Solvatochromic polarity measures

Another solvatochromic polarity measure, (30), is the transition energy for compound 8, which is 2,6-diphenyl-4-(2,4,6-triphenylpyridinio)phenolate, also referred to as Dimroth-Reichardt s betaine. 

Solvatochromic polarity measures, 436 Solvatochromism, 435 Solvent cage, 134... 

The relative importance of the hafide anion - HO - Cell interactions can be inferred from application of the Taft-Kamlet-Abboud equation to the UV-Vis absorbance data of solvatochromic probes, dissolved in cellulose solutions in different solvent systems, including LiCl/DMAc and LiCl/N-methyl-2-pyrrolidinone [96]. According to this equation, the microscopic polarity measured by the indicator, Ej (indicator), in kcalmol is correlated with the properties of the solvents by Eq. 1 ... 

Steady-State Solvatochromism. The majority of the reports on supercritical fluid solvation have used steady-state solvatochromic absorbance measurements (21-28). The original aim of these experiments was to determine the solvating power of supercritical fluids for chromatography and extraction (SFC and SFE) (26,28). To quantify solvent strength, researchers (21-28) adopted the Kamlet-Taft x solvent polarity scale (50-55). This scale best correlates solvatochromic effects on a- x and x- x electronic absorption transitions. 

As is shown below, the polarity measured by A T(30) for a protic solvent shows its ability to donate a hydrogen bond to a solute in addition to its polarity per se. A different solvatochromic polarity parameter, that is devoid of this complication (but has others), is Kamlet and Taft s 7t (Kamlet, Abboud and Taft 1977). This is based on the average of values of the 7C —> transition energies for several... 

Duchemin, C. R., Engle, N. L., Bonnesen, P. V., Haverlock, T. J., Delmau, L. H., and Moyer, B. A Solvatochromic Solvent Polarity Measurements of Alcohol Solvent Modifiers and Correlation with Cesium Extraction Strength, Solvent Extr. Ion Exch. 19(2001), 1037-1058. 

Polarity is one of the most important parameters of ILs for its effect on electrochemical reactions. It is important when we characterize ILs to measure not only thermal properties such as melting point but also solvent properties such as polarity [68-70]. The most common method of polarity measurement is a dielectric constant measurement. Weingartner et al. and Hefter et al. have shown, by applying appropriately high frequency methods, that the dielectric constants are uniformly around 10-15. Accordingly, the polarity of ILs should be estimated by other methods. Solvatochromism is heavily applied for this purpose due to its simplicity. 

A small change in the pressure of a SCF can produce a large change in the solvent strength (as measured by solubility parameter or solvatochromic polarity scales(2)), which can cause a large thermodynamic solvent effect on a rate or equilibrium constant. This phenomenon is unique to SCFs. An extremely pronounced pressure effect was discovered for the rate constant of the unimolecular decomposition of a-chlorobenzyl methyl ether in supercritical l,l>difluoroethane(12)... 

Since the solvatochromic standard betaine dye (see Figure 1) was numbered 30 in the first publication(9), its molar transition energies were also designated at E,j.(30)-values. This transition energy E-p or Ep(30) can be used either directly or in the form of a relative measure RPM (i.e. Relative Polarity Measure defined as follows(5) ... 

It should be born in mind, however, that the activation parameters calculated refer to the sum of several reactions, whose enthalpy and/or entropy changes may have different signs from those of the decrystalUzation proper. Specifically, the contribution to the activation parameters of the interactions that occur in the solvent system should be taken into account. Consider the energetics of association of the solvated ions with the AGU. We may employ the extra-thermodynamic quantities of transfer of single ions from aprotic to protic solvents as a model for the reaction under consideration. This use is appropriate because recent measurements (using solvatochromic indicators) have indicated that the polarity at the surface of cellulose is akin to that of aliphatic alcohols [99]. Single-ion enthalpies of transfer indicate that Li+ is more efficiently solvated by DMAc than by alcohols, hence by cellulose. That is, the equilibrium shown in Eq. 7 is endothermic ... 

The dielectric constant and refractive index parameters and different functions of them that describe the reactive field of solvent [45] are insufficient to characterize the solute-solvent interactions. For this reason, some empirical scales of solvent polarity based on either kinetic or spectroscopic measurements have been introduced [46,47]. The solvatochromic classification of solvents is based on spectroscopic measurements. The solvatochromic parameters refer to the properties of a molecule when its nearest neighbors are identical with itself, and they are average values for a number of select solutes and somewhat independent of solute identity. 

Molecular rotors with a dual emission band, such as DMABN or A/,A/-dimethyl-[4-(2-pyrimidin-4-yl-vinyl)-phenyl]-amine (DMA-2,4 38, Fig. 13) [64], allow to use the ratio between LE and TICT emission to eliminate instrument- and experiment-dependent factors analogous to (10). One example is the measurement of pH with the TICT probe p-A,A-dimethylaminobenzoic acid 39 [69]. The use of such an intensity ratio requires calibration with solvent gradients, and influences of solvent polarity may cause solvatochromic shifts and adversely influence the calibration. Probes with dual emission bands often have points in their emission spectra that are independent from the solvent properties, analogous to isosbestic points in absorption spectra. Emission at these wavelengths can be used as an internal calibration reference. 

