Solvatochromic measures

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). 

Keywords Metal surface Poly(vinyl amine) copolymers Polyelectrolyte adsorption Solvatochromic measurements X-ray photoelectron spectroscopy... 

Both, Moi and A/(0, can be evaluated by OP experiments. Change of state dipole moment Ap01 can be quantified from solvatochromic measurements using the Lippert-Mataga treatment, Eqs. (25a) and (25b) [59, 328-335]. 

P -Values can be measured by several techniques, including electric field induced second harmonic generation and solvatochromic measurements of absorption and fluorescence Moreover, quantum mechanical calculations can be used to estimate these nonlinearities For a more detailed description of the physical backgrounds of nonlinear optics we like to refer to excellent reviews and books< ° >. 

The preceding empirical measures have taken chemical reactions as model processes. Now we consider a different class of model process, namely, a transition from one energy level to another within a molecule. The various forms of spectroscopy allow us to observe these transitions thus, electronic transitions give rise to ultraviolet—visible absorption spectra and fluorescence spectra. Because of solute-solvent interactions, the electronic energy levels of a solute are influenced by the solvent in which it is dissolved therefore, the absorption and fluorescence spectra contain information about the solute-solvent interactions. A change in electronic absorption spectrum caused by a change in the solvent is called solvatochromism. 

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. 

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 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 ... 

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. 

P. A., Testa, B. Solvatochromic analysis of the retention mechanism of two novel stationary phases used for measuring lipophilicity by RP-HPLC./. Liquid Chromatogr. 1992, 35, 2133-2151. 

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. 

QSARs based on ionic compounds have thus been dramatically restricted due to the neglect of ion partitioning, which consequently meant that no technique was dedicated to such measurements and that modeling never took account of ionic species. To become fully accepted, potentiometry and electrochemistry at the ITIES need now to prove interesting in QSARs. As numerous lipophilicity data of ionizable compounds become available, one can expect that solvatochromic equations for ions will soon be developed in various solvent systems, which would greatly facilitate QSAR studies. 

When we perform experiment in such way that there is no interference of H-bonds or these bonds are stable and structure of solvent also does not varies essentially, solvatochromic plot demonstrates very good linearity as shown, for example, for some naphthylamine derivatives in ethanol-water mixtures. The linearity of solvatochromic plots is often regarded as an evidence for the dominant importance of nonspecific universal intermolecular interaction in the spectral shifts. Specific solvent effects lead to essential deviation of measured points from this linear plot. 

Molecular rotors are useful as reporters of their microenvironment, because their fluorescence emission allows to probe TICT formation and solvent interaction. Measurements are possible through steady-state spectroscopy and time-resolved spectroscopy. Three primary effects were identified in Sect. 2, namely, the solvent-dependent reorientation rate, the solvent-dependent quantum yield (which directly links to the reorientation rate), and the solvatochromic shift. Most commonly, molecular rotors exhibit a change in quantum yield as a consequence of nonradia-tive relaxation. Therefore, the fluorophore s quantum yield needs to be determined as accurately as possible. In steady-state spectroscopy, emission intensity can be calibrated with quantum yield standards. Alternatively, relative changes in emission intensity can be used, because the ratio of two intensities is identical to the ratio of the corresponding quantum yields if the fluid optical properties remain constant. For molecular rotors with nonradiative relaxation, the calibrated measurement of the quantum yield allows to approximately compute the rotational relaxation rate kor from the measured quantum yield [Pg.284]

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. 

Polymers can be characterized via the Kamlet-Taft approach which describes the ability of a species to act as a hydrogen bond acid (ai), the ability to act as a hydrogen bond base (Pj), the dipolarity/polarizability (7ti ), and the size of a species. These parameters are obtained by dissolving solvatochromic indicator dyes in the respective polymer and by measuring the shift of their absorbance maxima18. 

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 >. 

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