Showing papers on "Solvent effects published in 2007"
TL;DR: The rates of formal abstraction of phenolic hydrogen atoms by free radicals, Y* + ArOH --> YH + ArO*, are profoundly influenced by the hydrogen-bond-accepting and anion-solvation abilities of solvents, by the electron affinities and reactivities of radicals, and by the phenol's ring substituents.
Abstract: The rates of formal abstraction of phenolic hydrogen atoms by free radicals, Y• + ArOH → YH + ArO•, are profoundly influenced by the hydrogen-bond-accepting and anion-solvation abilities of solvents, by the electron affinities and reactivities (Y−H bond dissociation enthalpies) of radicals, and by the phenol's ring substituents. These apparently simple reactions can occur by at least three different, nonexclusive mechanisms: hydrogen atom transfer, proton-coupled electron transfer, and sequential proton-loss electron transfer. The delicate balance among these mechanisms depends on both the environment and the reactants. The main features of these mechanisms are described, together with some interesting kinetic consequences.
524 citations
04 Apr 2007
516 citations
TL;DR: A state specific (SS) model for the inclusion of solvent effects in time dependent density functional theory (TD-DFT) computations of emission energies has been developed and coded in the framework of the so called polarizable continuum model (PCM).
Abstract: A state specific (SS) model for the inclusion of solvent effects in time dependent density functional theory (TD-DFT) computations of emission energies has been developed and coded in the framework of the so called polarizable continuum model (PCM). The new model allows for a rigorous and effective treatment of dynamical solvent effects in the computation of fluorescence and phosphorescence spectra in solution, and it can be used for studying different relaxation time regimes. SS and conventional linear response (LR) models have been compared by computing the emission energies for different benchmark systems (formaldehyde in water and three coumarin derivatives in ethanol). Special attention is given to the influence of dynamical solvation effects on LR geometry optimizations in solution. The results on formaldehyde point out the complementarity of LR and SS approaches and the advantages of the latter model especially for polar solvents and/or weak transitions. The computed emission energies for coumarin derivatives are very close to their experimental counterparts, pointing out the importance of a proper treatment of nonequilibrium solvent effects on both the excited and the ground state energies. The availability of SS-PCM/TD-DFT models for the study of absorption and emission processes allows for a consistent treatment of a number of different spectroscopic properties in solution.
426 citations
TL;DR: The use of fluorinated alcohols as solvents, cosolvents or additives in homogeneous catalysis is reviewed in this paper, where the effect of these particular compounds on efficiency, regioselectivity and stereoselectivities of metal-catalyzed reactions, as well as on reactions mediated by organocatalysts, is described.
Abstract: The use of fluorinated alcohols as solvents, cosolvents or additives in homogeneous catalysis is reviewed. The effect of these particular compounds on efficiency, regioselectivity and stereoselectivity of metal-catalyzed reactions, as well as on reactions mediated by organocatalysts, is described. Where available in several cases, the effect of fluorous alcohols on the catalytic mechanism is analyzed in detail and rationalized by their unique physical and chemical properties. In most instances, a comparison to the effect of commonly employed solvents is elaborated. 1 Introduction 2 Physical Properties of Fluorinated Alcohols 3 Applications in Homogeneous Catalysis 3.1 Reduction Reactions 3.2 Oxidation Reactions 3.3 Carbon-Carbon Bond-Forming Reactions 3.4 Cycloaddition Reactions 3.5 Metathesis Reactions 3.6 Carbon-Heteroatom Bond-Forming Reactions 3.7 Isomerization Reactions 4 Conclusion
335 citations
245 citations
TL;DR: A short historical overview on the influence of solvents on chemical processes (equilibria, reaction rates) and physical processes (absorption spectra, crystallization) is presented in this paper.
209 citations
TL;DR: DFT and high-level ab initio calculations have been used to describe molecular reactions relevant for CO2 absorption in aqueous (alkanol)amine solutions and transition state energies agree well with experimental activation energies.
