Specific Solute-solvent Interactions Research Articles

Computational and spectroscopic studies of organic mixed-valence compounds: where is the charge?

This article discusses recent progress by a combination of spectroscopy and quantum-chemical calculations in classifying and characterizing organic mixed-valence systems in terms of their localized vs. delocalized character. A recently developed quantum-chemical protocol based on non-standard hybrid functionals and continuum solvent models is evaluated for an extended set of mixed-valence bis-triarylamine radical cations, augmented by unsymmetrical neutral triarylamine-perchlorotriphenylmethyl radicals. It turns out that the protocol is able to provide a successful assignment to class II or class III Robin-Day behavior and gives quite accurate ground- and excited-state properties for the radical cations. The limits of the protocol are probed by the anthracene-bridged system 8, where it is suspected that specific solute-solvent interactions are important and not covered by the continuum solvent model. Intervalence charge-transfer excitation energies for the neutral unsymmetrical radicals are systematically overestimated, but dipole moments and a number of other properties are obtained accurately by the protocol.

Dynamical Discrete/Continuum Linear Response Shells Theory of Solvation: Convergence Test for NH4+ and OH− Ions in Water Solution Using DFT and DFTB Methods

A new dynamical discrete/continuum solvation model was tested for NH(4)(+) and OH(-) ions in water solvent. The method is similar to continuum solvation models in a sense that the linear response approximation is used. However, different from pure continuum models, explicit solvent molecules are included in the inner shell, which allows adequate treatment of specific solute-solvent interactions present in the first solvation shell, the main drawback of continuum models. Molecular dynamics calculations coupled with SCC-DFTB method are used to generate the configurations of the solute in a box with 64 water molecules, while the interaction energies are calculated at the DFT level. We have tested the convergence of the method using a variable number of explicit water molecules and it was found that even a small number of waters (as low as 14) are able to produce converged values. Our results also point out that the Born model, often used for long-range correction, is not reliable and our method should be applied for more accurate calculations.

Shells theory of solvation and the long-range Born correction

A new solvation model, named shells theory of solvation, is proposed. In this approach, the solvent is divided in two regions, the S1 shell, close to the solute and describing specific solute–solvent interactions, and the S2 shell, representing the remain solvent and accounting for the long-range interaction contribution. A simple theoretical equation can be derived which allows the computation of the solvation free energy using two-point thermodynamic integration and configurations generated from molecular dynamics simulation. The discrete/continuum version of this theory provides rigorous theoretical foundations for the popular long-range Born correction and presents a new reliable expression for including this contribution. Further, it converges to the full discrete representation of the solvent when the number of solvent molecules goes to infinity. The method can be easily applied when the solute–solvent interaction (S1 shell) is treated by full quantum mechanics, while the S2 shell is described by a dielectric continuum solvation method. A simple test of the theory was done for solvation of fluoride ion in benzene solution. The S1 shell was composed of the fluoride ion plus 32 benzene molecules, and the interaction with the S2 shell was calculated at Hartree–Fock level with the MINI basis set and using the polarizable continuum model.

Synthesis, Spectral Characterization and Solvent Effects on the Electro-Reduction of Mixed Ligand Cobalt(III) Complexes

A series of cobalt(III) complexes of the type cis-β-[Co(ampy)2(4-R-Py)Cl]Cl2 where ampy = 2-aminomethylpyridine, R = H, Me, Et, t-Bu, COMe, and CN has been synthesized and characterized spectroscopically. The electro-reduction of these complexes has been studied in propan-2-ol/water and 1,4-dioxane/water media. The redox potential (E 1/2) data were correlated with solvent and structural parameters with an aim to shed some light on the mechanism of these reactions. Correlation of E 1/2 with macroscopic solvent parameters viz. relative permittivity, indicates that the reactivity is influenced by both specific and non-specific solute-solvent interactions. The Kamlet-Taft solvatochromic comparison method was used to separate and quantify the specific and non-specific solvent effects on the redox process. Increases in the percentage of organic co-solvent in both media makes the electro-reduction of Co(III) to Co(II) easier.

