Showing papers on "Solvent effects published in 2009"
TL;DR: This critical review provides an overview on the formation of pi-stacks of functional dyes in solution, aiming to acquaint young researchers with this topical research field and to stimulate further advance in supramolecular dye chemistry.
Abstract: This critical review provides an overview on the formation of π-stacks of functional dyes in solution, aiming to acquaint young researchers with this topical research field and to stimulate further advance in supramolecular dye chemistry. Different mathematical models that have been proposed and applied for the description of aggregation equilibria of π-systems in solution are discussed. The factors that have significant impact on the structural features of aggregates and the thermodynamics of π–π stacking such as electrostatic interactions, size and geometry of the dye molecules are covered in this review. A comparison of the binding strength is made for different classes of functional π-conjugated systems, from simple benzene to more extended polycyclic hydrocarbon molecules, including triphenylenes and hexabenzocoronenes, heteroaromatic porphyrins and phthalocyanines, quadrupolar naphthalene and perylene bisimides, dipolar or even zwitterionic merocyanines and squaraines, and some macrocyclic dyes. Solvent effects on binding constants are analysed by linear free energy relationships with various solvent polarity scales (98 references with multiple entries).
852 citations
TL;DR: The proposed methodology departs from the traditional single-parameter procedures for estimating nonspecific solvent effects by splitting them into a polarizability term and a dipolarity term, and is applied to understanding for the indole chromophore the feasible inversion of the electronic nature for the first electronic excited state due to solvent effects.
Abstract: This paper reports a methodology for analyzing the solvent effect from empirical measurements of solvent acidity (SA), basicity (SB), dipolarity (SdP), and polarizability (SP). The proposed methodology departs from the traditional single-parameter procedures for estimating nonspecific solvent effects by splitting them into a polarizability term and a dipolarity term. In this work, we examined the SA, SB, SP, and SdP values for 160 solvents, the gas phase (the absence of solvent) being the origin of these scales. As shown in this paper, this information allows one not only to accurately describe the solvent effect experienced by any solute—whether polar or nonpolar and exhibiting some or no specific interaction with the solvent—but also to understand the nature of the well-known solvent parameters ET(30), π*, S′, and SPP, which are frequently used to describe the overall nonspecific contribution of solvents in terms of a single parameter. The high potential of the proposed empirical methodology is illustra...
548 citations
TL;DR: In this article, the authors studied the impact of solvent choice on HMF yield using primary and secondary alcohols, ketones, and cyclic ethers in the C3-C6 range as extracting solvents in biphasic systems saturated with NaCl.
Abstract: Furan derivatives, such as 5-hydroxymethylfurfural (HMF), obtained from acid-catalyzed dehydration of carbohydrates, can serve as renewable chemical platforms for the production of fuels and chemical intermediates. Addition of an inorganic salt to concentrated aqueous solutions of fructose (30 wt% fructose on salt free basis) in biphasic systems containing an organic extracting phase improves HMF yields by increasing the partitioning of HMF into the extracting phase, as measured by the partition coefficient, R, equal to the concentration of HMF in the organic phase normalized by the concentration in the aqueous phase. We have studied the impact of solvent choice on HMF yield using primary and secondary alcohols, ketones, and cyclic ethers in the C3–C6 range as extracting solvents in biphasic systems saturated with NaCl. Biphasic systems containing C4 solvents generated the highest HMF yields within each solvent class. Tetrahydrofuran demonstrated the best combination of high HMF selectivity (83%) and high extracting power (R = 7.1) at 423 K. The presence of NaCl provided the additional benefit of creating biphasic systems using solvents that are completely miscible with water in the absence of salt. We have also studied the impact of different salts on HMF yield in systems using 1-butanol as the extracting solvent. Na+ and K+ showed the best combination of extracting power and HMF selectivity of the monovalent and divalent chloride salts tested. Changing the anion of the salt from Cl− to Br− resulted in R-values and HMF selectivity values resembling the non-salt system, while changing to the SO42− divalent species generated a high R-value (8.1), but a low HMF selectivity value (71%).
435 citations
222 citations
TL;DR: Quantum chemical calculations of various azobenzene (AB) derivatives have been carried out with the goal to describe the energetics and kinetics of their thermal cis --> trans isomerization, giving usually reliable activation energies and enthalpies when compared to experimentally determined values.