Abstract: DFT and high-level ab initio calculations (among them B3LYP and G3MP2B3) have been used to describe molecular reactions relevant for CO2 absorption in aqueous (alkanol)amine solutions. Reaction mechanisms for various reactions of CO2 with ammonia, monoethanolamine (MEA), and diethanolamine (DEA) to carbamic acid and ion pair products have been investigated and interpreted in light of experimental observations. Additional water, ammonia, MEA, and DEA molecules have also been added to the molecular complexes to simulate microsolvation effects. These extra molecules may act as catalysts for the desired reactions, and in several cases they have a large impact on activation and reaction energies. Solvent effects were estimated by applying electrostatic continuum models for selected systems. Our calculated transition state energies agree well with experimental activation energies.
164 citations
161 citations
TL;DR: In this article, the chemical reactions involved in CO 2 absorption in amine systems are studied and quantum mechanical calculations are carried out to predict the base strength and carbamate stability of different amine solvents with a useful degree of accuracy.
158 citations
158 citations
TL;DR: In this article, a simple synthetic methodology for the preparation of a polystyrene-supported L-proline material is reported, and this material has been used as catalyst in direct asymmetric aldol reactions between several ketones and arylaldehydes to furnish aldolic products in high yields and stereoselectivities.
TL;DR: Calculations carried out on water gives results that also are comparable with CC calculations in accuracy for ground-state and first-order properties, however, to obtain such accuracy an exchange-correlation functional capable of describing the diffuse Rydberg states must be chosen.
Abstract: A combined quantum mechanics/molecular mechanics (QM/MM) method is described, where the polarization between the solvent and solute is accounted for using a self-consistent scheme linear in the solvent polarization. The QM/MM method is implemented for calculation of energies and molecular response properties including the calculation of linear and quadratic response functions using the density-functional theory (DFT) and the Hartree-Fock (HF) theory. Sample calculations presented for ground-state energies, first-order ground-state properties, excitation energies, first-order excited state properties, polarizabilities, first-hyperpolarizabilities, and two-photon absorptions strengths of formaldehyde suggests that DFT may in some cases be a sufficiently reliable alternative to high-level theory, such as coupled-cluster (CC) theory, in modeling solvent shifts, whereas results obtained with the HF wave function deviate significantly from the CC results. Calculations carried out on water gives results that also are comparable with CC calculations in accuracy for ground-state and first-order properties. However, to obtain such accuracy an exchange-correlation functional capable of describing the diffuse Rydberg states must be chosen.
TL;DR: The structure and thermodynamic properties of a system of end-grafted flexible polymer chains grafted to a flat substrate and exposed to a solvent of variable quality are studied by molecular dynamics methods and the reduction of the surface tension between solvent and wall as a function of the grafting density of the brush is obtained.
Abstract: The structure and thermodynamic properties of a system of end-grafted flexible polymer chains grafted to a flat substrate and exposed to a solvent of variable quality are studied by molecular dynamics methods. The macromolecules are described by a coarse-grained bead-spring model, and the solvent molecules by pointlike particles, assuming Lennard-Jones-type interactions between pairs of monomers (ϵpp), solvent molecules (ϵss), and solvent monomer (ϵps), respectively. Varying the grafting density σg and some of these energy parameters, we obtain density profiles of solvent particles and monomers, study structural properties of the chain (gyration radius components, bond orientational parameters, etc.), and examine also the profile of the lateral pressure P‖(z), keeping in the simulation the normal pressure P⊥ constant. From these data, the reduction of the surface tension between solvent and wall as a function of the grafting density of the brush has been obtained. Further results include the stretching fo...
TL;DR: Treatment of solutions of P2 in tetrahydrofuran (THF), a nonselective solvent, with increasing amounts of solvents that are selective for the polar polythiophene blocks (water) or the nonpolar polyfluorene blocks (hexane), respectively, results in the formation of two different types of core-shell aggregates, which show rather different optical properties.
