Solvatochromic data, experimental

This clustering theory may be tested using solvatochromic data. By assuming a linear relation between V2 and kx, in accordance with experimental iesults(6.25). an important result is obtained... 

Experimental solvatochromic data, AExm AExi and AExs, and theoretical expressions for ZfcJzyi and ZaJ yi used to calculate the local compositions (local mole fractions) xi2 and X32 = 1 - X12. 

In Fig. 17.12 the solvatochromic data are reported with respect to the polarity functions, in comparison with the experimental data. Regarding the contributions due to the electrical dipole moment, as shown in Fig. 17.12a, b, if we plot the absorption transition energies as functions of one of the polarity functions, we do not obtain a picture that can be reasonably fitted with a linear regression. We have also tried to interpret the solvatochromic shift of the absorption transition energy, when increasing the solvent polarity, invoking the variation of the absorption... 

Fig. 17.12 Failure of the analytical models when employed to interpret the experimental solvatochromic data of Betaine-26. a Plot of the absorption transition energy as a function of the polarity function/2(e) (according to Eq. (4.14.6)). b (a) Plot of the absorption transition eneigy as a function of the polarity function F (s, n ) (according to Eq. (4.14.7)). c Comparison between computed versus experimental shift of the absorption transition energy, with respect to a reference solvent (1,4-dioxane, filled and unfilled circles refer to Eqs. (17.57)...

A sampling of appHcations of Kamlet-Taft LSERs include the following. (/) The Solvatochromic Parameters for Activity Coefficient Estimation (SPACE) method for infinite dilution activity coefficients where improved predictions over UNIEAC for a database of 1879 critically evaluated experimental data points has been claimed (263). (2) Observation of inverse linear relationship between log 1-octanol—water partition coefficient and Hquid... 

Combining the idea of solvent-induced changes in molecular structure with the concept of a solvent continuum around the solvatochromic molecule, a micro-structural model of solvatochromism has been developed by Dahne et al., which reproduces, qualitatively correctly and quantitatively satisfactorily, the solvatochromic behavior of simple merocyanine dyes [95b], The results obtained with this model for 5-(dimethylamino)penta-2,4-dienal are in good agreement with the solvent-dependent experimental data such as transition energies, oscillator strengths, r-electron densities, and r-bond energies [95b] cf. also [326, 327],... 

UV spectra usually involve electronic state transitions, so that simple Hartree-Fock and DFT calculations often are not sufficient PCM has been recently extended also to multi-configurational (MC-SCF) calculations [113] and to time-dependent approaches, allowing for the description of excited states and then the prediction of the so-called solvatochromic effects on these spectra. Nuclear magnetic resonance (NMR) and electron spin resonance (EPR) spectra are even more influenced by solute-solvent interactions moreover, the interpretation of experimental data is often very difficult without the support of reliable ab initio calculation, especially for EPR which is usually applied to unstable radical species. 

Three major approaches to the prediction of aqueous solubility of organic chemicals using Quantitative Structure Activity Relationship (QSAR) techniques arc reviewed. The rationale behind six QSAR models derived from these three approaches, and the quality of their fit to the experimental data are summarized. Their utility and predictive ability are examined and compared on a common basis. Three of the models employed octanol-water partition coefficient as the primary descriptor, while two others used the solvatochromic parameters. The sixth model utilized a combination of connectivity indexes and a modified polarizability parameter. Considering the case of usage, predictive ability, and the range of applicability, the model derived from the connectivity- polarizability approach appears to have greater utility value. 

The material presented in the chapter illustrates the difficulties connected with experimental determination and theoretical computation of dipole moments in general and heterocyclic compounds in particular. It is obvious that more work is needed on solute-solvent interactions, role of the polarity of the solvent, dipole moments in solution v. dipole moments in the gas phase, dielectric constant measurements V5. microwave spectroscopic data, improvements in calculations, better solvatochromic equations, etc. 

An integral equation formalism (lEF) has been developed as particularly suitable for the description of solvent effects on spectral transition energies within the PCM model. The respective theoretical equations have been applied for the calculation of solvatochromic shifts of several carbonyl-group containing molecules at the self-consistent field (SCF), configuration interaction (Cl) and multiconfiguration self-consistent (MC SCF) field level of theory. The calculated spectral shifts accompanying the transfer of a solvatochromic compound from the gas phase to water were comparable with the experimental data. In Table 11.1.4, the results of calculations are presented for three carbonyl compounds, formaldehyde, acetaldehyde and acetone. 

Table 11.1.5. The INDO/CI calculated solvatochromic shifts Av (from the gas phase to cyclohexane) of some aromatic compounds and the respective experimental data in low polarity solvents (cm )° ...

Fig. 6 (a) Calculated (CAM-B3LYP) excitation energy (eV) for the neutral and anionic form of RA in gas and solution phase compared to the experimental data in ethanol, (b) Calculated solvatochromic shift (eV) for the ground and excited state total energy of the neutral and anionic RA. 

All the solvatochromic contributions have been then evaluated following the analytical approaches presented in Sect. 4.1. The values of these quantities, as well as the estimation of the spherical cavity radius have been obtained from QM computations, and are reported in Table 17.3 and Fig. 17.11. In particular. Table 17.3 collects the numerical values of the different contributions, while in Fig. 17.11 the computed induction contributions are plotted together with the experimental data. It is apparent that both the contributions due to the transition dipole moment and to the electrical quadmpole moment are one order of magnitude less important than the contributions due to the electrical dipole moment (we remind that within Suppan-Ghoneim model the quadrupole moment [101] is treated as a scalar quantity this is tme just in case of linear and centro-symmetrical quadmpoles. In the other cases, 0 = Tr(0)/3. 

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