Abstract: Quantum chemical calculations of various azobenzene (AB) derivatives have been carried out with the goal to describe the energetics and kinetics of their thermal cis → trans isomerization. The effects of substituents, in particular their type, number, and positioning, on activation energies have been systematically studied with the ultimate goal to tailor the switching process. Trends observed for mono- and disubstituted species are discussed. A polarizable continuum model is used to study, in an approximate fashion, the cis → trans isomerization of azobenzenes in solution. The nature of the transition state(s) and its dependence on substituents and the environment is discussed. In particular for push−pull azobenzenes, the reaction mechanism is found to change from inversion in nonpolar solvents to rotation in polar solvents. Concerning kinetics, calculations based on the Eyring transition state theory give usually reliable activation energies and enthalpies when compared to experimentally determined valu...
212 citations
TL;DR: In this article, a thermodynamic scheme representing the atom transfer radical polymerization (ATRP) equilibrium as the formal sum of equilibria involving carbon−halogen bond homolysis and three additional distinct thermodynamic contributions related to the catalyst is rigorously evaluated.
Abstract: A thermodynamic scheme representing the atom transfer radical polymerization (ATRP) equilibrium as the formal sum of equilibria involving carbon−halogen bond homolysis and three additional distinct thermodynamic contributions related to the catalyst is rigorously evaluated. The reduction/oxidation of both the metal complex and the halogen atom, and the affinity of the higher oxidation state of the catalyst for halide anions (or “halidophilicity”), are measured. The validity and self-consistency of the model are verified by independently measuring, computing, or calculating the overall ATRP equilibrium constant and all four contributing equilibrium constants for one catalyst/alkyl halide combination in acetonitrile. As a thorough demonstration of the value and effectiveness of the scheme, the different equilibrium constants were measured or calculated in 11 different organic solvents, and a comparison of their values was used to both understand and predict catalyst activity in ATRP with high accuracy. The ...
196 citations
TL;DR: In this article, the disproportionation of colloidal Cu(I)X mediated by Me-6-TREN in various solvents was studied through UV-vis spectroscopy and Dynamic Light Scattering (DLS).
Abstract: Disproportionation of Cu(I)X is the major step in Single-Electron Transfer Living Radical Polymerization (SET-LRP). The disproportionation of Cu(I)X mediated by Me-6-TREN in various solvents was studied through UV-vis spectroscopy and Dynamic Light Scattering (DLS). UV-vis experiments reveal that disproportionation is dependent on both solvent composition and concentration of Me-6-TREN, consistent with a revised equilibrium expression and corroborated by mathematical models. Electrochemistry data do not accurately predict the extent of disproportionation in the presence of Me6-TREN. Exemplified by DMSO, a favored solvent for SET-LRP, LTV-vis spectroscopy shows that under certain conditions disproportionation is four-orders of magnitude greater than the value reported from electrochemistry experiments. Through LTV-vis and DLS analysis, it was demonstrated that DMSO, DMF, DMAC, and NMP, stabilize colloidal Cu(0), while acetone, EtOH, EC, MeOH, PC, and H2O facilitate agglomeration of Cu(0) particles. Additionally, for colloidal Cu(0) stabilizing solvents, the amount of ligand and solvent composition decide the particle size distribution. Therefore, the kinetics of SET-LRP are cooperatively and synergistically determined by the complex interplay of solvent polarity, the extent of disproportionation in the solvent/ligand mixture, and the ability of that mixture to stabilize colloidal Cu(0) or control particle size distribution. The implications of these results for SET-LRP are discussed. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47:5606-5628, 2009
187 citations
TL;DR: In this article, a library of solvents with different equilibrium constants for disproportionation of Cu(I)X such as DMSO, DMF, DMAC, EC, PC, EtOH, MeOH, methoxyethanol, NMP, acetone and in their binary mixtures with H 2 O was examined.
Abstract: SET-LRP is mediated by a combination of solvent and ligand that promotes disproportionation of Cu(I)X into Cu(0) and Cu(II)X 2 . Therefore, the diversity of solvents suitable for SET-LRP is limited. SET-LRP of MA in a library of solvents with different equilibrium constants for disproportionation of Cu(I)X such as DMSO, DMF, DMAC, EC, PC, EtOH, MeOH, methoxyethanol, NMP, acetone and in their binary mixtures with H 2 O was examined. H 2 O exhibits the highest equilibrium constant for disproportionation of Cu(I)X. The apparent rate constant of the polymerization exhibits a linear increase with the addition of H 2 O. This is consistent with higher equilibrium constants for disproportionation generated by addition of H 2 O to organic solvents. Furthermore, with the exception of alcohols and carbonates, the rate constant of polymerization in binary mixtures could be correlated with the Dimroth-Reichardt solvent polarity parameter. This is consistent with the single-electron transfer mechanism proposed for SET-LRP that involves a polar transition state. These experiments demonstrate that the use of binary mixtures of solvents with H 2 O provides a new, simple and efficient method for the elaboration of a large diversity of reaction media that are suitable for SET-LRP even when one of the two solvents does not mediate disproportionation of Cu(I)X.
