scispace - formally typeset
Search or ask a question

Showing papers on "Solvent effects published in 2001"


Journal ArticleDOI
11 Jan 2001-Nature
TL;DR: The technological utility of enzymes can be enhanced greatly by using them in organic solvents rather than their natural aqueous reaction media, and they have found numerous potential applications, some of which are already commercialized.
Abstract: The technological utility of enzymes can be enhanced greatly by using them in organic solvents rather than their natural aqueous reaction media. Studies over the past 15 years have revealed not only that this change in solvent is feasible, but also that in such seemingly hostile environments enzymes can catalyse reactions impossible in water, become more stable, and exhibit new behaviour such as 'molecular memory'. Of particular importance has been the discovery that enzymatic selectivity, including substrate, stereo-, regio- and chemoselectivity, can be markedly affected, and sometimes even inverted, by the solvent. Enzyme-catalysed reactions in organic solvents, and even in supercritical fluids and the gas phase, have found numerous potential applications, some of which are already commercialized.

1,926 citations


Journal ArticleDOI
TL;DR: In this paper, a procedure based on the polarizable continuum model (PCM) has been applied to reproduce solvent effects on electronic spectra in connection with the time-dependent density functional theory (TD-DFT).
Abstract: A procedure based on the polarizable continuum model (PCM) has been applied to reproduce solvent effects on electronic spectra in connection with the time-dependent density functional theory (TD-DFT). To account for solute-solvent interactions, a suitable operator has been defined, which depends on the solute electronic density and can be used to modify the TD-DFT equations for the calculation of molecular polarizabilities and of electronic transition energies. The solute-solvent operator has been derived from a PCM approach depending on solute electrostatic potential: Recently, it has been shown that such an approach also provides an excellent treatment of the solute electronic charge lying far from the nuclei, being particularly reliable for this kind of applications. The method has been tested for formaldehyde in water and in diethyl-ether, and then applied to the calculation of solvent effects on the n→π* transition of diazabenzenes in different solvents. The computed transition energies are in fairly...

1,829 citations


Journal ArticleDOI
TL;DR: In this article, the authors reviewed the kinetics of liquid-phase hydrogenation reactions with a special emphasis on α, β-unsaturated aldehydes as reactants and showed that surface coverage is independent of the liquid phase H2 concentration.
Abstract: The kinetics of liquid-phase hydrogenation reactions have been reviewed with a special emphasis on α, β-unsaturated aldehydes as reactants. These reactions can be complex and can be influenced by factors such as metal specificity, side reactions, and metal-support interactions as well as reaction parameters. The importance of results in the absence of heat and mass transfer limitations has been emphasized. Hydrogenation reactions are typically assumed to be structure-sensitive, but dependencies on metal crystallite size have been reported; however, this behavior has been attributed to side reactions which can inhibit activity. Finally, solvent effects can exist, but the effect of H2 concentration in the liquid phase has infrequently been isolated. Thermodynamic arguments indicate that a solvent effect can enhance the surface coverage of hydrogen on the catalyst surface at a constant H2 partial pressure, but in the absence of any solvent effects, surface coverage is independent of the liquid-phase H2 concentration.

311 citations


Journal ArticleDOI
TL;DR: From these studies, the association of electron-rich and electron-deficient aedamer monomers was seen to be driven primarily by hydrophobic interactions in polar solvents, but the magnitude of these interactions is modulated to a significant extent by the geometry of the donor--acceptor complex.
Abstract: One of the marquis challenges in modern Organic Chemistry concerns the design and synthesis of abiotic compounds that emulate the exquisite complex structures and/or functions of biological macromolecules. Oligomers possessing the propensity to adopt well-defined compact conformations, or foldamers, have been attained utilizing hydrogen bonding, torsional restriction, and solvophobic interactions.1 In this laboratory, aromatic electron donor−acceptor interactions have been exploited in the design of aedamersfoldamers that adopt a novel, pleated secondary structure in aqueous solution. Herein is reported detailed 1H NMR binding studies of aedamer monomers that were carried out in solvents and solvent mixtures covering a broad polarity range. Curve-fitting analysis of the binding data using a model that incorporated the formation of higher order and self-associated complexes yielded a linear free energy relationship between the free energy of complexation and the empirical solvent polarity parameter, ET(30)...

