Showing papers on "Solvent effects published in 2000"

Combined molecular mechanical and continuum solvent approach (MM-PBSA/GBSA) to predict ligand binding

Abstract: Significant progress has been achieved in computational methods to treat solvent effects in recent years. Among various techniques, the continuum solvent approach appears to be practically promising because it can be used to calculate reliable interaction and solvation energies in complex systems. A computational scanning mutagenesis method, one of such new approaches, has been recently developed. It combines the molecular mechanical and continuum solvent approaches and allows one to identify the `hotspots' in binding interfaces from a single trajectory of a wild type complex. Such techniques can be also used as a tool to optimize the interacting species for the binding, or as a ranking procedure in high throughput screening.

Solvent effect on vertical electronic transitions by the polarizable continuum model

Abstract: A recent extension of the polarizable continuum solvation model (PCM) to excited electronic states is applied to the study of solvent effects on electronic transitions, accounting for both electrostatic and nonelectrostatic solute–solvent interactions. A general formalism for nonequilibrium electrostatic solvation is developed, applicable to all PCM versions, and a recent procedure for the quantum-mechanical computation of dispersion and repulsion solute–solvent interactions is implemented and used for the first time in this context. The procedure is applied to the study of the n→π* transition of acetone in aqueous and nonaqueous solvents: nonequilibrium effects are very important in polar environments; also, the inclusion of dispersion and repulsion is mandatory to obtain the correct trend of the solvatochromic shifts. The effect of adding some explicit solvent molecules is also analyzed.

Phase Behavior of a Block Copolymer in Solvents of Varying Selectivity

Abstract: The effect of the thermodynamic selectivity of a solvent on the self-assembly of a styrene−isoprene (SI) diblock copolymer with block molecular weights of 1.1 × 104 and 2.1 × 104 g/mol is investigated. We explore the phase behavior from the melt state to dilute solution in solvents that are of varying selectivities for the two blocks. Bis(2-ethylhexyl) phthalate (DOP) is a neutral good solvent for SI. Di-n-butyl phthalate (DBP) and diethyl phthalate (DEP) are good solvents for PS but marginal and poor, respectively, for PI. Tetradecane (C14) is utilized as a complementary solvent that is good for PI but poor for PS. Small-angle X-ray scattering (SAXS) and static birefringence are used to locate and identify order−order (OOT) and order−disorder transitions (ODT). Dynamic and static light scattering were used to characterize the dilute solution micellar behavior. The neat polymer forms the gyroid (G1) morphology at low temperature, an OOT to hexagonal-packed cylinders (C1) occurs at 185 °C, and the ODT is l...

Theoretical Studies of the Properties and Solution Chemistry of AnO22+and AnO2+Aquo Complexes for An = U, Np, and Pu

Abstract: The structures and vibrational frequencies of UO2(H2O)52+, NpO2(H2O)52+, and PuO2(H2O)52+ corresponding to An(VI) oxidation states and UO2(H2O)5+, Np(H2O)5+, and Pu(H2O)5+ corresponding to An(V) have been calculated using density functional theory (DFT) and relativistic effective core potentials (RECPs) The resulting structures are compared to EXAFS solution studies, and the Raman and IR vibrational frequencies of the actinyl unit are compared to experimental studies in solution Free energies for reactions in solution are computed by combining thermodynamic free energies in the gas phase with a dielectric continuum model to treat solvent effects The hydrolysis reaction of UO2(H2O)52+ to form UO2(H2O)4(OH)+ and the reactions for removing or adding a water to the first shell in UO2(H2O)52+ are examined using this approach Multiplet and spin−orbit effects not included in a single-configuration DFT wave function are incorporated by model spin−orbit CI calculations PuO2q+ is used as a model for the aquo c

Origin of Graphite Exfoliation An Investigation of the Important Role of Solvent Cointercalation