Abstract: A novel, amphiphilic, conjugated block copolymer is described, which was prepared by a Suzuki-type cross-coupling of 2-bromo-[9,9-bis(2-ethylhexyl)fluorene]-7-pinacolato boronate as an AB-type monomer and monobromo-substituted poly[3-(6-bromohexyl)thiophene] (Br-P3BrHT) as a polymeric end capper in the key step. PF-P3PHT (P2; PF = poly(9,9-dialkylfluorene); P3PHT = poly[3-(6-diethylphosphonato-hexyl)thiophene]) as the amphiphilic target polymer was then generated in a polymer-analogous conversion of the alkyl bromide side chains of the PF-P3BrHT (P1) precursor into polar alkyl phosphonate groups by reaction with triethyl phosphite. P2 shows a strong influence of the solvent polarity on the optical spectra (absorption, emission). Treatment of solutions of P2 in tetrahydrofuran (THF), a nonselective solvent, with increasing amounts of solvents that are selective for the polar polythiophene blocks (water) or the nonpolar polyfluorene blocks (hexane), respectively, results in the formation of two different types of core-shell aggregates, which show rather different optical properties (photoluminescence quenching, excitation energy transfer).
TL;DR: In this article, the excited-state intramolecular proton transfer (ESIPT) mechanism in six amino 2-(2′-hydroxyphenyl)benzazole derivatives were investigated in different solvents by means of UV-vis absorption and steady-state fluorescence.
TL;DR: The entire ultraviolet-visible absorption spectrum of benzophenone in water is studied and compared with the same spectrum in gas phase, and the influence of the solute polarization in thesolute-solvent interaction and, in particular, in solute- solvent hydrogen bonds is analyzed.
Abstract: The entire ultraviolet-visible absorption spectrum of benzophenone in water is studied and compared with the same spectrum in gas phase. Five transitions are considered, and the corresponding solvatochromic shifts are obtained and compared to experiment. Using a sequential procedure of Monte Carlo simulations and quantum mechanical calculations, liquid configurations were generated and an averaged spectrum of the solution was calculated. The solute polarization was included by an iterative procedure where the atomic charges of the solute were obtained as an average with the solvent distribution. The calculated average dipole moment of benzophenone in water, with MP26-31++G(d,p), converges to the value of 5.84+/-0.05 D, 88% larger than the gas-phase value of 3.11 D. Using 100 statistically uncorrelated configurations and solvation shells with 235 explicit water molecules selected by a minimum-distance distribution of solvent shells, instead of the usual radial distribution, the excitation energies were obtained from solute-solvent all-valence-electron INDO/CIS calculations. The shift of the weak n-pi(*) transition is obtained as 2045+/-40 cm(-1) and the strong and broad pi-pi(*) shift as -1790+/-30 cm(-1). These results are in good agreement with the experimental values of 2200 and -1600 cm(-1), respectively. Standard procedure used by common force fields to generate atomic charges to describe the electrostatic moments of the solute, with HF6-31G(d), gives a dipole moment of 3.64 D. Using these standard charges in the simulation, the average shifts are calculated as 1395+/-35 and -1220+/-25 cm(-1), both about 600 cm(-1) smaller in magnitude than those obtained with the average converged fully polarized solute. The influence of the solute polarization in the solute-solvent interaction and, in particular, in solute-solvent hydrogen bonds is analyzed.
TL;DR: The dissymmetric ordering of solvent molecules around the chiral solute contributes to the chiroptical signature, as shown for (S)-methyloxirane in benzene.
Abstract: Solvent or solute dissymmetry? The dissymmetric ordering of solvent molecules around the chiral solute (see picture) contributes to the chiroptical signature. Indeed, the solvent can dominate the chiroptical response, as shown for (S)-methyloxirane in benzene. (Figure Presented). © 2007 Wiley-VCH Verlag GmbH & Co. KGaA.
TL;DR: In this article, a mixture of solid [Fe(η5-C5H4-4n) and solid pimelic acid HOOC(CH2)5COOH in the presence of solvent vapours generates co-crystals of different stoichiometry depending on the protic or aprotic nature of the solvent.