275 citations


Journal ArticleDOI
TL;DR: In this article, transient and steady state photoluminescence results along with absorption and NMR data were reported to support the existence of two distinct morphological species in MEH−PPV solutions.
Abstract: We report transient and steady state photoluminescence results along with absorption and NMR data to support the existence of two distinct morphological species in MEH−PPV solutions. NMR data provide evidence for the close packing of polymer chains, a consequence of solvent quality reduction. These data are correlated with optical properties of the aggregated species in poor solvents and the isolated chains in good solvents. We infer that steric hindrance of backbone motions increases effective conjugation length and leads to a spectral red shift in absorption and emission. At the same time, interchain excitations with negligible luminescence can be formed, leading to a dramatic reduction in photoluminescence quantum yield. While spectral changes are observed as packing is induced, we show that interchain state formation and its subsequent back-transfer to excitons are particularly sensitive to the interchain registry of the highly packed chains.

245 citations



Journal ArticleDOI
TL;DR: In this article, a general model for calculating the static dielectric constant of mixed-solvent electrolyte solutions is developed for mixtures of solvents without electrolyte components, based on an empirical modification of the Kirkwood theory for multicomponent systems.

234 citations


Journal ArticleDOI
TL;DR: Abraham et al. as mentioned in this paper derived room-temperature constants for hydrogen-atom abstraction in a number of solvents, where XH refers to several substituted phenols, tert-butyl hydroperoxide or aniline and Y• is a tert-alkoxyl radical.
Abstract: The rate of hydrogen-atom abstraction from XH by a radical, Y•, can be solvent-dependent. In many cases, the kinetic solvent effect (KSE) is directly related to hydrogen-bonding interactions between XH and the solvent. The relative hydrogen-bond acceptor (HBA) properties of solvents are given by constants of Abraham et al. (Abraham, M. H.; Grellier, P. L.; Prior, D. V.; Morris, J. J.; Taylor, P. J. J. Chem. Soc. Perkin Trans. 2 1990, 521−529). Room-temperature rate constants for hydrogen-atom abstraction, , have been determined in a number of solvents, S, where XH refers to several substituted phenols, tert-butyl hydroperoxide or aniline and Y• is a tert-alkoxyl radical. In all cases, plots of log( /M-1 s-1) versus gave excellent linear correlations, the slopes of which, MXH, were found to be proportional to the hydrogen-bond-donating (HBD) ability of XH, as scaled with parameters of Abraham et al. (Abraham, M. H.; Grellier, P. L.; Prior, D. V.; Duce, P. P.; Morris, J. J.; Taylor, P. J. J. Chem. Soc., Per...

212 citations


Journal ArticleDOI
TL;DR: In this paper, the thermal and photochemical ring opening and ring closure of spiropyran/merocyanine couple of three nitrogen-substituted SPs were studied and the results were compared with those of other 6-NO2====== spirobenzopyranindolines (BIPS) (1-5).
Abstract: The thermal and photochemical ring opening and ring closure of the spiropyran/merocyanine couple of three nitro-substituted spiropyrans (6-NO2: 6, 7 and 8-NO2: 8) were studied and the results were compared with those of other 6-NO2 spirobenzopyranindolines (BIPS) (1–5). The photocolouration, which occurs in the triplet manifold throughout, and the photochemical conversion and thermal relaxation of the two observable photomerocyanines into the closed spiropyran (Sp) form (decolouration) were quantified as a function of solvent polarity and temperature. The relaxation time (τt-Sp) at 25°C ranges from 2 s for 5 in methylcyclohexane to 104 s for 7 in ethanol. This large variation in τt-Sp is due to changes in both the activation energy (Ea = 75–105 kJ mol−1, increasing with polarity) and the pre-exponential factor (A = 1012–1015 s−1). The quantum yield of colouration with 308 nm pulses is substantial in solvents of low polarity (Φcol = 0.3–0.8) and decreases (<0.2) with increasing polarity. The observed merocyanine triplet state is rather short-lived (<10 μs) and the precursor of the trans- and a cis-merocyanine. The same intermediates, the triplet state and the cis-isomer, were observed when the trans-merocyanine was excited at 530 nm. The thermal cis → trans isomerization takes place in the μs–ms range and has an activation energy of Ec → t = 30–40 kJ mol−1. The effects of substituents and medium properties are described and the mechanism of photochromism is discussed.