Abstract: To elucidate the origin of graphite exfoliation, we have investigated the influence of various material parameters relevant to solvent co‐intercalation, such as the cation, the electrolytic solvents, and the structure of graphite, on the solvent decomposition behavior. By electrochemically probing changes in the electrode, we demonstrated that a large increase of surface area accompanies the decomposition of propylene carbonate (PC). Furthermore, such a change in surface area is dramatically amplified when is replaced by tetrabutylammonium ion. A slight structural modification of PC exerts a profound influence on the solvent decomposition behavior, as demonstrated with cis‐ and trans‐2,3‐butylene carbonate. These reaction behaviors are also altered significantly by the choice of graphite. Such an influence of graphite structure is particularly surprising for t‐BC electrolyte, in which SFG44 graphite undergoes extensive exfoliation, whereas SFG6 graphite and MCMB25 can be cycled reversibly. These results can be best explained by incorporating the co‐intercalation of cyclic carbonate as a critical process in the solid electrolyte interphase formation mechanism. © 2000 The Electrochemical Society. All rights reserved.

Powerful solvent effect of water in radical reaction: Triethylborane-induced atom-transfer radical cyclization in water

Abstract: Triethylborane-induced atom-transfer radical cyclization of iodo acetals and iodoacetates in water is described. Radical cyclization of iodo acetal proceeded smoothly both in aqueous methanol and in water. Atom-transfer radical cyclization of allyl iodoacetate (3a) is much more efficient in water than in benzene or hexane. For instance, treatment of 3a with triethylborane in benzene or hexane at room temperature did not yield the desired lactone. In contrast, 3a cyclized much more smoothly in water and yielded the corresponding γ-lactone in high yield. The remarkable solvent effect of water was observed in this reaction, although the medium effect is believed to be small in radical reactions. Powerful solvent effects also operate in the preparation of medium- and large-ring lactones. Water as a reaction solvent strikingly promoted the cyclization reaction of large-membered rings. Stirring a solution of 3,6-dioxa-8-nonenyl iodoacetate in water in the presence of triethylborane at 25 °C for 10 h provided a ...

Importance of gas-phase proton affinities in determining the electrospray ionization response for analytes and solvents.

Abstract: The effect of gas-phase proton transfer reactions on the mass spectral response of solvents and analytes with known gas-phase proton affinities was evaluated. Methanol, ethanol, propanol and water mixtures were employed to probe the effect of gas-phase proton transfer reactions on the abundance of protonated solvent ions. Ion-molecule reactions were carried out either in an atmospheric pressure electrospray ionization source or in the central quadrupole of a triple-quadrupole mass spectrometer. The introduction of solvent vapor with higher gas-phase proton affinity than the solvent being electrosprayed caused protons to transfer to the gas-phase solvent molecules. In mixed solvents, protonated solvent clusters of the solvent with higher gas-phase proton affinity dominated the resulting mass spectra. The effect of solvent gas-phase proton affinity on analyte response was also investigated, and the analyte response was suppressed or eliminated in solvents with gas-phase proton affinities higher than that of the analyte.

Gibbs energies of transfer of anions from water to mixed aqueous organic solvents.

Abstract: A large number of quantitative studies have been made of the Gibbs energy of transfer (the solvent medium effect) for cations transferring from water into mixed aqueous−organic solvent systems, mainly at 25 °C. Nevertheless, no systematic effort appears to have been made to compile and analyze these data, particularly for multivalent cations. A critical review of this information and its presentation in a manner that permits comparison of different cations in a given solvent mixture and of a given cation transferring into different mixtures is therefore of value.