Changes induced by solvent polarity in electronic absorption spectra of some azo disperse dyes

The electronic absorption spectra of some azo disperse dyes in various solvents of different polarities have been studied at room temperature. The solvent effects on the wavenumbers of the absorption band maxima ( ṽ max) were discussed using the following solvent parameters, refractive index, n, dielectric constant, ε and empirical Kamlet–Taft solvent parameters, π* (dipolarity/polarizability), α (hydrogen bond donating capacity) and β (hydrogen bond accepting ability). The solute–solvent interactions were determined on the basis of multilinear solvation energy relationships concept. The fitting coefficients obtained from this analysis allowed us to estimate the contribution of each type of interactions to the total spectral shifts in solution. The established dependences between ṽ max and the solvent parameters emphasize that the visible band of the studied molecules is affected by both non-specific and specific solute–solvent interactions.

Efficient Photochemical Oxidation of Anisole in Protic Solvents: Electron Transfer driven by Specific Solvent–Solute Interactions

The dynamics of the bimolecular quenching of triplet excited benzophenone by anisole was studied by nanosecond flash photolysis. We carried out a detailed study of the solvent dependence of the reaction rates and efficiencies in a number of protic and non-protic solvents. These studies were augmented by theoretical modelling and experimental investigation of solute/solvent interactions in the triplet excited and the ground state, respectively. The triplet quenching that follows Stern-Volmer kinetics in all cases is profoundly dependent on the nature of the solvent, with the highest reactivity being consistently found in protic solvents. The results in non-protic solvents are compatible with unproductive quenching via a charge-transfer state, whereas the generally fast quenching in protic solvents is accompanied by efficient formation of free-radical products. Analysis of the solvent dependence in terms of Marcus theory reveals the impact of specific solvation of benzophenone by protic solvents on the ET driving force and kinetics. Specific solvation is found to support efficient free radical ion formation in media of moderate and low polarity as well.

Thermodynamics of Complexation of Alkali Metal Cations by a Lower-Rim Calix[4]arene Amino Acid Derivative

Complexation of alkali metal cations with 5,11,17,23-tetra-tert-butyl-26,28,25,27-tetrakis(O-methyl-d-α-phenylglycylcarbonylmethoxy)calix[4]arene (L) in methanol and acetonitrile was studied by means of direct and competitive microcalorimetric titrations at 25 °C. The thermodynamic parameters of complexation reactions showed that all the reactions investigated were enthalpically controlled. In both solvents the reaction enthalpy was most favorable for Na+ binding with L leading to the highest affinity of the examined calix[4]arene derivative towards this cation. The solubilities (and consequently the solution Gibbs energies) of the ligand were determined, as were the corresponding solution enthalpies and entropies. No significant difference was observed between the solution thermodynamic quantities of L in the two solvents, whereas the transfer of complex species from methanol to acetonitrile was found to be quite favorable. The interactions of solvent molecules with the free and the complexed ligand were investigated by 1H NMR spectroscopy. It was concluded that in both cases inclusion of an acetonitrile molecule into the hydrophobic cavity of L occurred, which significantly affected the cation complexation in this solvent. The thermodynamic data were discussed regarding the structural properties of the ligand, the free and the complexed cations as well as the solvation abilities of the solvents examined. In this respect, the specific solvent-solute interactions and the intramolecular NH⋅⋅⋅O=C hydrogen bonds at the lower rim of L were particularly addressed.

Ionic liquids for acetylene and ethylene separation: Material selection and solubility investigation

Potential applications of ionic liquids (ILs) for the green separation process of acetylene in ethylene and for the storage of acetylene were investigated. To deal with this proposal, the solubilities of the unsaturated hydrocarbons in various ionic liquids were evaluated. The solubility of ethylene shows a solubility parameter-dependent behavior as indicated by the proportional relationship between the natural log value of Henry's law constant and the inverse molar volume of ILs. This correlation suggests the most important role of voids formation within IL to accommodate the solutes and the applicability of regular solution theory to model the solubility behavior. Whereas, in addition to the free-volume contribution of ILs, the solubility of acetylene is largely controlled by a specific solute–solvent interaction as a result from the association of the acidic hydrogen character in acetylene and the relative basicity of the anion. Those two different solubility behaviors result in a high absorption selectivity of acetylene over ethylene in the basic ILs. 1H NMR experiment clearly demonstrated the presence of a substantial interaction between the acetylene and the anion of IL. Interestingly, this solute–solvent interaction is reversible as indicated by the absorption–desorption test of acetylene in [BMIM][Me 2PO 4].