173 citations


Journal ArticleDOI
TL;DR: Solvent effect studies show that the optical transition energy (E(op) does not behave as dielectric continuum theory predicts but that solvent reorganization energy may be usefully separated from the vibrational reorganizationEnergy by including linear terms in both the Pekar factor (gamma) and the Gutmann donor number (DN) in correlating the solvent effect.
Abstract: Electron transfer parameters are extracted from the optical spectra of intervalence bis(hydrazine) radical cations. Compounds with 2-tert-butyl-3-phenyl-2,3-diazabicyclo[2.2.2]octyl-containing charge-bearing units that are doubly linked by 4-σ-bond and by 6-σ-bond saturated bridges are compared with ones having tert-butylisopropyl- and diphenyl-substituted charge bearing units and others having the aromatic units functioning as the bridge. Solvent effect studies show that the optical transition energy (Eop) does not behave as dielectric continuum theory predicts but that solvent reorganization energy may be usefully separated from the vibrational reorganization energy by including linear terms in both the Pekar factor (γ) and the Gutmann donor number (DN) in correlating the solvent effect. Solvation of the bridge for these compounds is too large to ignore, which makes dielectric continuum theory fail to properly predict solvent effects on either Eop or the free energy for comproportionation.

154 citations


Journal ArticleDOI
TL;DR: In this article, the results of variations in the bulk solvent type and the addition of cosolvents on the growth rate of metallic copper nanoparticles produced through a reduction reaction within AOT reverse micelles were discussed in terms of an absorbance ratio, which provides a correlation between UV−vis absorption spectra and particle size.
Abstract: In this paper, we present the results of variations in the bulk solvent type and the addition of cosolvents on the growth rate of metallic copper nanoparticles produced through a reduction reaction within AOT reverse micelles. The metallic copper particles were characterized using both UV−vis spectroscopy and TEM microscopy. Growth rates are discussed in terms of an absorbance ratio, which provides a correlation between UV−vis absorption spectra and particle size. Time-resolved measurements showed that the intermicellar exchange rate and subsequent particle growth rate is a function of water content, bulk solvent, and the addition of cosolvents and cosurfactants. The copper particle growth rate was found to increase with increasing water content, but essentially the same particle size was eventually approached at all water contents. Copper particle growth was faster in isooctane than in cyclohexane because of the structure of the bulk solvent and the resulting degree of solvation into the micellar tail re...

Journal ArticleDOI
TL;DR: In this paper, the effect of the solvent media on the cyclohexane oxidation rate and reaction selectivity was investigated in various solvents, including acetone, isopropanol, dichloromethane, chloroform, carbon tetrachloride, benzene, and n-hexane.
Abstract: The photo-oxidation of cyclohexane on titanium dioxide was investigated in neat cyclohexane and in various solvents to determine the effect of the solvent media on the cyclohexane oxidation rate and reaction selectivity to cyclohexanol and cyclohexanone. The solvents that were used in this study include acetone, isopropanol, dichloromethane, chloroform, carbon tetrachloride, benzene, and n-hexane. It was found that the reaction rate and selectivity to the formation of cyclohexanol and cyclohexanone in various solvents depend upon the relative strengths of adsorption of cyclohexane, cyclohexanone, cyclohexanol, the solvent, and the partially oxidized solvent species on the titanium dioxide particles. In non-polar solvents, cyclohexanol preferentially adsorbed onto the titanium dioxide particles and underwent deep oxidation, ultimately to carbon dioxide and water. Therefore, in non-polar solvents, the selectivity of the reaction to cyclohexanol was very low. However, in polar solvents, cyclohexanol adsorbed to the titanium dioxide particles to a lesser extent due to the competition for adsorption sites with the solvent, and the selectivity of the reaction to cyclohexanol significantly increased. Competitive adsorption, in part, determined the overall rate of cyclohexane oxidation and selectivity to the desired products, cyclohexanone and cyclohexanol. The highest product-formation rate in this study was observed in dichloromethane, whereas chloroform and isopropanol significantly inhibited the desired reactions. The ideal solvent for the photo-oxidation of cyclohexane is one that minimizes the strengths of adsorption of the desired products on titanium dioxide and either does not compete with cyclohexane and oxygen for adsorption sites or is strongly adsorbed but is non-reactive with itself upon forming a radical on the illuminated titanium dioxide surface.

