Solvatochromic shift theoretical models

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],...

As shown in the other contributions, continuum models have been significantly modified and improved with respect to the older versions the same improvements have also been achieved for their extensions to the study of vertical excitation/de-excitation processes. These extensions will be reviewed here but before that, a brief overview will be given on the main physical aspects to be accounted for in any theoretical model aimed at reliably reproducing solvatochromic shifts. 

The study of solvatochromic shifts is of great importance and has received enormous theoretical attention in recent years. Progress has been achieved in the use of the self-consistent reaction field and cavity models. These advances have also shown several limitations. It is thus of great interest to have alternative procedures to calculate solvent effects. In this respect the use of Monte Carlo/Molecular Dynamics simulations has been growing. In this paper we suggest a procedure to allow a full quantum mechanical calculation of the solute-solvent system. The basic idea is to treat the solute, the solvent and its interaction by quantum mechanics. First a Monte Carlo simulation is performed to characterize the liquid structure. These structures are then used in the quantum mechanical calculation. As a liquid has not one but a great number of structures equally possible within a... 

Quantum chemical methods were employed with variable success in theoretical studies of solvatochro-mism. Electronic transition energies of molecules in the gas phase may be calculated by ab-initio and semiempirical methods. The latter are more frequently used because of their much smaller computational cost and also because of the existence of programs, like CNDO/S and INDO/S, that were parametrized for spectral studies. Calculation of solvent effects poses a major problem in the prediction of solvatochromic shifts. Several methods exist that are based on a continuum model, where changes in the electronic levels of the molecule are treated as perturbations by the solvent dielectric constant. This model was extended to include the effect of an added salt. Alternatively, the bulk solvent may be replaced by point charges. ... 

All theoretical treatments of solvatochromic shifts proceed from modelling the solvational interactions in the liquids and solutions. Theoretically, the interaction potential between a solute molecule and the surrounding solvent molecules 0 is given by the following integral... 

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. 

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