Hydrogen Atom Abstraction Kinetics from Intramolecularly Hydrogen Bonded Ubiquinol-0 and Other (Poly)methoxy Phenols

Abstract: The effect of methoxy substitution on the abstraction of the phenolic hydrogen atom involved in intramolecular hydrogen bonding by tert-butoxyl and cumyloxyl radicals has been investigated by laser flash photolysis. Also transition state calculations for methoxyl radical and 2-methoxyphenol have been carried out by a density functional theory (DFT) method. Hydrogen atom abstraction is surprisingly easy from intramolecularly hydrogen bonded methoxyphenols, in contrast to intermolecularly hydrogen bonded molecules. The kinetic solvent effect, investigated in six solvents with different hydrogen bond accepting properties, on the hydrogen atom abstraction reaction from o-methoxy phenols was shown to be smaller than for non-hydrogen bonded phenols, and is independent of further methoxy substitution. The high rate constant for hydrogen atom abstraction from ubiquinol-0 (2.8 × 109 M-1 s-1 in CCl4) and the small kinetic solvent effect make it a good antioxidant, even in a polar environment.

Photoinduced Electron Transfer in Carotenoporphyrin−Fullerene Triads: Temperature and Solvent Effects

Abstract: A carotenoid (C) porphyrin (P) fullerene (C60) molecular triad (C−P−C60) has been synthesized and found to undergo photoinduced electron transfer from the porphyrin first excited singlet state or to the fullerene first excited singlet state to yield C−P•+−C60•-. Electron transfer from the carotenoid then gives a C•+−P−C60•- final charge-separated state. This state is formed with quantum yields up to 0.88 and has a lifetime of up to 1 μs, depending upon the conditions. The various electron transfer rate constants are relatively insensitive to solvent and temperature. The quantum yield of C•+−P−C60•- is relatively constant under conditions ranging from fluid solutions at ambient temperatures to a rigid organic glass at 8 K. In most solvents, recombination of C•+−P−C60•- yields the carotenoid triplet state, rather than the ground state. The results suggest that the energies of the charge-separated states of fullerene-based systems are only about half as sensitive to changes in solvent dielectric constant as ...

Tautomerism of 2-hydroxynaphthaldehyde Schiff bases

Abstract: A UV–Vis spectroscopic study based on the recently developed chemometric approach for quantitative analysis of undefined mixtures is performed on a series of donor and acceptor substituted Schiff bases of 2-hydroxynaphthaldehydes. In CCl4 solution all compounds preferentially exist as the phenol tautomer independent of the nature of the respective substituent. With increasing polarity the tautomeric equilibrium is shifted towards the quinone form. In CHCl3 and, especially, ethanol a clear distinction between the effect of donors (stabilization of the quinone form) and acceptors (stabilization of the phenol tautomer) is evident. Ab initio calculations including solvent effects via the polarized continuum model of solvation as well as the supermolecule approach are used to rationalize the experimental findings.

Do secondary orbital interactions really exist

Abstract: A revision of the most typical examples used to illustrate the existence of secondary orbital interactions (SOI) has been achieved. However, our analysis indicates that no conclusive evidence can be obtained from these cases. All five examples proposed by Woodward and Hoffmann in The Conservation of Orbital Symmetry have been revisited. A combination of well-known mechanisms (such as solvent effects, steric interactions, hydrogen bonds, electrostatic forces, and others) can be invoked instead to justify the endo/exo selectivity of Diels−Alder reactions.

Theoretical study of solvent effect on intramolecular proton transfer of glycine

Abstract: The intramolecular proton transfer pathways for the passage from the neutral form of NH 2 –CH 2 –COOH (GN) to the zwitterionic form + NH 3 –CH 2 –COO − (GZ) of glycine hydrated by three water molecules are computed using DFT and ab initio methods at high levels of theory. The three water molecule cluster yields a zwitterion minimum of about the same energy as the neutral form. The transfer barrier and the GZ–GN energy difference are strongly sensitive to the correlation effects. The solvent effect on the unhydrated and the trihydrated proton transfer surfaces are treated using a continuum model. As modeled in water, the solvent stabilizes the zwitterionic cis conformation of glycine with regard to the neutral cis form. The free energy stabilization of GZ( cis ) over the GN( cis ) form is 5.4 kcal mol −1 for the solvated trihydrated complex compared to an experimental value of 7 kcal mol −1 . Also computed is the small free energy barrier of 2.2 kcal mol −1 for the conversion of GN( cis ) to GZ( cis ). Rationalization of why this barrier persists at all levels of calculation is found in the fact that the solvent effect only becomes important when the structure is close to the zwitterionic configuration.