Journal ArticleDOI
TL;DR: Solvent effects were shown to give the main contribution to the observed structure variations between the uranium(VI) tetrahydroxide and the tetrafluoride complexes.
Abstract: The structures of the complexes UO2Fn(H2O)5-n2-n, n = 3−5, have been studied by EXAFS. All have pentagonal bipyramid geometry with U−F of and U−H2O distances equal to 2.26 and 2.48 A, respectively. On the other hand the complex UO2(OH)42- has a square bipyramid geometry both in the solid state and in solution. The structures of hydroxide and fluoride complexes have also been investigated with wave function based and DFT methods in order to explore the possible reasons for the observed structural differences. These studies include models that describe the solvent by using a discrete second coordination sphere, a model with a spherical, or shape-adapted cavity in a conductor-like polarizable continuum medium (CPCM), or a combination of the two. Solvent effects were shown to give the main contribution to the observed structure variations between the uranium(VI) tetrahydroxide and the tetrafluoride complexes. Without a solvent model both UO2(OH)4(H2O)2- and UO2F4(H2O)2- have the same square bipyramid geometry...

Journal ArticleDOI
TL;DR: The effects of polar (acetonitrile and tert-butyl alcohol) and apolar (cyclohexane) solvents on the peroxyl-radical-trapping antioxidant activity of some flavonoids, catechol derivatives, hydroquinone, and monophenols have been studied.
Abstract: The effects of polar (acetonitrile and tert-butyl alcohol) and apolar (cyclohexane) solvents on the peroxyl-radical-trapping antioxidant activity of some flavonoids, catechol derivatives, hydroquinone, and monophenols have been studied. The inhibition rate constants kinh of the antioxidants have been determined by following the increase in absorbance at 234 nm of a dilute solution of linoleic acid at 50 °C containing small amounts of antioxidant and radical initiator. Despite the low concentration of linoleic acid, the peroxidation process has been confirmed to be a free radical chain reaction described by the classical kinetic laws for this process. However, in the evaluation of kinh, a careful analysis of the peroxidation curve, absorbance versus time, must be done because the final oxidation products of phenols may absorb at 234 nm. Phenols with two ortho-hydroxyls are the most active antioxidants, with inhibition rate constants in the range of (3−15) × 105 M-1·s-1 (in cyclohexane). Nevertheless, it ha...