177 citations
TL;DR: Nonstandard hybrid density functional calculations with 35% Hartree-Fock-like exchange combined with continuum solvent models are suggested as a practical protocol for the quantum-chemical characterization of organic mixed-valence systems and provides a basis for the evaluation of other computational methods.
Abstract: Structures, dipole moments, electron-transfer barriers, and spin density distributions of a series of mixed-valent bistriarylamin radical cations have been studied systematically by hybrid density functional methods with variable exact-exchange admixture combined with a continuum solvent model. The chosen systems differ in their bridging units and are all relatively close, from both sides, to the class II/III borderline of the Robin-Day classification of mixed-valence systems. Solvent effects are found to have a dramatic influence on the localized vs delocalized character of these cations. While gas-phase calculations or computations in a nonpolar solvent place all systems on the delocalized class III side, a more polar solvent like acetonitrile enables observation of symmetry breaking and charge localization with moderate exact-exchange admixtures in a hybrid functional for the systems on the class II side (with diphenylbutadiyne and diphenylethyne bridges). In contrast, the cations with the shortest bridges (phenylene, biphenylene) are characterized as class III. The comparison of computed intervalence charge-transfer excitation frequencies with experiment confirms the system with the diphenylbutadiyne bridge, and probably the system with the diphenylethyne bridge, to be class II, whereas in the dichloromethane solvent employed for spectroscopic measurements, the two other systems are on the class III side. Nonstandard hybrid density functional calculations with 35% Hartree-Fock-like exchange combined with continuum solvent models are suggested as a practical protocol for the quantum-chemical characterization of organic mixed-valence systems. This approach should allow closer examinations and provides a basis for the evaluation of other computational methods.
167 citations
TL;DR: In this article, the electronic parameters of plain 1,2,4-triazole and three isomeric forms of amino-1, 2, 4, 5 triazole have been studied at the B3LYP/6-31G(d,p) level of theory.
166 citations
TL;DR: In this article, the solvent-induced polymorphism, coadsorption effect, as well as solvent effects on chirality and electronic structures are discussed in view of the polarity, solubility and viscosity of the solvent, the hydrogen bonding formation between solute and solvent, and the solvophobic and solvophilic effects.
Abstract: Self-assembly structures investigated by using scanning tunneling microscopy (STM) at liquid/solid interface have been a topic of broad interest in surface science, molecular materials, molecular electronics. The delicate balance among the adsorbate–solvent, adsorbate–adsorbate, solvent–solvent interactions would give rise to the coadsorption or competitive deposition of solvent with adsorbate. The solvents at the interface enable dynamic absorption and desorption of the adsorbates leading to the controlled assembly of the molecular architectures. The solvent-induced polymorphism, coadsorption effect, as well as solvent effects on chirality and electronic structures are discussed in this report in view of the polarity, solubility and viscosity of the solvent, the hydrogen bonding formation between solute and solvent, and the solvophobic and solvophilic effects. The systematic studies on the solvent effects would shed light on better control of assembly structures for design of new molecular materials and molecular electronics.
TL;DR: A triple fluorescent compound, N-salicylidene-3-hydroxy-4-(benzo[d]thiazol-2-yl)phenylamine (SalHBP), was dispersed in solid polymers and was developed as a white-light-emitting source in LED by using it as the first simple single compound with different configurations.