Thermal Reversion Mechanism of N-Functionalized Merocyanines to Spiropyrans: A Solvatochromic, Solvatokinetic, and Semiempirical Study

Abstract: In continuing studies of the effect of solvent and molecular structure on the behavior of photochromic and thermochromic dye molecules, especially spiropyran (SP)−merocyanine (MC) interconversions, we have examined a series of 6‘-nitrobenzoindolinospiropyrans (6-nitro-BIPS) with varying N-functionalities (R = CH3, CH2CH2COOH, CH2CH2CH2SO3-, CH2CH2COO-Cholesteryl). The solvent effect was assessed by following the thermal decay of the photochemically ring-opened merocyanine to the spiropyran (MC ⇋ SP) via UV/vis spectroscopy at the λmax of the MC form. It was found that while modification of the N-moiety produced no perturbations in the solvatochromic behavior of these dyes, there was a marked effect on the solvatokinetic behavior. In nonpolar solvents, where the MCs possess predominantly quinoid character (unit central bond order), a constant thermal reversion rate was observed for the MCs with electron-rich N-ligands. This was attributed to electronic and steric interactions between the ligands and the ph...

Effect of reinforcement and solvent content on moisture absorption in epoxy composite materials

Abstract: The effect of fibrous reinforcement and solvent content on moisture uptake in composite laminate was investigated. Two materials using identical epoxy resin systems but different reinforcements—glass vs. carbon fibers—and of different solvent content—low vs. normal—were examined. Samples were characterized in terms of water absorption and desorption. Mechanical and thermal properties including flexural modulus, flexural strength, and glass transition temperature were measured. Results clearly show the contribution of the fiber reinforcement as well as solvent content on the water absorption rate and mechanical property changes.

Reaction Pathways and Energy Barriers for Alkaline Hydrolysis of Carboxylic Acid Esters in Water Studied by a Hybrid Supermolecule-Polarizable Continuum Approach

Abstract: Reaction pathways, solvent effects, and energy barriers have been determined for the base-catalyzed hydrolysis of two representative alkyl esters in aqueous solution, using a hybrid supermolecule-polarizable continuum approach. Four solvent water molecules were explicitly included in the supermolecular reaction coordinate calculations; the remaining solvent water was modeled as a polarizable dielectric continuum surrounding the supermolecular reaction system. Two competing reaction pathways were observed, sharing a common first step, i.e. the formation of the tetrahedral intermediate. One pathway involves a direct proton transfer in the second step, i.e. the decomposition of the tetrahedral intermediate. A second pathway involves a water-assisted proton transfer during the decomposition of the tetrahedral intermediate. The direct participation of the solvent water molecule in the proton-transfer process significantly drops the energy barrier for the decomposition of the tetrahedral intermediate. Thus, the...