Journal ArticleDOI
TL;DR: In this article, the authors compared the properties of dendrimers with respect to the radius of gyration (Rg) and the hydrodynamic radius (Rh), and showed that the latter is more sensitive to the solvent properties.
Abstract: Controlling the molecular architecture of dendrimers is crucial for their intended use in applications such as drug delivery, biocides, gene transfer, catalyst supports, and processing aids (see refs 1-3). The molecular conformation in solution and the melt, which will control efficacy in such applications, has been the subject of theory,4,5 molecular,6 and coarse-grained simulations7-11 and extensive experimentation by X-ray, light and neutron scattering,12-21 and rheology.12,13,22-27 Of fundamental interest is the location of the terminal groups and molecular density profile as a function of the generation number. Direct, quantitative comparison of theory, experiment, and simulation is often complicated by the various levels of approximation used in the modeling and the need to accurately specify the physical properties of chemical groups and their interactions with the solvent. However, one useful, quantitative indicator of molecular conformation is the ratio of the radius of gyration (Rg) to the viscosimetric radius (Rη) or the hydrodynamic radius (Rh). These ratios can be computed from the molecular architecture without specifying the chemical details and depend primarily on the mass distribution. As such, they are an indication of overall macromolecular geometry and hydrodynamic draining. The radius of gyration is typically calculated from static scattering measurements, while the hydrodynamic radius (or equivalent sphere radius) is determined by dynamic light scattering or other diffusion measurements. For reference, a solid sphere yields a value of Rg/Rh ) Rg/Rη ) x(3/5) ≈ 0.77. Core weighting of the density distribution decreases these ratios, and values much higher (∼1.22 ln(L/D), with L/D the aspect ratio) can be achieved by a slender-rod geometry. Larger ratios are generally observed for polymers in solution; the experimental results of Schmidt and Burchard28 for linear polystyrene and poly(methyl methacrylate) in Θ solvents yield ratios of Rg/Rh ) 1.27 and 1.16, respectively. The viscosimetric radius is also an effective hydrodynamic sphere radius but is determined from intrinsic viscosity measurements. Although equal for hard spheres, Rη is generally different from Rh due to the different flow fields generated during the two measurements. The ratio Rg/Rη is inversely related to the Flory-Fox parameter and can be thought of as a measure of how well the molecule is drained by the solvent. Molecules that are easily drained have small viscosimetric radii and Flory-Fox parameters; thus, Rg/Rη becomes large. Using relationships in Flory,29 the ratio Rg/Rη is calculated as 1.17 for a Gaussian coil in a Θ solvent, based on experimental data for several linear polymers. For highly branched molecules the density of segments is higher than for linear polymers.30 This results in a larger Flory-Fox parameter (larger Rη) and a smaller value of Rg/Rη. It is expected that these results also depend on the solvent quality; collapsing the polymer in poor solvents should reduce the value toward that of a solid sphere, while expansion of the polymer in good solvents is expected to increase the ratio. It is clear that these simple comparisons of measurable molecular dimensions can provide valuable information about the conformation of polymers in solution. Here, we extend this idea to explore the effects of extreme branching on polymer conformation, where monodisperse dendrimers of varying generation provide clean model systems for investigation. To begin, we analyze the available experimental and theoretical data in the literature. The two most heavily studied systems are poly(propyleneimine) (PPI) and poly(amidoamine) (PAMAM) dendrimers. We then present new smallangle neutron scattering data for several generations of poly(benzyl ether) monodendrons in a good solvent. Comparing this information with previously published intrinsic viscosity results sheds light on the effect of dendritic architecture, solvent quality, and steric crowding on molecular conformation. The new results and the literature results are compared with recent theoretical calculations and simulations of these geometric properties to assess their validity.

Journal ArticleDOI
TL;DR: In this paper, the resonance Raman intensities of p-nitroaniline, a prototypical push-pull chromophore with a large first hyperpolarizability (β), have been measured in dilute solution in five solvents having a wide range of polarities (cyclohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol) at excitation wavelengths spanning the strong near-ultraviolet charge-transfer absorption band.
Abstract: Resonance Raman intensities of p-nitroaniline, a prototypical “push–pull” chromophore with a large first hyperpolarizability (β), have been measured in dilute solution in five solvents having a wide range of polarities (cyclohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol) at excitation wavelengths spanning the strong near-ultraviolet charge-transfer absorption band. The absolute Raman excitation profiles and absorption spectra are simulated using time-dependent wave packet propagation techniques to determine the excited-state geometry changes along the five or six principal Raman-active vibrations as well as estimates of the solvent reorganization energies. The total vibrational reorganization energy decreases and the solvent reorganization energy increases with increasing solvent polarity in all solvents except methanol, where specific hydrogen-bonding interactions may be important. The dimensionless normal coordinate geometry changes obtained from the resonance Raman analysis are conve...

Journal ArticleDOI
TL;DR: This work has studied the binding of an octapeptide ligand to the murine MHC class I protein using both explicit solvent and implicit solvent models, and finds electrostatic interactions are found to enhance the binding affinity.
Abstract: Solvent effects play a crucial role in mediating the interactions between proteins and their ligands. Implicit solvent models offer some advantages for modeling these interactions, but they have not been parameterized on such complex problems, and therefore, it is not clear how reliable they are. We have studied the binding of an octapeptide ligand to the murine MHC class I protein using both explicit solvent and implicit solvent models. The solvation free energy calculations are more than 103 faster using the Surface Generalized Born implicit solvent model compared to FEP simulations with explicit solvent. For some of the electrostatic calculations needed to estimate the binding free energy, there is near quantitative agreement between the explicit and implicit solvent model results; overall, the qualitative trends in the binding predicted by the explicit solvent FEP simulations are reproduced by the implicit solvent model. With an appropriate choice of reference system based on the binding of the discharged ligand, electrostatic interactions are found to enhance the binding affinity because the favorable Coulomb interaction energy between the ligand and protein more than compensates for the unfavorable free energy cost of partially desolvating the ligand upon binding. Some of the effects of protein flexibility and thermal motions on charging the peptide in the solvated complex are also considered. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 591–607, 2001

Journal ArticleDOI
TL;DR: In this paper, the role of the external surface of TS-1 in the phenol hydroxylation was investigated by inertization of external surface, using cycles of lowtemperature chemical vapor deposition (CVD) of tetraethoxysilane followed by high-temperature calcination.