Abstract: A triple fluorescent compound, N-salicylidene-3-hydroxy-4-(benzo[d]thiazol-2-yl)phenylamine (SalHBP), was dispersed in solid polymers and was developed as a white-light-emitting source in LED by using it as the first simple single compound with different configurations The CIE coordinates were at (029, 035), close to those of pure white light To explore speciation mechanisms in this single compound white light, SalHBP was dissolved in protic, nonpolar, and moderate polar solvent, respectively Upon excitation, blue, green, and yellowish green emissions were observed from the three solutions at various temperatures The conformation of SalHBP at room temperature was described by a Car-Parrinello molecular dynamics simulation With the aid of hybrid density functional theory at the B3LYP/TZVP and PBE0/TZVP levels, three observed emission bands of SalHBP were assigned from the five most probable excited state conformations that were derived from four ground state conformations The effect of solvent on the emission of SalHBP was summarized as a possibility for forming intermolecular hydrogen bonds between solvent and SalHBP molecules and competition between intra- and intermolecular hydrogen bonds
TL;DR: In this article, the second-order rate constants k2 for the attack on the electrophiles by the amines were determined from the linear free energy relationship log(k2 (20 °C) = s(N + E).
TL;DR: In this article, it was shown that a similar effect is observed in binary mixtures of organic solvents, wherein the addition of a more polar solvent to a less polar solvent provides a linear increase in the apparent rate constant of propagation, k.
Abstract: SET-LRP requires a combination of ligand and solvent that mediates the disproportionation of Cu(I)X into Cu(O) activator, and Cu(II) deactivator. The solvent also modulates the kinetics of the reaction. More polar solvents, including mixtures of water and organic solvents enhance the rate of polymerization in accord with the Dimroth-Reichardt parameter. Here, it is demonstrated that a similar effect is observed in binary mixtures of organic solvents, wherein the addition of a more polar solvent to a less polar solvent provides a linear increase in the apparent rate constant of propagation, k . However, this linear relationship does not hold for the entire range of volume fraction for binary mixtures when ethylene carbonate (EC) or MeOH are one of the two components. Results herein, suggest that the kinetics of SET-LRP in these solvent mixtures is cooperatively and synergistically determined by polarity, degree of disproportionation, and also by another parameter related to the ability of the solvent to stabilize colloidal Cu(0) and determine its particle size.
TL;DR: Theoretical work was essentially focused on the evaluation of the Nonrandom Two-Liquid Segment Activity Coefficient (NRTL-SAC) model, which has been referred as a simple and practical thermodynamic framework for drug solubility estimation.
TL;DR: It is shown that the alkyl chain length of the cations and type of anion affect the ionic association constants and limiting molar conductivities of the ILs, and similar to the classical electrolytes, a linear relationship is observed between ln KA of theILs and the reciprocal of the dielectric constants of the molecular solvents.
Abstract: Considering the ionic nature of ionic liquids (ILs), ionic association is expected to be essential in solutions of ILs and to have an important influence on their applications. Although numerous studies have been reported for the ionic association behavior of ILs in solution, quantitative results are quite scarce. Herein, the conductivities of the ILs [Cnmim]Br (n=4, 6, 8, 10, 12), [C4mim][BF4], and [C4mim][PF6] in various molecular solvents (water, methanol, 1-propanol, 1-pentanol, acetonitrile, and acetone) are determined at 298.15 K as a function of IL concentration. The conductance data are analyzed by the Lee-Wheaton conductivity equation in terms of the ionic association constant (KA) and the limiting molar conductance (Lambda(m)(0)). Combined with the values for the Br- anion reported in the literature, the limiting molar conductivities and the transference numbers of the cations and [BF4]- and [PF6]- anions are calculated in the molecular solvents. It is shown that the alkyl chain length of the cations and type of anion affect the ionic association constants and limiting molar conductivities of the ILs. For a given anion (Br-), the Lambda(m)(0) values decrease with increasing alkyl chain length of the cations in all the molecular solvents, whereas the KA values of the ILs decrease in organic solvents but increase in water as the alkyl chain length of the cations increases. For the [C4mim]+ cation, the limiting molar conductivities of the ILs decrease in the order Br- > [BF4]- > [PF6]-, and their ionic association constants follow the order [BF4]- > [PF6]- > Br- in water, acetone, and acetonitrile. Furthermore, and similar to the classical electrolytes, a linear relationship is observed between ln KA of the ILs and the reciprocal of the dielectric constants of the molecular solvents. The ILs are solvated to a different extent by the molecular solvents, and ionic association is affected significantly by ionic solvation. This information is expected to be useful for the modulation of the IL conductance by the alkyl chain length of the cations, type of anion, and physical properties of the molecular solvents.
TL;DR: The flexible, electropositive cavity of linear 1,4-diaryl-1,2,3-triazole oligomers provides a suitable host for complexation of various anions, and a picture emerges in which anion binding can be effectively interpreted in terms of a competition between two solvation spheres.