Catalytic reaction rates in thermodynamically non-ideal systems

Abstract: Chemical reactions reflect the universal tendency of systems to approach equilibrium. The dynamics towards equilibrium, reflected in rates of chemical reactions, are therefore influenced only by thermodynamic properties, such as reaction affinity and the chemical potential, activity, or fugacity of reactants and products. Reaction rates depend on concentrations only in ideal reaction mixtures, because here, concentration appears in the defining equations for all relevant thermodynamic properties. Catalytic reactions in gas–liquid–solid systems involve molecules solvated in a non-ideal environment and reacting on surfaces. Transition state treatments show that such reactions on surfaces detect the presence and identity of a liquid phase only when a liquid solvates kinetically relevant adsorbed intermediates and activated complexes or when its presence prevents gas–liquid equilibrium by imposing transport restrictions. Chemisorption energies are much larger than typical intermolecular interactions in liquids; therefore, inert liquids rarely influence the structure or reactivity of chemisorbed reactants and activated complexes. However, solvent effects become possible on heterogeneous catalysts when adsorption or desorption steps are rate-determining or kinetically relevant. Here, the reaction coordinate involves molecules in solution, and the corresponding activated complexes can become solvated by the surrounding fluid phase. In the special case of identical solvation of a reactant and an activated complex, a fortuitous cancellation of activity coefficients leads to reaction rates that depend on concentration rather than the thermodynamic activity of reactants. This stringent requirement makes concentration-driven reaction rates unusual exceptions to the general case of chemical reaction rates that depend on the thermodynamic activity of reactants and products. We have used transition state treatments of reaction rates in non-ideal systems to explain observed solvent effects for cyclohexene hydrogenation on Pt and Pd catalysts. A dihydrogen dissociative-adsorption rate-determining step on Pt leads to solvation of the kinetically relevant activated complex. Its activity coefficient and that for dissolved H2 cancel. As a result, the hydrogenation rate on Pt depends on H2 concentration in the liquid phase. On Pd, the rate-determining step involves chemisorbed species that are not influenced by the solvent; the reaction rate depends only on H2 partial pressure in the gas phase and not on the nature of the liquid. A similar treatment shows that the presence of liquid products in three-phase Fischer–Tropsch synthesis reactors cannot increase the rate of olefin readsorption, unless the liquid introduces transport restrictions that prevent rapid removal of olefins from catalyst pores. Higher solubility of larger olefins cannot account for enhanced readsorption. In fact, increasing solubility either has no effect or, under certain circumstances, increases the propensity for desorption rather than readsorption. Finally, we show mechanistic implications of the dependence of paraffin cracking rates on intrazeolite paraffin activities or concentrations. For example, for dependence on the latter, we involve a molecule in a precursor state that is solvated by the environment within zeolite channels to the same extent as a paraffin absorbed within the structure. Since zeolites introduce non-ideality to a reaction system, experimental observations need to be rationalized using the approach we have proposed.

Effect of various water-miscible solvents on enzymatic activity of fungal laccases

Abstract: The effect of acetonitrile, acetone and dimethyl sulfoxide (DMSO) on the catalytic activity of laccase from P. radiata and the effect of DMSO on the catalytic activity of laccase from P. oryzae in the oxidation of 2,6-dimethoxyphenol were studied. The results obtained in the studied solvent–water mixtures were satisfactorily described using the equations equivalent to the mixed enzyme inhibition. The fitted inhibition parameters were correlated with various physical and chemical parameters characterizing the mixed systems or used solvents, such as: effective pH of the reaction mixture, log P and E T N of the solvent, solubility of the substrate and thermodynamic activity of water in mixed systems. Moreover, a good correlation between the inhibitory effects and the denaturation capacity of the studied solvents was observed.

Glycine and alanine: a theoretical study of solvent effects upon energetics and molecular response properties

Abstract: In the present work, we show that polarisable continuum model is able to reproduce the stability of zwitterionic forms of amino acids alanine and glycine in water solution at B3-LYP/6-31G (d) level of theory. The model is then extended to the calculation of vibrational frequencies, Vibrational circular dichroism spectra and nuclear magnetic resonance chemical shifts. The agreement with experimental data is good, except in the case of vibrations where specific hydrogen bond interactions are involved. In the latter case, a supermolecular approach may help in the predictions of some vibrational frequencies of the groups which form hydrogen bonds.