Journal ArticleDOI
TL;DR: In this article, the effects of solvents on nuclear magnetic shielding parameters derived from NMR spectroscopy were studied, focusing on a specific nucleus, nitrogen, in two molecular solutes, acetonitrile and pyridine, immersed in different solvent types.
Abstract: This paper presents a study on the effects of solvents on nuclear magnetic shielding parameters derived from NMR spectroscopy. In particular, the study focuses on a specific nucleus, nitrogen, in two molecular solutes, acetonitrile and pyridine, immersed in different solvents. Among the solvents, particular attention is devoted to chloroform; its specific characteristics (low polarity and proticity), in fact, make it a very challenging application for theoretical solvation models. Here, we exploit a coupling scheme of solute−solvent cluster structures generated through MD simulations and high-level quantum chemical calculations in which a continuum solvation model is also introduced. This scheme permits the study of the competitive effects due to short-range and highly directional H-bonds and to long-range electrostatic forces and of the way these two effects are taken into account through a discrete, a continuum, or a coupled description of the solvent. Natural bond analysis of computed results has been ...

Journal ArticleDOI
TL;DR: Calculations show that a polar solvent tends to lower the relative energies of the transition states in the bromine atom-transfer and radical addition steps, and the coordination of a carbonyl group to a proton in a protic solvent, like a Lewis acid, would increase the efficiency of the propagation.
Abstract: A mixture of ethyl bromoacetate and 1-octene was treated with triethylborane in water at ambient temperature to provide ethyl 4-bromodecanoate in good yield. The bromine atom-transfer radical addition in benzene was not satisfactory. The addition proceeded smoothly in polar solvents such as DMF and DMSO, protic solvents such as 2,2,2-trifluoroethanol and 1,1,1,3,3,3-hexafluoro-2-propanol, and aqueous media. Ab initio calculations were conducted to reveal the origin of the solvent effect of water in the addition reaction. The polar effect of solvents, which is judged by the dielectric constant, on the transition states in the bromine atom-transfer and radical addition steps is moderately important. Calculations show that a polar solvent tends to lower the relative energies of the transition states. The coordination of a carbonyl group to a proton in a protic solvent, like a Lewis acid, would also increase the efficiency of the propagation.

Journal ArticleDOI
TL;DR: Car-Parrinello molecular dynamics simulations were performed to model the combined effects of temperature (dynamic averaging) and solvent (charge polarization) and both effects are shown to be rather small, of the order of a few dozen ppm.
Abstract: Car-Parrinello molecular dynamics simulations were performed for [H2VO4], [VO2(OH2)4]+, and [VO(O2)2(OH2)]- in periodic boxes with 30, 28, and 29 water molecules, respectively, employing the BLYP density functional. On the timescale of the simulations, up to 2 ps, well-structured first solvation spheres are discernible for [H2VO4]- and [VO(O2)2(OH2)]- containing, on average, eight and ten water molecules, respectively. One of the four water molecules directly attached to the metal in [VO2(OH2)4]+ is only loosely bound, and the average coordination number of vanadium in aqueous VO2+ is between five and six. 51V chemical shifts were evaluated at the B3LYP level for representative snapshots along the trajectories, including the water molecules of the solvent by means of point charges. The resulting averaged delta(51V) values are proposed to model the combined effects of temperature (dynamic averaging) and solvent (charge polarization). Both effects are shown to be rather small, of the order of a few dozen ppm. The observed shielding of 51V in the bis(peroxo) complex with respect to the vanadate species is not reproduced computationally.

Journal ArticleDOI
TL;DR: In this paper, it was demonstrated that the length per monomer also depends on the solvent quality, i.e., the cylinders are shorter in a poor solvent as compared to a good solvent.
Abstract: Communication: Polymacromonomers with a main chain much larger than the side-chain length adopt the form of cylindrical brush polymers the contour length per mainchain monomer of which depends on the side-chain length. In the present investigation it is demonstrated that the length per monomer also depends on the solvent quality, i.e., the cylinders are shorter in a poor solvent as compared to a good solvent. It is argued that the repulsion of the side chains represents the extension force, which acts against the entropic contraction force of the main chain. Thus, cylindrical brushes may be suitable as responsive materials for sensors, actuators or soft machines.

