondary structure. Thus, hydrophobic amino acids prefer fi-strands (and fi-sheet conformations) and typically are buried in protein structures, whereas hydrophilic residues are found commonly in turns (coil structure) at the protein surface. 

The use of a single parameter (e.g., E(A) or E(F) or any other photophysical parameter) to describe solvent polarity is based on the assnmption that it is necessary to take only one mechanism of solute-solvent interaction into acconnt. The inadequacy of the dielectric model of solvent to represent the solvent effect on the varions properties of solutes aud proliferation of empirical polarity scales point to the existence of specific solute-solvent interaction. According to Equation 7.1 any solvent-dependent property (A) of a solute 5" in a solvent T can be represented as... 

---