Ab Initio Study of the Electronic Excited States in 4-(N,N-Dimethylamino)benzonitrile with Inclusion of Solvent Effects: The Internal Charge Transfer Process

Abstract: The lower singlet excited states for (dimethylamino)benzonitrile (DMABN) have been studied as a function of the twisting and wagging motion with inclusion of solvent effects. Theoretical calculations have been performed using a multireference perturbed CI method, and the solvent effects have been described within the polarizable continuum model (PCM). In the methodology we have used, solvent interactions are explicitly included in the CI scheme and in the following perturbative corrections, through proper operators corresponding to electrostatic and repulsion interactions. The results obtained including solvent interactions both on geometries and energies support the twisted intramolecular charge transfer (TICT) model proposed to explain the dual fluorescence phenomenon occurring in DMABN when immersed in polar solvents. Calculated transition energies (absorption and emission) obtained for both the isolated and the solvated system are in agreement with available experimental information.

Effect of Solvent on the Activation Rate Parameters for Polystyrene and Poly(butyl acrylate) Macroinitiators in Atom Transfer Radical Polymerization

Abstract: The activation rate parameters of polystyrene (kSact) and poly(butyl acrylate) (kBAact) macroinitiators in ATRP catalyzed by CuBr/4,4‘-di-n-heptyl-2,2‘-bipyridine have been determined at 110 °C using a combination of nitroxide exchange reactions with HPLC analysis. The method is based on exchange of the bromide end group with a hydroxy-TEMPO end group. The depletion of the macroinitiator is monitored as a function of time. The method is applicable in different solvents, which implies the possibility to investigate the effect of solvent characteristics. Solvent polarity has a negative influence on kSact and a positive effect on kBAact. The ability of the solvent to act as ligand in the copper complex hampers the activation process for both macroinitiators, while the presence of hydroxy groups in the solvent increases both activation rate parameters.

Solvent effects on the electronic spectra of transition metal complexes.

Abstract: 1. Introduction 7752. Approaches to Treatment of Solvent Effects inCharge-Transfer-Type Excitations7773. The ZHR−SS Computational Model for SolventEffects on MLCT Spectra7784. Calculations for Azine Solutions Using theZHR−SS Method7805. Results from ZHR−SS for MLCT Transitions inRu Complexes7816. Photochemistry of Fe(H

A multiconfiguration self-consistent field/molecular dynamics study of the (n→π*)1 transition of carbonyl compounds in liquid water

Abstract: A model is presented for the electrostatic component of solvatochromic shifts in vertical electronic excitation energies. The model, which makes use of the mean-field approximation, combines quantum mechanics (QM) in the description of the solute molecule and molecular mechanics (MM) in the description of the solvent. The method is implemented at the multiconfigurational self-consistent field level. We present illustrative applications to the (n→π*)1 transitions of formaldehyde, acetaldehyde, and acetone in liquid water. The solvent shifts obtained compare well with other ab initio QM/MM calculations and when the electron correlation components are included with the experimental solvent shift, but differ from the results obtained with semiempirical QM/MM and continuum models.

Mechanism of SN2 Alkylation Reactions of Lithium Organocuprate Clusters with Alkyl Halides and Epoxides. Solvent Effects, BF3 Effects, and Trans-Diaxial Epoxide Opening

Abstract: The B3LYP density functional studies on the mechanism of the SN2-substitution reaction of methyl halides and epoxides with lithium organocuprates(I), (CH3)2CuLi·LiCl and [(CH3)2CuLi]2, revealed the energetics and the geometries of important transition states and intermediates along the reaction pathway. In the absence of solvent coordination on the copper atom, the reaction takes place in a single step through rate determining cleavage of the C−X bond (X = leaving group) involving nucleophilic participation of the CH3−Cu bond composed of the copper 3dz2 orbital and carbon 2s+2p orbitals. Consideration of solvent polarity and coordination of an explicit (CH3)2O molecule to a lithium atom in the cuprate cluster lowers the activation energy to <20 kcal/mol, which is a reasonable value for the reaction taking place below 0 °C. Solvation of the copper atom does not change much the geometry or the energy of the transition state, but modifies the pathway afterward. The origin of the “trans-diaxial opening of cyc...