Journal ArticleDOI
TL;DR: The origin of the C-N rotational barrier and its relation to the concept of amide "resonance" is examined and the calculated solution-phase barriers for the thioamides using the above solvents are in good agreement with the observed barriers.
Abstract: The solvent effect on the C−N rotational barriers of N,N-dimethylthioformamide (DMTF) and N,N-dimethylthioacetamide (DMTA) has been investigated using ab initio theory and NMR spectroscopy. Selective inversion recovery NMR experiments were used to measure rotational barriers in a series of solvents. These data are compared to ab initio results at the G2(MP2) theoretical level. The latter are corrected for large amplitude vibrational motions to give differences in free energy. The calculated gas phase barriers are in very good agreement with the experimental values. Solvation effects were calculated using reaction field theory. This approach has been found to give barriers that are in good agreement with experiment for many aprotic, nonaromatic solvents that do not engage in specific interactions with the solute molecules. The calculated solution-phase barriers for the thioamides using the above solvents are also in good agreement with the observed barriers. The solvent effect on the thioamide rotational b...

Journal ArticleDOI
TL;DR: In this paper, the effect of solvent on the collapse dynamics of polymers is studied using computer simulation, and two cases are investigated, one where the solvent is incorporated through a pairwise additive attraction between the polymer beads and a random force on each polymer bead, and another where the solvers are incorporated in an explicit fashion as a second component.
Abstract: The effect of solvent on the collapse dynamics of polymers is studied using computer simulation. Two cases are investigated, one where the solvent is incorporated through a pairwise additive attraction between the polymer beads and a random force on each polymer bead, and another where the solvent is incorporated in an explicit fashion as a second component. Brownian dynamics and molecular dynamics simulations are used in the former and latter model, respectively, with intermolecular interactions chosen so that the equilibrium size of the polymer is similar in both models at similar conditions. In the Brownian dynamics simulations, at short times local blobs of monomers are found separated by linear segments. With time the blobs grow in size and coalesce to form sausage like shapes. These sausages gradually become thicker and shorter until the final shape of a spherical globule is reached. The first stage is rapid whereas the second sausage-sphere stage is slow. In this stage the polymer often gets trapped in local minima and the change in size with time occurs through discrete jumps, and the equilibrium conformation is often not reached. In contrast, in the molecular dynamics simulations with explicit solvent, the size of the polymer changes smoothly with time, and the polymer does not get trapped in local minima for the cases investigated, although the sequence of polymer shapes is similar. This suggests that incorporating solvent molecules explicitly is important in the computer simulations of collapse and folding of polymers.

Journal ArticleDOI
TL;DR: The rate of oxygen depletion, as measured by electron spin resonance spectroscopy (oximetry using a spin probe), in a homogeneous solution of peroxidating methyl linoleate in the presence or absence of antioxidants was converted to second-order rate constants for the inhibiting reaction of quercetin and epicatechin.
Abstract: The rate of oxygen depletion, as measured by electron spin resonance spectroscopy (oximetry using a spin probe), in a homogeneous solution of peroxidating methyl linoleate (initiated by an azo initiator) in the presence or absence of antioxidants was converted to second-order rate constants for the inhibiting reaction of quercetin and epicatechin. In the non-hydrogen-bonding solvent chlorobenzene at 50 °C, kinh had values of 4.3 × 105 M-1 s-1 for quercetin and 4.2 × 105 M-1 s-1 for epicatechin, respectively. In the hydrogen-accepting “water-like” solvent tert-butyl alcohol, the values were 2.1 × 104 and 1.7 × 104 M-1 s-1, respectively. The solvent effect (factor of 20) is more significant than for α-tocopherol (factor of 4), and the two flavonoids have efficiencies comparable to that of α-tocopherol in scavenging peroxyl radicals in the nonpolar solvent but not in the hydrogen-bonding solvent. Keywords: Quercetin; epicatechin; antioxidant mechanism; hydrogen atom transfer; solvent effect