Abstract: The flexible, electropositive cavity of linear 1,4-diaryl-1,2,3-triazole oligomers provides a suitable host for complexation of various anions. The binding affinities for various combinations of oligomer and anion were determined by (1)H NMR titrations. Effective ionic radius is found to be a primary determinant of the relative binding interactions of various guests, with small but measurable deviations in the case of nonspherical anions. Solvent effects are significant, and the strength of the binding interaction is found to depend directly on the donor ability of the solvent. A picture emerges in which anion binding can be effectively interpreted in terms of a competition between two solvation spheres: one provided by the solvent and a second dominated by a folded cavity lined with electropositive 1,2,3-triazole CH protons. Implications for rigid macrocycles and other multivalent hosts are discussed.
TL;DR: In this paper, the concept of cooperative dehydration was applied to study the collapse of a poly(N-isopropylacrylamide) (PNIPAM) chain upon heating in aqueous solutions.
Abstract: The concept of cooperative dehydration, defined as the simultaneous dissociation of the water molecules bound in correlated sequences to a polymer chain, has been applied to study the collapse of a poly(N-isopropylacrylamide) (PNIPAM) chain upon heating in aqueous solutions. We examined the applicability of this concept in three situations: (i) PNIPAM in water (in the absence of added force), (ii) PNIPAM in water subjected to a tension applied to the chain ends, and (iii) PNIPAM in a mixed solvent of water and a second water-miscible solvent. The transition becomes sharper as the cooperativity parameter of hydration increases. The tension−elongation curve of a hydrated chain at various temperatures, calculated following an approach similar to the classical theory of coil−helix transition, presents a flat plateau corresponding to the tension for which collapsed segments reel out of the globules. The reeled-out segments are hydrated immediately upon exposure to water. The calculations suggest a possible shi...
TL;DR: The influence of the reaction medium (organic solvents, water, ionic liquids, supercritical CO(2) ) on the propagation rate in radical polymerizations has very different causes, e.g., hindered rotational modes, hydrogen bonding or electron pair donor/acceptor interactions.
Abstract: The influence of the reaction medium (organic solvents, water, ionic liquids, supercritical CO(2) ) on the propagation rate in radical polymerizations has very different causes, e.g., hindered rotational modes, hydrogen bonding or electron pair donor/acceptor interactions. Depending on the origin of the solvent influence propagation rate coefficients, k(p) , may be enhanced by up to an order of magnitude associated with changes in the pre-exponential or the activation energy of k(p) . In contrast, non-specific interactions, size and steric effects lead to rather small changes in the vicinity of the radical chain end and are reflected by modest variations in k(p) .
TL;DR: Measured TPA cross-sections of rhodamine dyes are consistently higher in nonpolar solvents, which could be a result of vibronic couplings that depend on solvent environment.
Abstract: For a series of rhodamine dyes, two-photon absorption (TPA) and two-photon fluorescence (TPF) have been performed in different solvents. Solvent-dependent TPA spectra of these dyes were measured with open aperture z-scan method and compared to their respective single-photon spectra at equivalent energies. In the TPA spectra, relative peak intensities and positions are highly solvent dependent, which could be a result of vibronic couplings that depend on solvent environment. Measured TPA cross-sections of rhodamine dyes are consistently higher in nonpolar solvents. Certain complementary and similarity between TPA and TPF are also elucidated. Finally, a two-photon figure-of-merit is presented for these dyes in different solvents as a function of wavelength.
TL;DR: It is shown that the conformational properties of the all-atom dipeptide in explicit solvent can be accurately reproduced with the CG model, and preliminary simulations of peptide self-assembly performed with theCG model illustrate good agreement with results obtained from all- atom, explicit solvent simulations.
Abstract: We discuss the development of a coarse-grained (CG) model for molecular dynamics (MD) simulation of a hydrophobic dipeptide, diphenylalanine, in aqueous solution. The peptide backbone is described with two CG beads per amino acid, the side groups and charged end groups are each described with one CG bead. In the derivation of interaction functions between CG beads we follow a bottom-up strategy where we devise potentials such that the resulting CG simulation reproduces the conformational sampling and the intermolecular interactions observed in an atomistic simulation of the same peptide. In the CG model, conformational flexibility of the peptide is accounted for through a set of intra-molecular (bonded) potentials. The approach followed to obtain the bonded potentials is discussed in detail. The CG potentials for nonbonded interactions are based on potentials of mean force obtained by atomistic simulations in aqueous solution. Following this approach, solvent mediation effects are included in the effective bead-bead nonbonded interactions and computationally very efficient (solvent-free) simulations of self-assembly processes can be performed. We show that the conformational properties of the all-atom dipeptide in explicit solvent can be accurately reproduced with the CG model. Moreover, preliminary simulations of peptide self-assembly performed with the CG model illustrate good agreement with results obtained from all-atom, explicit solvent simulations.
