Solvatochromism model

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. 

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 ... 

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. 

A number of models have been proposed to describe the solution formation process [505-509], some of which can be extended to Include chromatographic processes and other solvent-dependent phenomena. In terms of chromatographic aiqplications the most useful are the solubility parameter concept, solvatochromic parameters and Snyder s solvent strength and selectivity... 

Tables 11-6, 11-7, and 11-8 show calculated solvatochromic shifts for the nucle-obases. Solvation effects on uracil have been studied theoretically in the past using both explicit and implicit models [92, 94, 130, 149, 211-214] (see Table 11-6). Initial studies used clusters of uracil with a few water molecules. Marian et al. [130] calculated excited states of uracil and uracil-water clusters with two, four and six water molecules. Shukla and Lesczynski [122] studied uracil with three water molecules using CIS to calculate excitation energies. Improta et al. [213] used a cluster of four water molecules embedded into a PCM and TDDFT calculations to study the solvatochromic shifts on the absorption and emission of uracil and thymine. Zazza et al. [211] used the perturbed matrix method (PMM) in combination with TDDFT and CCSD to calculate the solvatochromic shifts. The shift for the Si state ranges between (+0.21) - (+0.54) eV and the shift for the S2 is calculated to be between (-0.07) - (-0.19) eV. Thymine shows very similar solvatochromic shifts as seen in Table 11-6 [92],...

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. 

J. Li, C. J. Cramer, and D. G. Truhlar, A two-response-time model based on CM2/INDO/S2 electrostatic potentials for the dielectric polarization component of solvatochromic shifts on vertical excitation energies, Int. J. Quan. Chem. 77 264 (2000). 

If solvent (or environment) relaxation is complete, equations for the dipole-dipole interaction solvatochromic shifts can be derived within the simple model of spherical-centered dipoles in isotropically polarizable spheres and within the assumption of equal dipole moments in Franck-Condon and relaxed states. The solvatochromic shifts (expressed in wavenumbers) are then given by Eqs (7.3) and (7.4) for absorption and emission, respectively ... 

Rettig W. (1982) Application of Simplified Microstructural Solvent Interaction Model to the Solvatochromism of Twisted Intramolecular Charge Transfer (TICT) States, /. Mol. Struct. 8, 303-327. 

The solvatochromism of [Fe(phen)2(CN)2] and [Fe(phen)2(CN)2] has been discussed, with particular reference to solvent hydrogen-bond donor properties. Various solvation models have been applied to solvatochromism of [Fe(diimine)2(CN)2], and cormections between solvatochromism, electronic absorption spectra, and color perception parameters discussed in relation to [Fe(phen)2(CN)2]. Solvatochromic properties have been documented for [Fe(bipy)2(CN)2]>... 

However, it was noted that the discrepancy between the micellar rate constant and the rate constant in the model compound solution was largest for 4, which was known to be more sensitive toward hydrophobic interactions than 5. In addition, the solvatochromic Ex30 probe indicated a much more hydrophobic environment and other authors similarly found systems for which a model solution only mimicking the surfactant headgroups was insufficient to reproduce properties of the micellar pseudophase. The Hammett p-value for hydrolysis of substituted 1-benzoyl-1,2,4-triazoles 6a-f could similarly not be reproduced using a model solution mimicking ionic interactions only. " ... 

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. 

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... 

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 ... 

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. 

For the fluorescence solvatochromism emitted from a TICT state with the dipole moment fie to a FC ground state with dipole moment fj.Fgc 0 the CPC model results, in improving Eq. (2.20),... 

Transition moments, ioi> to the first excited state can be calculated from the integrated absorption of the linear electronic spectrum. This can be used to calculate -Af poi n the first term (defined here as yc) of the two-level model. The second term (Af p.oiAjioi m) from Equation (12) (defined here yn) involves Ajioi, which can be determined directly from solvatochromism( 19), or from a two-level analysis of a molecular EFISH measurement of p. 

Yonker and co-workers (60) used near-and mid-IR spectroscopy to study supercritical C02 and binary supercritical fluid systems composed of C02/H20, Kr/H20, and Xe/H20. The C02 results are consistent with increased intermolecular interaction between C02 molecules with increasing density. This parallels previous results using UV-Vis solvatochromism (21-28,32). For an ideal gas/water system an Onsanger electrostatic model (dipole-induced-dipole) sufficed to describe the spectral shifts. In contrast, the C(VH20 system exhibited density-dependent changes in specific intermolecular interactions. 

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