Abstract: Vapour digestion of a mixture of solid [Fe(η5-C5H4–C5H4N)2] and solid pimelic acid HOOC(CH2)5COOH in the presence of solvent vapours generates co-crystals of different stoichiometry depending on the protic or aprotic nature of the solvent. The nature of the products has been ascertained by a combined use of SSNMR and X-ray diffraction.
TL;DR: Computational protocols based on DFT and molecular dynamics simulations were applied to the prediction of the alkyl 1H and 13C chemical shifts of alpha-d-glucose in water, finding that 1H chemical shifts do not show a marked solvent effect.
Abstract: We have applied computational protocols based on DFT and molecular dynamics simulations to the prediction of the alkyl 1H and 13C chemical shifts of α-d-glucose in water. Computed data have been compared with accurate experimental chemical shifts obtained in our laboratory. 13C chemical shifts do not show a marked solvent effect. In contrast, the results for 1H chemical shifts provided by structures optimized in the gas phase are only fair and point out that it is necessary to take into account both the flexibility of the glucose structure and the strong effect exerted by solvent water thereupon. Thus, molecular dynamics simulations were carried out to model both the internal geometry as well as the influence of solvent molecules on the conformational distribution of the solute. Snapshots from the simulation were used as input to DFT NMR calculations with varying degrees of sophistication. The most important factor that affects the accuracy of computed 1H chemical shifts is the solute geometry; the effect...
TL;DR: The synthesis and spectroscopic properties of the following tetra and octa-substituted aryloxy zinc(II) phthalocyanines are reported for the first time as discussed by the authors.
TL;DR: The activation strain model is used to arrive at a qualitative understanding of how the competition between OxIn and SN2 pathways is determined by the halogen atom in the activated C-X bond, by anion assistance, and by solvation.
Abstract: We have theoretically studied the oxidative addition of all halomethanes CH3X (with X = F, Cl, Br, I, At) to Pd and PdCl-, using both nonrelativistic and zeroth-order-regular-approximation-relativistic density functional theory at BLYP/QZ4P. Our study covers the gas phase as well as the condensed phase (water), where solvent effects are described with the conductor-like screening model. The activation of the C*−X bond may proceed via two stereochemically different pathways: (i) direct oxidative insertion (OxIn) which goes with retention of the configuration at C* and (ii) an alternative SN2 pathway which goes with inversion of the configuration at C*. In the gas phase, for Pd, the OxIn pathway has the lowest reaction barrier for all CH3X's. Anion assistance, that is, going from Pd to PdCl-, changes the preference for all CH3X's from OxIn to the SN2 pathway. Gas-phase reaction barriers for both pathways to C−X activation generally decrease as X descends in group 17. Two striking solvent effects are (i) th...
TL;DR: Destruction of urea's resonance stabilization during hydrolysis via an addition/elimination mechanism and its preservation in the rearrangement to the H3NCONH intermediate were identified as important factors in determining the preferred reaction route.
Abstract: A joint QM/MM and ab initio study on the decomposition of urea in the gas phase and in aqueous solution is reported. Numerous possible mechanisms of intramolecular decomposition and hydrolysis have been explored; intramolecular NH3 elimination assisted by a water molecule is found to have the lowest activation energy. The solvent effects were elucidated using the TIP4P explicit water model with free energy perturbation calculations in conjunction with QM/MM Monte Carlo simulations. The explicit representation of the solvent was found to be essential for detailed resolution of the mechanism, identification of the rate-determining step, and evaluation of the barrier. The assisting water molecule acts as a hydrogen shuttle for the first step of the elimination reaction. The forming zwitterionic intermediate, H3NCONH, participates in 8−9 hydrogen bonds with water molecules. Its decomposition is found to be the rate-limiting step, and the overall free energy of activation for the decomposition of urea in water...
TL;DR: Small molecule diversity can be achieved in a single synthetic operation from bifunctional substrates in the absence of additives and under the influence of microwaves with complete control of pathway selectivity.