TL;DR: For a fast exploration, design, and screening of effective solvents it is highly desirable to adopt a general-purpose thermodynamic approach that computes the chemical potential of a solute (CO2 in this case) in any solvent at an arbitrary dilution.
Abstract: With global warming established as a critical problem, it is extremely important to cut down on emissions of CO2 into the atmosphere. Prior to sequestration, it is necessary to separate the CO2 from its emission source, for example, flue gas in a coal-fired power plant. The few coal plants with commercial CO2 capture capability all use processes based on chemical absorption with a monoethanolamine (MEA) solvent. Unfortunately, MEA is a nonselective solvent prone to degradation and equipment corrosion, and mandates large equipment sizes, thereby increasing costs. Ionic liquids (ILs) constitute an alternative solvent system and offer distinct advantages over traditional solvents such as MEA, some of which include: (1) high chemical stability; (2) low corrosion; (3) almost zero vapor pressure (i.e. , “green”) ; (4) supportable on membranes; and (5) a huge library of anion and cation choices, which can be potentially optimized for CO2 solubility and selectivity. Over the last few years several ILs have been experimentally demonstrated to be efficient solvents for CO2. [3–8] This data provides useful trends that can be used to optimize the choice of ILs for CO2 capture. However, each new experiment costs time and money and is often hindered because a specific IL may not be readily available. Thus, it is highly desirable to have a computational/theoretical tool that can quickly and accurately compute CO2 solubility in any solvent (as a function of pressure and temperature). Atomic-level simulations, either molecular dynamics or binding-energy calculations, can provide useful insights into the interactions of CO2 with the cation and the anion. However, an accurate computation of solubility in such complex fluids faces many challenges, including accurate force-field development, clever Monte Carlo moves, and very long simulation times required for good statistical averaging. For a fast exploration, design, and screening of effective solvents it is highly desirable to adopt a general-purpose thermodynamic approach that computes the chemical potential of a solute (CO2 in this case) in any solvent at an arbitrary dilution. A widely used method is the “conductor-like screening model for real solvents” (COSMO-RS), which uses the statistical distribution (histogram) of the surface charge density of individual molecules, called the s profile, to derive an expression for the ensemble-averaged Gibbs free energy of an interacting system of molecules (solute+ solvent at specified mole fractions) in the condensed (liquid) phase. From this a pseudochemical potential (m) of each species (i.e. , the Gibbs free energy per molecule without the mixing entropy contribution) is obtained. If the pseudo-chemical potential of a solute molecule at a temperature T in a solution containing a mole-fraction x of the solute is msolutionACHTUNGTRENNUNG(x, T), and that in the solute’s own liquid environment is mself(T), then assuming ideal mixing law we have under equilibrium (kB=Boltzmann constant):
TL;DR: It is concluded from kinetic data together with theoretically calculated OH bond dissociation enthalpies of phenols that the OH position played a crucial role in addition to the temperature and nature of the medium in determining the rate of the radical scavenging ability of polyphenols.
Abstract: In this work we have demonstrated the free radical scavenging ability of two-hydroxy (catechol, hydroquinone, resorcinol) and three-hydroxy (phloroglucinol, pyrogallol, 1,2,4-benzenetriol) phenols against the diphenylpicrylhydrazyl radical at various temperatures (15-40 degrees C) and in different solvent media. Kinetic measurements, made by the stopped-flow method, showed that the phenols with OH groups in the ortho positions have the largest rate coefficients compared to those with OH groups in the meta and para positions at all temperatures and in all solvent media. Among the ortho-structured phenols catechol, pyrogallol, and 1,2,4-benzenetriol, pyrogallol (three OH groups ortho to each other) had the greatest radical scavenging ability. This suggested that intramolecular hydrogen bonding in phenols controlled the rate of radical scavenging ability. The radical scavenging ability of phenols was fastest in methanol and slowest in THF, which emphasized the importance of the interactive behavior of the phenolic OH with the solvent. We concluded from our kinetic data together with our theoretically calculated OH bond dissociation enthalpies of phenols that the OH position played a crucial role in addition to the temperature and nature of the medium in determining the rate of the radical scavenging ability of polyphenols.