Journal ArticleDOI
TL;DR: In this article, the structural and electronic properties of a series of imidazoline derivatives R1-I-R2 have been studied by means of density functional theory, and the results indicate a high reactivity for both the tertiary and the iminic nitrogen atoms of the imininic ring.
Abstract: The structural and electronic properties of a series of imidazoline (I) derivatives R1-I-R2 have been studied by means of density functional theory. The chosen compounds have different chain lengths (R1) and pendant (R2) groups, where R1=CH3, (CH2)nCH3 (n=2,4,…,16) and R2=H, CH3, C2H5, C3H7, C2H5OH, C2H5NH2, C2H5SH, C2H5COOH, and C2H5Cl. All-electron calculations were carried out by means of the Gaussian-98 program. Natural and Mulliken population analysis were determined for each of the neutral and charged molecules. The reactivity was analyzed in terms of the Fukui indices. The results indicate a high reactivity for both the tertiary (showing the biggest) and the iminic nitrogen atoms of the imidazoline ring. Although the N site of the pending groups has a much lower reactivity, this is significantly increased when a sulfur atom replaces it. In general, the reactivity of the N atoms of the ring depends significantly on the substituents. As expected, the frontier molecular orbitals, highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), also describe the reactivity behavior. As the size of the R1 chain increases, the two N atoms of the ring increase their nucleophilic character, since the HOMOs become more highly located. The obtained results help to explain the observed increased activity of these derivatives when they are used as inhibitors of corrosion moieties on metallic surfaces. Finally, the solvent effects on the ground-state geometry were studied for one of the most promising reactive imidazolines; for which its interaction with an iron oxide surface was also addressed. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001


Journal ArticleDOI
TL;DR: In this article, the microscopic solvent structure of dimethyl sulfoxide-water mixtures and its influence on the solvation structure of solute from a clustering point of view, by means of a specially designed mass spectrometric system, was studied.
Abstract: We have studied the microscopic solvent structure of dimethyl sulfoxide-water mixtures and its influence on the solvation structure of solute from a clustering point of View, by means of a specially designed mass spectrometric system. It was observed that the propensity to the cluster formation is nonlinearly dependent on the solvent composition, exhibiting the existence of a critical value of mixing ratios where drastic changes in the microscopic solvent structure occur. It was also demonstrated that in such a solvent mixture the solvation structure of solutes such as 2-butanol, cyclopentanol, cyclohexanol, and phenol is greatly related to the microscopic solvent structures, implying that solute species interact with already established solvent clusters, rather than with individual solvent molecules.

Journal ArticleDOI
TL;DR: In this article, the average potential energy of the solvent molecules near the surfaces was found to be higher than in the solvent bulk, which indicated that the surface induced potential energy change might be an adequate solvent effect parameter to use in a prediction of the morphological importance of a crystal surface.

Journal ArticleDOI
29 Jun 2001-Langmuir
TL;DR: Enthalpy et al. as discussed by the authors found that aqueous solution of β- or γ-cyclodextrin can form polypseudorotaxanes upon threading of linear hydrophobic polymers such as poly(propylene glycol) bis-2-aminopropyl ether, H3CCH(NH2)CH2(OCH2CH(CH3))nNH2 (PPG-Am2; MW ≈ 2000; nav ≈ 33), and pluronic 105.
Abstract: Aqueous solutions of β- or γ-cyclodextrin were found to form polypseudorotaxanes upon threading of linear hydrophobic polymers such as poly(propylene glycol) bis-2-aminopropyl ether, H3CCH(NH2)CH2(OCH2CH(CH3))nNH2 (PPG-Am2; MW ≈ 2000; nav ≈ 33), and pluronic 105, HO(CH2CH2O)34(CH2CH(CH3)O)61(CH2CH2O)34H (PLU; MW ≈ 6500). The kinetics of the threading process was determined as a function of the temperature and solvent composition (water, heavy water, and urea). When the water solution of cyclodextrin was added to the polymer's dispersion, a thick solid precipitate was promptly formed, depending on the concentration and temperature. Turbidity measurements allowed us to determine the time necessary for a complete threading of the linear polymer (the “threading time”); this parameter depends on the solution temperature and on the solvent nature. A simple kinetic model provides the activation free energy of the process, and the number of cyclodextrin molecules threaded around a single linear chain. Enthalpy, e...