Substituent and solvent effects on the photo-physical properties of some coumarin dyes

Absorption and fluorescence spectra of three coumarin dyes with various substituents and alkyl groups have been recorded in solvents in the range of 200–730 nm. The photo-physical behavior of dissolved dye depends on the nature of its environment, i.e. the solvatochromic behaviors of coumarin dyes and solvent/solute hydrogen bonding interactions can be analyzed by means of linear solvation energy relationships concept proposed by Kamlet and Taft. The intensity, shape, and maximum wavelength of the absorption and fluorescence band of these dyes in solution depend strongly on the solvent–solute interactions and solvent nature. Hydrogen bonding interactions (specific solute–solvent interactions) between these dye–solvent complex and dipolarity/polarizability (non-specific solute–solvent interactions) control reorientation of solvent molecules around the dye.

Solvent and structural effects on the photo-reduction of cobalt(III) complexes in aquo-organic solvent media possessing different H-bonding properties

The photo-reduction of a series of cobalt(III) complexes of the type cis-β-[Co(ampy) 2 (4-R-Py)Cl]Cl 2 where ampy = 2-aminomethylpyridine and R = H, Me, Et, t-Bu, COMe, and CN has been studied in propan-2-ol/water and 1,4-dioxane/water medium. The photo-reduction quantum yield (Φ Co II ) data were correlated with solvent and structural parameters with an aim to shed some light on the mechanism of these reactions. Increase in the percentage of organic co-solvent in both the medium increased the photo-reduction quantum yield. Correlation of Φ Co II with macroscopic solvent parameters viz. relative permittivity indicated that the reactivity is influenced by both specific and non-specific solute-solvent interactions. Kamlet-Taft solvatochromic comparison method was used to separate and quantify these effects. H-bonding property of the mixture was found to dominate the solvation and consequently the magnitude of the quantum yield.

Differential Pulse Voltammetric Studies of Some Mixed-ligand Co(III) Complexes in Aquo-Organic Solvent Media Possessing Different Hydrogen Bonding Properties

The electro-reduction of a series of Co(III) complexes of the type cis-β -[Co(trien)(4-R-Py)Cl]Cl2, where trien = triethylenetetramine and R = H, Me, Et, t-Bu, COMe, and CN, has been studied in propan-2-ol/water and 1,4-dioxane/water binary mixtures. The redox potential (E1/2) data were correlated with solvent and structural parameters with an aim to shed some light on the mechanism of these reactions. Correlation of E1/2 with macroscopic solvent parameters indicated that the reactivity is influenced by both specific and non-specific solute-solvent interactions. The reduction of Co(III) to Co(II) in these complexes was observed to become increasingly easier with an increase in the dipolarity/ polarizability of the medium as evidenced by the Kamlet-Taft correlation. The difference in the relative lenience of the reduction has been explained using the difference in H-bonding properties of the medium.

Structural and Spectroscopic Studies of the Photophysical Properties of Benzophenone Derivatives

The effect a solvent has on the photophysical properties of a series of benzophenone derivatives, all FDA approved for use in sunscreens, is examined. Experimentally significant differences in the solvatochromic behavior are found to be dependent upon the substituents on the parent benzophenone molecule. The spectral trends do not appear to originate from only changes in the solvent polarity but indicate that specific solvent-solute interactions influence the absorbance energies of some benzophenones. Computational investigations examine the structure and electronic excitation energies of the molecules. Specific interactions of the solvent and solute are modeled to evaluate structural changes that result from solvent-solute complexation and the impact of the changes upon absorbance properties. The viability of an intramolecular excited state proton transfer is theoretically evaluated. The combination of experimental and computational analysis provides a more complete understanding of the molecular level origin of the unique photophysical properties of this class of UV absorbers.

Solvent hydrogen bonding and structural influences on the CrVI oxidation of anilines in aqueous acetic acid medium

The oxidation of meta- and para-substituted anilines by Cr VI oxidant, imidazolium fluorochromate (IFC), in aqueous acetic acid mixtures of varying compositions in the presence of p-toluenesulfonic acid (PTS) is first order in IFC and PTS. Michaelis-Menten type kinetics is observed with all of the anilines. The IFC oxidation of 15 meta- and para-substituted anilines at 299-322 K complies with the isokinetic relationship but not to any of the linear free energy relationships. The isokinetic temperature lies within the experimental range. The rate data failed to correlate with macroscopic solvent parameters such as relative permittivity, e r , and ionizing power, Y, correlation of rate data with Kamlet-Taft solvatochromic parameters (hydrogen bond donor acidity, α, hydrogen bond acceptor basicity, β. and dipolarity/polarizability, π * ) is linear which suggests that the specific solute-solvent interactions play a dominating role in governing the reactivity.