TL;DR: Spectral and photophysical properties of the alkaloid berberine were studied in solvents with different solvent parameters, using UV/Vis absorption, emission and excitation spectroscopy, suggesting that B is a molecule attractive as a probe for solvent polarity and hydrogen bonding properties.
Abstract: Spectral and photophysical properties of the alkaloid berberine (B) were studied in solvents with different solvent parameters, using UV/Vis absorption, emission and excitation spectroscopy. The absorption and emission maxima were found to be between 421–431 nm and 514–555 nm, respectively, leading to Stokes’ shifts between 4099 and 5735 cm−1. The fluorescence quantum yields varied between 10−2–10−4, depending on the solvent. Different solvent scales have been used to study the solvatochromism of B. Linear solvation energy relationships (LSER) proposed by Kamlet–Taft suggest that B is a molecule attractive as a probe for solvent polarity and hydrogen bonding properties.
TL;DR: In this paper, the effects of solvent properties on the morphology and dimensions of solidified particles were systematically studied, and the size of the solidified spherical particles with smooth surfaces reflected that of the initially formed liquid droplets, which could partially be estimated by theoretical equations developed for pure liquids.
Abstract: Electrospraying technology has been studied in many fields to produce particles of various substances from nanoscale to microscale sizes. Unlike pure liquids, droplets formed by electrospraying that are comprised of polymer solutions undergo additional solidification processes involving solvent evaporation, which primarily determine the particle size and morphology. Herein, the effects of the solvent properties on the morphology and dimensions of solidified particles were systematically studied. In general, the size of the solidified spherical particles with smooth surfaces reflected that of the initially formed liquid droplets, which could partially be estimated by theoretical equations developed for pure liquids. Particle sizes increased with an increase in polymer content and a decrease in the boiling point of the volatile solvent. Inhomogeneous drying processes related to phase separation or skin formation resulted in hollow, cuplike, and porous particle structures, with particle sizes and morphologies that were outside of the scope of the theoretical treatments. The selection of a proper solvent or solvent mixture seemed to be a convenient way to control the particle morphologies, such as hollow, cuplike, or porous structures. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009
TL;DR: It is shown that the polarity of the solvent influences the thermal stability of the gel, the hydrogen-bonding network, and finally the structure of gel network.
Abstract: The solvent effect on organogel formation in nitrobenzene and chlorobenzene using 1,2-O-(1-ethylpropylidene)-α-d-glucofuranose (1) as the gelator is presented. Fourier transform infrared (FTIR) spectroscopy revealed that hydrogen bonding between the molecules of gelator 1 is the main driving force for gelator self-aggregation. The gels are characterized by different hydrogen-bonding patterns, which are reflected in a different microstructure of the networks. The morphology of fibers of nitrobenzene organogel consists of straight, rod-like, and thinner fibers, in comparison to the elongated but generally not straight and thicker fibers in chlorobenzene organogel. The thermal stability of gels also differs, and the ΔH is equal to 50.1 and 65.0 kJ/mol for nitrobenzene and chlorobenzene gels, respectively. The properties of the gels reported here were compared to benzene and toluene gels of 1 presented in previous work and correlated with different solvent parameters: e, δ, and ET(30). We have shown that the ...
TL;DR: QM-MM calculations indicate that the greatly increased quenching found in water relative to dioxane can be attributed mainly to the larger fluctuations of the energy gap in water.