TL;DR: In this article, a level-set method is developed for numerically capturing the equilibrium solute-solvent interface that is defined by the recently proposed variational implicit solvent model, and the method is implemented and applied to the solvation of nonpolar molecules such as two xenon atoms, two paraffin plates, helical alkane chains, and a single fullerence C(60).
Abstract: A level-set method is developed for numerically capturing the equilibrium solute-solvent interface that is defined by the recently proposed variational implicit solvent model [Dzubiella, Swanson, and McCammon, Phys. Rev. Lett. 104, 527 (2006); J. Chem. Phys. 124, 084905 (2006)]. In the level-set method, a possible solute-solvent interface is represented by the zero level set (i.e., the zero level surface) of a level-set function and is eventually evolved into the equilibrium solute-solvent interface. The evolution law is determined by minimization of a solvation free energy functional that couples both the interfacial energy and the van der Waals type solute-solvent interaction energy. The surface evolution is thus an energy minimizing process, and the equilibrium solute-solvent interface is an output of this process. The method is implemented and applied to the solvation of nonpolar molecules such as two xenon atoms, two parallel paraffin plates, helical alkane chains, and a single fullerence C(60). The level-set solutions show good agreement for the solvation energies when compared to available molecular dynamics simulations. In particular, the method captures solvent dewetting (nanobubble formation) and quantitatively describes the interaction in the strongly hydrophobic plate system.
TL;DR: The authors have performed the Langevin dynamics simulation to investigate the unforced polymer translocation through a narrow nanopore in an impermeable membrane and show that the translocation time changes from a wide and asymmetric distribution with a long tail to a narrow and symmetrical distribution with the increase of the difference of the solvent qualities.
Abstract: The authors have performed the Langevin dynamics simulation to investigate the unforced polymer translocation through a narrow nanopore in an impermeable membrane. The effects of solvent quality controlled by the attraction strength λ of the Lennard-Jones cosine potential between polymer beads and beads on two sides of the membrane on the translocation processes are extensively examined. For polymer translocation under the same solvent quality on both sides of the membrane, the two-dimensional and three-dimensional simulations confirm the scaling law of τtrans∼N1+2υ for the translocation in the good solvent, where τtrans is the translocation time, N is the chain length, and υ is the Flory exponent. For the three-dimensional polymer translocation under different solvent qualities on two sides of the membrane, the translocation efficiency may be notably improved. The scaling law between τtrans and N varies from τtrans∼N1+2υ to τtrans∼N with the increase of the difference of solvent qualities, and the crosso...
TL;DR: In this paper, a series of lanthanum benzoate complexes have been prepared and structurally characterised as DMF or DMSO solvates, and the structures of these complexes vary considerably as a function of the steric bulk of the ligands and the degree of solvation.
Abstract: A series of lanthanum benzoate complexes have been prepared and structurally characterised as DMF or DMSO solvates. The structures of these complexes vary considerably as a function of the steric bulk of the ligands and the degree of solvation. With one exception, all of the complexes form coordination polymers. These experimental results are compared to reported data from the Cambridge Structural Database (CSD) in order to study the role of sterically bulky substituents in the structures of lanthanum benzoate complexes. Binding modes vary from the η1 coordination of a carboxylic acid co-ligand to carboxylate ligands adopting a bridging μ-η2 : η1 geometry. The nature of the bridging environment between adjacent lanthanum ions varies significantly in the structures and often several different environments exist within the same structure. Solvent effects are also observed, particularly in cases where the solvent is able to partake in intra- or intermolecular hydrogen bonding.
TL;DR: In this paper, a poly(styrene-b-dimethylsiloxane) diblock copolymer with block molecular weights of 4 and 12 kDa, respectively, were prepared in a series of styrene-selective dialkyl phthalates.