Experimental and theoretical investigation of 5-para-nitro-benzylidene-thiazolidine-2-thione-4-one molecule

Experimental methods (FT-IR, Raman and NMR spectroscopies, and X-ray diffraction technique) coupled with quantum chemical calculations based on density functional theory (DFT) are used for structural and electronic characterization of 5-para-nitro-benzylidene-thiazolidine-2-thione-4-one (5pN-BTT). X-ray diffraction technique indicates that this compound crystallizes with one tetrahydrofuran (THF) solvent molecule, forming a 1:1 5pN-BTT·THF complex, in the monoclinic space group C2/ c, with Z = 8 and cell parameters a = 29.496(9) Å, b = 7.276(2) Å, c = 14.873(4) Å, α = 90.0°, β = 95.161(6)° and γ = 90.0°. The lowest energy optimized geometry of the investigated compound in gas-phase corresponds to thionic tautomer being consistent with that obtained by X-ray technique. Tautomeric equilibrium between the thione, thiol and enol forms of the compound have been considered and analyzed by theoretical methods. The continuum PCM solvation model fails to reproduce the experimental chemical shift associated with the NH proton in the thione form, but a very good correlation between experiment and theory was obtained by taking into account the specific solute–solvent interactions. Proton NMR spectrum of 5pN-BTT in DMSO solution shows an unexpected proton chemical shift at 14 ppm which is attributed to the proton bound to the nitrogen atom and explained by assuming a relatively strong hydrogen bond between 5pN-BTT and the solvent molecule, realized through NH group. The molecular vibrations of 5pN-BTT were investigated by FT-IR and FT-Raman spectroscopies and the vibrational spectrum of the solid state 5pN-BTT compound has been assigned based on DFT calculations at B3LYP level of theory using the standard 6-31G(d) basis set.

Dendritic Molecular Switch: Chiral Folding and Helicity Inversion

Appropriately designed chemical architectures can fold to adopt well-defined secondary structures without the need for structural motifs of biological origin. We have designed tris(N-salicylideneaniline)-based hyperbranched molecules that spontaneously collapse to compact three-blade propeller geometry of either (P)- or (M)-handedness. For a homologous series of compounds, a direct correlation was established between the absolute screw sense, either (P)- or (M)-, of this helical folding and the absolute configuration, either (R)- or (S)-, of the chiral alcohol groups introducing local asymmetric bias to the conformationally restricted molecular backbone. 1H NMR and CD spectroscopic studies provided significant insights into structural folding and unfolding of these chiral molecules in solution, which proceed via reversible assembly and disassembly of the C3-symmetric hydrogen-bonding network. Notably, solvents profoundly influenced this dynamic process. A strong correlation between the solvent donor number (DN) or solvent basicity (SB) parameters and the change in the Cotton effects pointed toward specific O-H...solvent interactions that drive structural unfolding and eventual refolding to apparently opposite helicity. This unusual chirality inversion process could also be induced by installation of chemical protecting groups that simulate specific solvent-solute interactions. Removal of this covalent mimic of the solvent shell restored the original screw sense of the parent molecule, thus establishing the feasibility of covalently triggered helicity inversion as a new mode of operation for chiroptical molecular switches.

A joint experimental and theoretical study of kinetic and mechanism of rearrangement of allyl p-tolyl ether

A joint theoretical and experimental study of the kinetic and mechanism of the rearrangement of allyl p-tolyl ether was performed in order to study the kinetic and mechanism of the reaction. Experimental studies were performed in gas phase over a temperature range of 493.15–533.15 K. The experimental Arrhenius parameters of this reaction were measured to be E a = 36.08 kcal mol −1, Δ S # = −7.88 cal mol −1 K −1, and Log A = 11.74, experimentally. Using GC for the mixture of the reaction with and without cyclohexene demonstrated that the reaction is clean without any radical intermediates. The experimental results show that the studied reaction is unimolecular and proceeds through a concerted pathway. Theoretical calculation were performed at RHF and B3LYP levels of theory using 6-31G ∗, 6-31++G ∗∗ and 6-311G ∗ basis sets. These calculations showed that the reaction proceeds through an asynchronous concerted mechanism. The calculated kinetics parameters especially at B3LYP/6-31G ∗ level of theory are in good agreement with the experimental data. Also polarizable continuum model was used to study solvent effect on the reaction rate. These calculations indicated that the rate enhancement of the reaction in polar and protic solvents arise from specific solute–solvent interactions such as hydrogen bonding and the hydrophobic effects of solvent.