Abstract: Hybrid quantum mechanical/molecular mechanics (QM-MM) calculations [Callis and Liu, J. Phys. Chem. B 2004, 108, 4248-4259] make a strong case that the large variation in tryptophan (Trp) fluorescence yields in proteins is explained by ring-to-backbone amide electron transfer, as predicted decades ago. Quenching occurs in systems when the charge transfer (CT) state is brought below the fluorescing state (1L(a)) as a result of strong local electric fields. To further test this hypothesis, we have measured the fluorescence quantum yield in solvents of different polarity for the following systems: N-acetyl-L-tryptophanamide (NATA), an analogue for Trp in a protein; N-acetyl-L-tryptophan ethyl ester (NATE), wherein the Trp amide is replaced by an ester group, lowering the CT state energy; and 3-methylindole (3MI), a control wherein this quenching mechanism cannot take place. Experimental yields in water are 0.31, 0.13, and 0.057 for 3MI, NATA, and NATE, respectively, whereas, in the nonpolar aprotic solvent dioxane, all three have quantum yields near 0.35, indicating the absence of electron transfer. In alkyl alcohols the quantum yield for NATA and NATE is between that found for water and that found for dioxane, and it is surprisingly independent of chain length (varying from methanol to decanol), revealing that microscopic H-bonding, and not the bulk dielectric constant, dictates the electron transfer rate. QM-MM calculations indicate that, when averaged over the six rotamers, the greatly increased quenching found in water relative to dioxane can be attributed mainly to the larger fluctuations of the energy gap in water. These experiments and calculations are in complete accord with quenching by a solvent stabilized charge transfer from ring to amide state in proteins.
TL;DR: A pyrene-tagged ruthenium carbene 8 was synthesized and immobilized on single-walled carbon nanotubes (SWNTs) via pi-pi stacking, offering a new reversible immobilization model that can be controlled by reaction temperature in polar solvents, such as acetone.
Abstract: A pyrene-tagged ruthenium carbene 8 was synthesized and immobilized on single-walled carbon nanotubes (SWNTs) via π−π stacking. These π−π interactions were greatly affected by the reaction temperature and the solvent polarity, thus, offering a new reversible immobilization model that can be controlled by reaction temperature in polar solvents, such as acetone. SWNTs-supported ruthenium carbene 8 is a robust and recyclable catalyst system. Six to seven cycles were achieved for ring-closing metathesis of selected substrates. Importantly, after the complete loss of activity, the SWNTs can be easily recycled by washing with tetrahydrofuran. The recycled SWNTs can then be reloaded with 8. The reusability of the catalyst supported on recycled SWNTs is comparable with that supported on fresh SWNTs.
TL;DR: Several examples of non-linear Eyring plots in stereoselective nucleophilic additions, cycloadditions, photochemical and enzymatic reactions are reported, leading to the formulation of a hypothesis on the inversion temperature phenomenon as being due to an equilibrium between distinct solute-solvent clusters, which are the real reactive species in solution.
Abstract: Stereoselectivity is a major topic in organic synthesis. Intensive investigations into the role of solvents on diastereo- and enantioselective reactions, as well as temperature-dependent measurements of diastereomeric and enantiomeric ratios, have shed light on the existence of dynamic solvation effects. In this tutorial review, several examples of non-linear Eyring plots in stereoselective nucleophilic additions, cycloadditions, photochemical and enzymatic reactions are reported. Experimental data and spectroscopic analyses obtained in aliphatic and aromatic hydrocarbons, halohydrocarbons, ethers and mixtures lead to the formulation of a hypothesis on the inversion temperature phenomenon as being due to an equilibrium between distinct solute–solvent clusters, which are the real reactive species in solution.
TL;DR: The maximum protection strategy for the synthesis of human parathyroid hormone indicates that fully protected peptide segments in the form of Boc-peptide phenacyl (Pac) ester are relatively soluble in ordinary organic solvents such as DMF, NMP or DMSO, which are suitable for coupling segments.
Abstract: Our maximum protection strategy for the synthesis of human parathyroid hormone(1-84) indicates that fully protected peptide segments in the form of Boc-peptide phenacyl (Pac) ester are relatively soluble in ordinary organic solvents such as DMF, NMP or DMSO, which are suitable for coupling segments. However, about 1% of such segments synthesized were found to be insoluble even in the most polar solvent, DMSO. Thus, a more powerful solvent which can be used for their peptide synthesis was pursued. Among the solvent systems tested, a mixture of trifluoroethanol (TFE) or hexafluoroisopropanol (HFIP) and trichloromethane (TCM) or dichloromethane (DCM) was found to be most powerful for dissolving such sparingly-soluble protected peptides. These solvent systems were confirmed to be useful for the removal reaction of the carboxy-terminal Pac esters from the sparingly-soluble segments. They were then tested for the coupling reactions of fully protected Boc-peptides with other sparingly-soluble peptide esters. The TFE/TCM or TFE/DCM system was extremely useful for coupling segments without danger of racemization and of trifluoroester formation, if WSCI was used as the coupling reagent in the presence of 3,4-dihydro-3-hydroxy-4-oxo-1,2,3-benzotriazine (HOOBt).