Solvatochromic approach

Solvatochromic Approach Solvatochromic relationships are multivariate correlations between a property, usually solubility or partitioning property (see Sections 11.4 and 13.3), and solvatochromic parameters, parameters that account for the solutes interaction with the solvent. In the case of vapor pressure, the solvatochromic parameters only have to account for intermolecular interaction such as selfassociation between the solute (i.e., pure compound) molecules themselves. The following model has been reported for liquid and solid compounds, including hydrocarbons, halogenated hydrocarbons, alkanols, dialkyl ethers, and compounds such as dimethyl formamide, dimethylacetamide, pyridine, and dimethyl sulfoxide 

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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. 

Solvatochromic Approach The solvatochromic approach describes a solvent-dependent property, XYZ, as a function of a cavity term, a dipolar term, and terms that account for hydrogen bonding [21] ... 

The solvatochromic approach has been criticized by Yalkowsky et al. [24]. In particular, they claim n to be an insignificant parameter for the estimation of aqueous solubilities and they contend that models in which the solubility is correlated with Kov/ and Tm (models 11.4.3 to 11.4.5, 11.4.10 and 11.4.11) are more versatile and have a firmer thermodynamic basis. 

The correlation between aqueous solubility and molar volume discussed by McAuliffe [5] for hydrocarbons, and the importance of the cavity term in the solvatochromic approach, indicates a significant solubility dependence on the molecular size and shape of solutes. Molecular size and shape parameters frequently used in quantitative structure-water solubility relationships (QSWSRs) are molecular volume and molecular connectivity indices. Moriguchi et al. [33] evaluated the following relationship to estimate Cw of apolar compounds and a variety of derivatives with hydrophilic groups ... 

Yalkowsky SH, Pinal R, Baneijee SJ (1988) Water solubility a critique of the solvatochromic approach. Pharm Sci 77 74-77... 

A. de Juan, G. Fonrodona and E. Casassas, Solvent classification based on solvatochromic parameters a comparison with the Snyder approach. Trends Anal. Chem., 16(1) (1997) 52-62. T. Naes, The design of calibration in near infra-red reflectance analysis by clustering. J. Chemom., 1 (1987) 129-134. 

A question which has intrigued colour chemists for years is why indigo, a relatively small molecule, absorbs at such long wavelengths. The colour of indigo depends crucially on its environment. It is known that, in the vapour phase, the only situation in which it approaches a monomolecular state, indigo is red. In solution, indigo exhibits pronounced positive solvatochromism in non-polar solvents it is violet, while in polar solvents it is blue. In the solid state, and when applied to fabric as a vat dye, it is... 

Solvatochromic shifts for cytosine have also been calculated with a variety of methods (see Table 11-7). Shukla and Lesczynski [215] studied clusters of cytosine and three water molecules with CIS and TDDFT methods to obtain solvatochromic shifts. More sophisticated calculations have appeared recently. Valiev and Kowalski used a coupled cluster and classical molecular dynamics approach to calculate the solvatochromic shifts of the excited states of cytosine in the native DNA environment. Blancafort and coworkers [216] used a CASPT2 approach combined with the conductor version of the polarizable continuous (CPCM) model. All of these methods predict that the first three excited states are blue-shifted. S i, which is a nn state, is blue-shifted by 0.1-0.2 eV in water and 0.25 eV in native DNA. S2 and S3 are both rnt states and, as expected, the shift is bigger, 0.4-0.6eV for S2 and 0.3-0.8 eV for S3. S2 is predicted to be blue-shifted by 0.54 eV in native DNA. 

There are single- and multiparameter approaches for determining the polarity and separation of contribution of different interactions to the total effect of polarity on spectroscopic characteristics. They are based on different theories of solvatochromic shifts of absorption and fluorescence bands. 

Mukherjee, S., Chattopadhyay, A., Samanta, A. and Soujanya, T. (1994). Dipole-moment change of Nbd group upon excitation Studied using solvatochromic and quantum-chemical approaches - Implications in membrane research. J. Phys. Chem. 98, 2809-2812. 

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. 

Lavastre s approach is valid, but the use of (a) less reactive brominated arenes and (b) the obvious lack of solubilizing groups restricts the value of this approach. Perhaps all of the 96 polymers would have to be made in a conventional approach and compared to the polymers obtained by the high-through-put method to draw further conclusions. In addition, the fluorescence was measured in situ, in a THF/diisopropylamine mixture in the presence of the Pd-Cu catalyst. That approach may have an intrinsic problem too, because it is known that PAEs are solvatochromic, and as a consequence, the obtained fluorescence data might be biased. However, with some tweaking this is a powerful method to obtain a cornucopia of different structures quickly and without too much synthetic effort, if the monomers are easily available. 

Use of Solvatochromic Equations Solvatochromic analysis for log P calculations appears to be one of the more useful approach built using linear free energy relationship (LEER) described by the general Equation (5.6) [58] ... 

Solvatochromic probes have found applications in polymer characterisation, where they can be used to look for localised polar features at the molecular level. Two distinct approaches can be adopted ... 

QM/MM approaches where the solute is QM and the solvent MM are in principle useful for computing the effect of the slow reaction field (represented by the solute point charges) but require a polarizable solvent model if electronic equilibration to the excited state is to be included (Gao 1994). With an MM solvent shell, it is no more possible to compute differential dispersion effects directly than for a continuum model. An option is to make the first solvent shell QM too, but computational costs for MC or MD simulations quickly expand with such a model. Large QM simulations with explicit solvent have appeared using the fast semiempirical INDO/S model to evaluate solvatochromic effects, and the results have been promising (Coutinho, Canute, and Zemer 1997 Coutinho and Canute 2003). Such simulations offer the potential to model solvent broadening accurately, since they can compute absorptions for an ensemble of solvent configurations. 

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