The solvatochromic phenolbetaine Reichardt s Dye (RD) allows to calculate a single parameter that indicates the overall polarity of the polymer. It is obtained by dissolving the dye in the polymer and measuring the absorbance maximum. The molar transition energy (Ex(30)) of RD is an empirical parameter to scale solvent polarity and is obtained by calculating18 Et(30) - hcvmaxNA OT 2.859vmaX R >. 

Monte Carlo calculations of interactions of 1 //-bcnzotriazole with water reveal significant electronic polarization of the heterocycle. The dipole moment is increased by 2.89 D for the ground state and 2.75 D for the excited state to the total values of 6.89 and 6.40 D, respectively. Direct measurements of dipole moments in water are not possible, but these numerical results are supported by experimental solvatochromic blue shift of the 7t —> Jt transition <2003IJQ572>. 

The properties of organic liquids relevant to their use as solvating agents have also been reviewed [76]. The ability of liquids to solvate a solute species depends mainly on their polarity and polarizability properties, ability to hydrogen bond, and cohesive electron density. These molecular properties are best measured by the Kamlet-Taft solvatochromic parameters, and the square of Hildebrand s solubility parameter. 

X0 is the value of the property in the gas phase. (In practice, X and X0 are often the logarithm of the property in question.) The parameters a and p are measures of a solvent s ability to donate and accept hydrogen bonds, respectively, and tt is an index of its polarity/polarizability. They were initially assigned on the basis of ultraviolet spectral shifts of certain dyes in a variety of solvents, and hence were labeled solvatochromic parameters.186"188... 

The first point to be addressed is the increase in dipole moment, Apt, on excitation of PRODAN (Formula in Figure 7.4). The determinations of A/x reported in the literature, apart from one, are based on solvatochromic shifts analyzed with the Lippert-Mataga equation. In the original paper by Weber and Farris Apt was estimated to be 20 D, but this value was later recognized to be overestimated and recalculation led to a value of 8 Da) b). Another study yielded a consistent value of 7 Dc) d). A completely different method based on transient dielectric loss measurement provided a somewhat lower value 4.4-5.0 Dd. From all these results, it can be concluded that the increase in dipole moment on excitation is not responsible for the high sensitivity of PRODAN to solvent polarity. 

The polarity of some l-alkyl-3-methylimidazolium ionic liquids has been determined using the solvatochromic dyes Nile Red and Reichardt s dye [16, 17], Measurements with Nile Red do not give absolute values of polarity but provide a useful scale to estimate the relative polarity of the ionic liquids. Similar measurements have been made using a number of other solvatochromic dyes (dansylamide, pyrene, pyrenecarboxyaldehyde, and bromonapthalene) for [BMIM][PF6], and gave results consistent with those obtained with Nile Red. Values for Ej obtained for ionic liquids generally fall between the values of 0.6 and 1.0, as shown in Table 4.3. 

In this respect, the solvatochromic approach developed by Kamlet, Taft and coworkers38 which defines four parameters n. a, ji and <5 (with the addition of others when the need arose), to evaluate the different solvent effects, was highly successful in describing the solvent effects on the rates of reactions, as well as in NMR chemical shifts, IR, UV and fluorescence spectra, sol vent-water partition coefficients etc.38. In addition to the polarity/polarizability of the solvent, measured by the solvatochromic parameter ir, the aptitude to donate a hydrogen atom to form a hydrogen bond, measured by a, or its tendency to provide a pair of electrons to such a bond, /, and the cavity effect (or Hildebrand solubility parameter), S, are integrated in a multi-parametric equation to rationalize the solvent effects. 

Parameters of the Kamlet-Taft solvatochromic relationship. These parameters measure the contributions to overall solvent polarity of the hydrogen bond donor, the hydrogen bond acceptor, and the dipolarity/polarizability properties of solvents. 

The variations in the absorption energies of various dyes have been used to characterise the polarity of various media and create empirical scales. For this purpose, the most widely used dye is the highly, negatively solvatochromic betaine (1.98), known as Reichardt s dye, whose transition energy f .j,(30) in kcal mol, measured in a particular solvent, characterises the polarity of that solvent. 

An obvious outlet for solvatochromism is the measurement of the concentrations of small amounts of polar molecules in non-polar environments, e.g. methanol in naphtha. Fuel oil is obviously a key market, as the various fractions are often marked ... 

Spectroscopic measurements of solvatochromic and fluorescent probe molecules in room temperature ILs provide an insight into solvent inter-molecular interactions, although the interpretation of the different and generally uncorrelated polarity scales is sometimes ambiguous [23]. It appears that the same solvatochromic probes work in ILs as well [24], but up to now only limited data are available on the behavior of electronic absorption and fluorescence solvatochromic probes within ILs and IL-organic solvent mixtures. 

Figure 3.49 Examples of solvatochromic plots of absorption spectra, (a) The first absorption band of 4-nitroaniline (charge transfer band), (b) The first absorption band of acetone (n-n band). The ordinates are in units of 103 cm, measured at the band maximum the abcissa are in units of Onsager solvent polarity function f(D)...

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