Abstract: Dilute solutions of a poly(styrene-b-dimethylsiloxane) diblock copolymer with block molecular weights of 4 and 12 kDa, respectively, were prepared in a series of styrene-selective dialkyl phthalates: dioctyl phthalate (DOP), dibutyl phthalate (DBP), and diethyl phthalate (DEP). The phthalates were chosen because the interfacial tension between the core block and the solvent can be continuously varied by mixing the solvents in varying proportions. The morphologies were characterized by cryogenic transmission electron microscopy (cryo-TEM) and dynamic light scattering (DLS). By increasing the selectivity of the mixed solvent at room temperature the equilibrium micelle morphology changed from spheres (DOP) to cylinders (DBP) to vesicles (DEP). The selectivity of the solvent was then reduced by increasing the temperature, and we observed the reverse transitions: cylinders → spheres (in DBP) and vesicles → cylinders → spheres (in DEP). Since the core block is always above its glass transition temperature (Tg...
TL;DR: Front- and back-side nucleophilic attack on the substrates is proposed mainly on the basis of the primary kinetic isotope effects, and a hydrogen-bonded, four-center-type transition state is suggested for a front-side attack, while the trigonal bipyramidal pentacoordinate transitionState is proposed for a back- side attack.
Abstract: Kinetic studies of the reactions of aryl phenyl chlorothiophosphates (1) and aryl 4-chlorophenyl chlorothiophosphates (2) with substituted anilines in acetonitrile at 55.0 °C are reported. The negative values of the cross-interaction constant ρXY (ρXY = −0.22 and −0.50 for 1 and 2, respectively) between substituents in the nucleophile (X) and substrate (Y) indicate that the reactions proceed by concerted SN2 mechanism. The primary kinetic isotope effects (kH/kD = 1.11−1.13 and 1.10−1.46 for 1 and 2, respectively) involving deuterated aniline nucleophiles are obtained. Front- and back-side nucleophilic attack on the substrates is proposed mainly on the basis of the primary kinetic isotope effects. A hydrogen-bonded, four-center-type transition state is suggested for a front-side attack, while the trigonal bipyramidal pentacoordinate transition state is suggested for a back-side attack. The MO theoretical calculations of the model reactions of dimethyl chlorothiophosphate (1‘) and dimethyl chlorophosphate (...
TL;DR: A comprehensive basicity study of alpha,omega-alkanediamines and related bases has been carried out and the basicity order of N-methylated 1,3-propanediamines is practically inverse to that in the gas phase.
Abstract: A comprehensive basicity study of alpha,omega-alkanediamines and related bases has been carried out. Basicities in acetonitrile (AN, pK(a) values), tetrahydrofuran (THF, pK(alpha) values), and gas phase (GP, GB values), were measured for 16, 14, and 9 diamine bases and for several related monoamines. In addition the gas-phase basicities and equilibrium geometries were computed for 19 diamino bases and several related monoamines at the DFT B3LYP 6-311+G** level. The effects of the different factors (intrinsic basicity of the amino groups, formation of intramolecular hydrogen bonds, and molecular strain) determining the diamine basicities were estimated by using the method of isodesmic reactions. The results are discussed in terms of molecular structure and solvation effects. The GP basicity is determined by the molecular size and polarizability, the extent of alkylation, and the energy effect of intramolecular hydrogen bond formation in the protonated base. The basicity trends in the solvents differ very much from those in GP: 1) The solvents severely compress the basicity range of the bases studied (3.5 times for the 1,3-propanediamine family in AN, and 7 times in THF), and 2) while stepwise alkylation of the basicity center leads to a steady basicity increase in the gas phase, the picture is complex in the solvents. Significant differences are also evident between THF and AN. The high hydrogen bond acceptor strength of THF leads to this solvent favoring the bases with "naked" protonation centers. In particular, the basicity order of N-methylated 1,3-propanediamines is practically inverse to that in the gas phase. The picture in AN is intermediate between that of GP and THF.
TL;DR: In this paper, a combined quantum/classical force field has been used to investigate the properties of hydrogen peroxide in aqueous solution, and it has been shown that H 2 O 2 is a better proton donor than H O 2 O but a weaker proton acceptor.