Solvent and structural effects on the electro- and photo-reduction of cis - β -Co(trien)(RC6H4NH2)Cl]Cl2 type complexes in water/1,4-dioxane media

The electro- and photo-reduction of a series of cobalt(III) complexes of the type cis-β-[Co(trien)(RC6H4NH2)Cl]Cl2 where R=H, p-OMe, p-OEt, p-Me, p-Et, p-F and m-Me has been studied in 1,4-dioxane/water. The redox potential (E 1/2) and photo-reduction quantum yield (φCo(II)) data were correlated with solvent and structural parameters to shed light on the mechanism of these reactions. Correlation of E 1/2 and φCo(II) with macroscopic solvent parameters viz. relative permittivity indicates that the reactivity is influenced by both specific and non-specific solute-solvent interactions. The Kamlet-Taft solvatochromic comparison method was used to separate and quantify these effects. Increasing the percentage of organic co-solvent in the medium enhances both reduction processes; there exists good correlation between E 1/2 and φCo(II) suggesting a similar solvation in these redox processes.

Correlation Analysis of Reactivity in the Photo- and Electro-Reduction of Cobalt(III) Complexes in Binary Organic Solvent/Water Mixtures

The photo- and electro-reduction of a series of cobalt(III) complexes of the type cis-β - [Co(trien)(RC6H4NH2)Cl]Cl2 with R = H, p-OMe, p-OEt, p-Me, p-Et, p-F, and m-Me has been studied in binary propan-2-ol/water mixtures. The redox potential (E1/2) and photo-reduction quantum yield (ΦCo(II)) data were correlated with solvent and structural parameters with the aim to shed some light on the mechanism of these reactions. The correlation of E1/2 and ΦCo(II) with macroscopic solvent parameters, viz. relative permittivity, indicated that the reactivity is influenced by both specific and non-specific solute-solvent interactions. The Kamlet-Taft solvatochromic comparison method was used to separate and quantify these effects: An increase in the percentage of organic cosolvent in the medium enhances both reduction processes, and there exists a good linear correlation between E1/2 and ΦCo(II), suggesting a similar solvation of the participants in these redox processes.

A method for calculating the Gibbs energy of nonspecific solvation

A method for calculating the Gibbs energy of nonspecific solvation of nonelectrolytes was suggested. The new equation for the Gibbs energy of nonspecific solvation contains one solvent parameter that characterize nonspecific solvent-solute interactions and two experimental Gibbs energies of solvation in two standard solvents. The method is applicable to a wide range of solutes and solvents. It was successfully used to describe some 800 Gibbs energies of solvation for systems without specific solvent-solute interactions.

Photoisomerization of cyanine derivatives in 1-butyl-3-methylimidazolium hexafluorophosphate and aqueous glycerol: Influence of specific interactions

Photoisomerization of two cyanine derivatives, 3,3(')-diethyloxadicarbocyanine iodide (DODCI) and merocyanine 540 (MC 540), has been investigated in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate and aqueous glycerol (93 wt % glycerol +7 wt % water) by measuring fluorescence lifetimes and quantum yields. The aim of this work is to understand how the rates of photoisomerization of DODCI and MC 540 are influenced by specific solute-solvent interactions besides the viscosity of the medium. For DODCI, it has been observed that the nonradiative rate constants, which represent the rates of photoisomerization, are almost identical in the ionic liquid and aqueous glycerol at given temperature, indicating that viscosity is the sole parameter that governs the rate of photoisomerization. In contrast, the photoisomerization rate constants of MC 540 have been found to be a factor of 2 higher in aqueous glycerol compared to the ionic liquid. The observed behavior is due to the zwitterionic character of MC 540, a consequence of which, the twisted state gets stabilized by the solute-solvent hydrogen bonding interactions in aqueous glycerol, thus lowering the barrier for isomerization.