Solvatochromic probe in molecular solvents: implicit versus explicit solvent model

Abstract

the ground and of the excited states of the molecule, and therefore, they induce shifts of the maxima in absorption or emission spectra. Two important factors contributing to the overall effect are the long-range electrostatic interactions with the solvent polarizing the solute and the shortrange contributions such as dispersion or specific solutesolvent interactions, e.g., hydrogen bonding. Properties of the solvent which affect the spectra of the solute are commonly described by empirical parameters quantifying solvent polarity/polarizability or its ability to act as a hydrogen bond donor or acceptor. examples of such scale are the Kamlet-Taft π * , α and β parameters [1-3] derived experimentally from the values of solvent-induced shifts in absorption spectra of molecules serving as solvatochromic probes [4].Two extreme approaches used to account for the solvent effect in quantum-chemical calculations are implicit and explicit solvent models. In the implicit approach, the solute is inserted into a cavity in a continuous solvent treated as an effective medium. In such a way, the electrostatic component of the solvent effect can be estimated from the interactions of the solute with surrounding polarizable medium; corrections to the energy resulting from van der Waals interactions can be also parametrized. Common examples of implicit solvent models are polarizable continuum model (PCM) [5] or conductor-like screening model (COSMO) [6]. Such methods can provide corrections to the energy of the solute in the solvent or values of solvatochromic shifts; they are computationally cheap and therefore widely used. Significant drawback of implicit approaches is that they do not account for specific interactions and the dispersion and repulsion contributions are treated via an effective parameterization.At the other end, there are the approaches in which solvent molecules are explicitly included in calculations.Abstract Solvent-induced shifts in the absorption spectrum of N,N-diethyl-4-nitroaniline were studied by quantum-chemical methods in water, dimethylsulfoxide, acetonitrile and acetone. TDDFT methodology and semiempirical ZINDO/S and PM6-CIS approaches were used to calculate excitation energies. Solvent effect was modeled in implicit solvent model by different variants of the PCM approach. Classical molecular dynamics was applied to obtain solute-solvent geometries used in explicit solvent modeling. Most implicit solvent models fail to reproduce the sequence of solvatochromic shifts for four studied solvents, usually yielding too small effect for water. The best result of the PCM method was obtained with SMD atomic radii. Semiempirical quantum-chemical methods in explicit solvent model did not provide satisfactory description of solvatochromic shifts with the largest disagreement to experiment observed for water. TDDFT explicit solvent calculations performed the best in modeling of spectral shifts. Problems with reproduction of experimental data were attributed to specific interactions.

Modeling absorption spectra of molecules in solution

“…Given that a QM treatment of solvent might be desirable, the careful researcher may next wonder how much solvent should be treated quantum mechanically. A number of studies have explored this question for different molecular properties, and the study of convergence of excitation energies with increasing size of molecular environment has become an area of active research . Although in some systems a large amount of QM solvent may not be critical for accurately computing the property of interest, we have found that for excited states, values within approximately 0.05 eV of the converged value can be obtained with a full solvation shell treated with QM, as shown in Figure .…”

“…A number of studies have explored this question for different molecular properties, [61][62][63][64][65][66] and the study of convergence of excitation energies with increasing size of molecular environment has become an area of active research. [10][11][12][67][68][69] Although in some systems a large amount of QM solvent may not be critical for accurately computing the property of interest, we have found that for excited states, values within approximately 0.05 eV of the converged value can be obtained with a full solvation shell treated with QM, as shown in Figure 3. [13] The amount of solvent required for this level of accuracy was similar for both polar and less-polar solutes, although this was somewhat dependent on the choice of the electronic structure method.…”

“…For example, implicit solvent approaches are unable to reproduce the significant difference in solvatochromic shift between Nile red in acetone and Nile red in ethanol, whereas an explicit solvent approach yields results in close agreement with experiment . Another study showed that implicit solvent approaches are unable to reproduce the strong solvatochromic shift of N , N ‐diethyl‐4‐nitroaniline in water, whereas explicit solvent yields much improved results. Explicit solvent models can also lead to improved solvatochromic shifts in nonpolar solvents, where the small dielectric constant of the solvent produces small shifts relative to vacuum calculations for implicit solvent approaches …”

Near UV-Visible electronic absorption originating from charged amino acids in a monomeric protein

“…Environmental/solvation effects are typically included in spectra calculations through continuum dielectric or quantum mechanics (QM)/molecular mechanics (MM) models. 40 , 43 – 45 These models are best suited for the description of isolated (spectrally distinct) chromophores ( e.g. dyes or aromatic amino acids) embedded in a solvent and/or protein medium.…”

Stereochemistry of Simple Molecules inside Nanotubes and Fullerenes: Unusual Behavior of Usual Systems

“…This statement is one of the main components of solute-solvent interaction research strategy. Its further development has resulted in two solvent models: implicit (continuum) and explicit (discrete); their comparative effectiveness has been discussed in recent publications [78][79][80][81][82][83][84]. In this connection, the endohedral complexes of fullerenes and nanotubes can be viewed as a variety of explicit models.…”