Basicity solvatochromism

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... 

The a scale of solvent acidity (hydrogen-bond donor) and the (3 scale of solvent basicity (hydrogen-bond acceptor) are parameters derived from solvatochromic mea-siuements used in adsorption chromatography [51,54,55]. 

The correlation of Snyder s solvent strength e° with molecular dipolarity and polarizability (7t ) and the hydrogen-bond acidity (a) and the hydrogen-bond basicity ((3) solvatochromic parameters for adsorption chromatography can be achieved, although most papers on solvatochromic parameters deal with reversed-phase systems [18]. 

Stella, C., Galland, A., liu, X., Testa, B., Rudaz, S., Veuthey, J. L, Carrupt, P. A. Novel RPLC stationary phases for lipophilicity measurement solvatochromic analysis of retention mechanisms for neutral and basic compounds. /. Sep. Sci. 2005, 28, 2350-2362. 

The Lewis basicity of the solvents was found to be more important for the solvatochromism than the solvent polarity, since such Si... solvent and Si... F interactions are competitive in the presence of solvents with donor atoms. In contrast, the analogous non-fluorinated polysilane, poly(methyl-/z-propylsilylene), revealed a disordered conformation in both coordinating and non-coordinating solvents. Additionally, the UV spectra of various molecular weight fractions of 87 showed an unusual molecular weight dependency an isosbestic point is apparent, suggesting an equilibrium between globule- and rod-like conformations at room temperature, which was also evident from the... 

Hydrogen bond donor solvents are simply those containing a hydrogen atom bound to an electronegative atom. These are often referred to as protic solvents, and the class includes water, carboxylic acids, alcohols and amines. For chemical reactions that involve the use of easily hydrolysed or solvolysed compounds, such as AICI3, it is important to avoid protic solvents. Hydrogen bond acceptors are solvents that have a lone pair available for donation, and include acetonitrile, pyridine and acetone. Kamlet-Taft a and ft parameters are solvatochromic measurements of the HBD and HBA properties of solvents, i.e. acidity and basicity, respectively [24], These measurements use the solvatochromic probe molecules V, V-die lliy I -4-n i in tan iline, which acts as a HBA, and 4-nitroaniline, which is a HBA and a HBD (Figure 1.17). 

TABLE 9. Solvent solvatochromic parameters0 and the relative basicities (SAG) of propylamines in the gas phase and various solvents (values are in kcal mol-1 at 298 K and relative to diethylamine)97. Reprinted with permission from Reference 97. Copyright (1995) American Chemical Society... 

In 1976, Kamlet and Taft introduced their solvatochromic comparison method [25, 26], The hydrogen-bond donor acidity a and basicity /3 together with the solvent polarity and polarizability jv were employed to correlate the solvent effects on reaction rates, equilibria, and spectroscopic properties XYZ according to equations of the form... 

Kamlet MJ, Taft RW (1976) The solvatochromic comparison method. 1. The /6-scale of solvent hydrogen-bond acceptor (HBA) basicities. J Am Chem Soc 98 377-383. 

FIGURE 9.7 Variation of Kamlet-Taft solvatochromic parameters for methanol/C02 mixtures as a function of added CO2 at 298 K and 17.2 MPa (A)n (dipolarity/polarizability, ( ) (H-bond acidity), and (T)/8 (H-bond basicity). (Adapted from Y. Cui, S. V. Olesik, Anal. Chem., 63 1812(1991).)... 

Logarithmic bioconcentration factors have been shown to be correlated with the logarithmic octa-nol/water partition coefficient in aquatic organisms (Davies and Dobbs, 1984 de Wolf et al., 1992 Isnard and Lambert, 1988) and fish (Davies and Dobbs, 1984 Kenaga, 1980 Isnard and Lambert, 1988 Neely et al., 1974 Ogata et al., 1984 Oliver and Niimi, 1985). In addition, bioconcentration factors are well correlated by a linear solvation energy relationship (coimnonly known as LSER) that includes the intrinsic solute molecular volume and solvatochromic parameters that measnre hydrogen bond acceptor basicity and donor acidity of the componnd (Park and Cho, 1993). 

The ability of molecules to accept a hydrogen bond is measured by the Taft-Kamlet solvatochromic parameter, P, (or P for the monomer of self-associat-ing solutes) (see Table 2.3). This, too, is a measure of their basicity (in the Lewis sense), also measured by the Gutmann donor number DN (discussed later). Thus, pyridine has P = 0.64, compared with 0.40 for acetonitrile, but... 

Different di- and trihydroxy bile salts and drugs that differ in lipophilicity, basicity, and structure (HBA and HBD) have been compared in order to examine solvatochromic equilibria (30,31). 

LSER Model of Leahy In the LSER model of Leahy [22], the cavity term is substituted by the molar volume, Vm, at 25°C in g cm-3 or by the intrinsic molecular volume, V), in mLmoL1. The dipolar term and the hydrogen-bonding terms are represented by the dipole moment, n, and the HBA basicity, (3, respectively. Group contribution schemes have been developed to calculate the solvatochromic parameters from molecular structure input [23]. Leahy [22] gives the following equation derived with a diverse set of monofunctional liquids ... 

Since solvatochromic parameters are derived from direct measurements of the energy resulting from intermolecular interaction, they can be used to predict solubility, which is determined by solute-solute, solvent-solvent, and solute-solvent interaction energies. For nonself-associated liquid aliphatic compounds with a weak or nonhydrogen-bond donor (Taft etal., 1985 Kamlet etal., 1986), the solubility in water at 29S was related to molar volunWjf, hydrogen-bond basicity j and polarity/polarizability (jf) by a linear solvation energy relationship (LSER) as in Equation 3.55 ... 

The dye is positively solvatochromic, the absorption maximum in a polar solvent such as dimethyl sulfoxide being 620 nm, that is, 12 rnn higher than in a nonpolar solvent such as carbon tetrachloride [5],Characteristic of indigo is the unusually deep shade compared with other conjugated systems of similar size [6], This is explicable in terms of the special arrangement of the atoms in the basic... 

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