Showing papers on "Solvent effects published in 1996"

An effective fragment method for modeling solvent effects in quantum mechanical calculations

Abstract: An effective fragment model is developed to treat solvent effects on chemical properties and reactions. The solvent, which might consist of discrete water molecules, protein, or other material, is treated explicitly using a model potential that incorporates electrostatics, polarization, and exchange repulsion effects. The solute, which one can most generally envision as including some number of solvent molecules as well, is treated in a fully ab initio manner, using an appropriate level of electronic structure theory. In addition to the fragment model itself, formulae are presented that permit the determination of analytic energy gradients and, therefore, numerically determined energy second derivatives (hessians) for the complete system. Initial tests of the model for the water dimer and water‐formamide are in good agreement with fully ab initio calculations.

Monolithic, “Molded”, Porous Materials with High Flow Characteristics for Separations, Catalysis, or Solid-Phase Chemistry: Control of Porous Properties during Polymerization

Abstract: The porosity and flow characteristics of macroporous polymer monoliths that may be used to prepare separation media, flow-through reactors, catalysts, or supports for solid-phase chemistry can be controlled easily during their preparation. Key variables such as temperature, composition of the pore-forming solvent mixture, and content of cross-linking divinyl monomer allow the tuning of average pore size within a broad range spanning 2 orders of magnitude. The polymerization temperature, through its effects on the kinetics of polymerization, is a particularly effective means of control, allowing the preparation of macroporous polymers with different pore size distributions from a single composition of the polymerization mixture. The choice of pore-forming solvent is also important, larger pores being obtained in a poor solvent due to an earlier onset of phase separation. Increasing the proportion of the cross-linking agent present in the monomer mixture not only affects the composition of the final monolit...

Molecular Dynamics Study of Dendrimer Molecules in Solvents of Varying Quality

Abstract: The properties of dendrimers (“starburst” molecules) under varying solvent conditions are studied using molecular dynamics simulations. The dendrimers are found to have a compact (space filling) structure under all solvent conditions, with a radius of gyration which scales with the number of monomers as RG ∝ N1/3. For high generation number dendrimers, there is a distinct region of constant monomer density. The density in this region depends only on the solvent quality and is independent of the generation number. When the contributions of the different generations to the overall density profile are separated, we find that the monomers which belong to the first few generations are stretched and spatially localized. Later generations are less localized and penetrate well into the central regions of the dendrimer. The different primary branches (“dendrons”) of the dendrimers are found to be segregated. The amount of spatial overlap between the different dendrons decreases with increasing generation number of...

A Generalized Solvent Basicity Scale: The Solvatochromism of 5‐Nitroindoline and Its Homomorph 1‐Methyl‐5‐nitroindoline

Abstract: A total of 202 organic solvents and the gas phase were placed on a solvent basicity scale for hydrogen bond acceptor based on parameter SB. The value of such a parameter can readily be determined from the UV/Vis spectrum for an appropriate acid probe (5-nitroindoline) (NI) and its non-acid homomorph (1-methyl-5-nitroindoline) (MNI). The proposed scale can advantageously substitute the more widely used solvent scales such as Gutmann's donor number (DN), the Koppel-Palm B(MeOD) scale, and the Taft-Kamlet β scale. While data for the proposed scale are derived only from electronic transitions, they are accurately descriptive of solvent basicity in both spectroscopy (UV/Vis, IR and NMR) and miscellaneous chemical areas (thermodynamic, kinetics, and electrochemistry).

Transition State Dynamics and Relaxation Processes in Solutions: A Frontier of Physical Chemistry

Abstract: The theoretical framework for describing solvent effects on solution phase reactions is summarized. When possible, the results are related to experiments involving geometric isomerization, to photoinduced reactive processes that are initiated at the transition state, and to the bimolecular reactions between molecules that are at van der Waals separations in liquids. Key differences between solution and gas phase transition state dynamics are pointed out, and the relaxation processes that determine wavepacket propagation in the solution phase are discussed. Quantum activated rate processes are described theoretically, and some results on proton transfer are given.

Precursor of the Iaq− charge‐transfer‐to‐solvent (CTTS) band in I−⋅(H2O)n clusters

Abstract: Ultraviolet charge‐transfer‐to‐solvent (CTTS) bands are general spectral features of inorganic salts dissolved in dipolar (e.g., aqueous) solvents. The bands originate from excitations of the anions, and the excited states are thought to involve a delocalized electronic state supported by the arrangement of solvent molecules around the ion in the ground state. These diffuse states have fascinated physical chemists since the 1930s, who first used continuum models to explain the phenomenon and then later refined the treatment to include local molecular interactions. In this paper, we report that CTTS bands evolve from features present in very small gas phase clusters, I−⋅(H2O)n, n=1–4, and discuss how localization of the continuum wave function of the ejected electron acts as a precursor to the bound excited state in the liquid.

Macromolecular Engineering of Polylactones and Polylactides. 19. Kinetics of Ring-Opening Polymerization of ε-Caprolactone Initiated with Functional Aluminum Alkoxides

Abstract: End-functional aliphatic polyesters have been synthesized by ring-opening polymerization (ROP) of e-caprolactone (e-CL) and lactides (LA). Polymerization initiated with any of the functional aluminum alkoxides of the general structure Et3-pAl(OCH2X)p, where p = 1 and 3, obeys a living “coordination−insertion” mechanism, under suitable conditions (temperature, concentration, and solvent), with the formation of α-functional chains of a predictable molecular weight. A kinetic study has emphasized the importance of the cyclic monomer (e-CL or LA) and the key role of the initiator structure: the number of functional alkoxides per aluminum and the nature of the functional group (X = CH2Br, (CH2)2CHCH2, (CH2)2NEt2). Activation enthalpy and entropy have been calculated for the e-CL polymerization initiated by several aluminum alkoxides at various temperatures. The effects of solvent (toluene or tetrahydrofuran) and purification of the aluminum monoalkoxides by distillation under reduced pressure have also been c...

Kinetic Solvent Effects on Hydrogen Atom Abstraction from Phenol, Aniline, and Diphenylamine. The Importance of Hydrogen Bonding on Their Radical-Trapping (Antioxidant) Activities1

Abstract: Absolute rate constants for hydrogen atom abstraction by alkoxyl radicals from phenol, aniline, and diphenylamine have been measured in 14 solvents at room temperature by laser flash photolysis. For all three substrates the rate constants decline as the solvent becomes a stronger hydrogen-bond acceptor (HBA). Thus, on changing the solvent from CCl4 to CH3CN the rate constants decline by factors of 148, 7.1, and 4.7 for PhOH, PhNH2, and Ph2NH, respectively. The kinetic solvent effect for phenol correlates rather well with Abraham's scale of relative HBA activities of the solvents we have employed as measured as solutes in CCl4 solvent. This correlation is not quite so good with aniline and it is almost nonexistent for diphenylamine. With all three substrates the “deviant” solvents produce higher rate constants than would be expected from the value of the solvent and, generally, these are the solvents in which steric hindrance to hydrogen-bond formation would appear probable. The kinetic data for the three ...

Extension of MST/SCRF method to organic solvents: Ab initio and semiempirical parametrization for neutral solutes in CCl4

Abstract: The self-consistent reaction field (SCRF) method proposed by Miertus, Scrocco, and Tomasi (MST) was extended to solutions of neutral solutes in CCl4. A detailed parametrization of the solute/solvent interface and of the “hardness” atomic parameters determining the van der Waals interactions was performed from comparison with experimental data and Monte Carlo simulations. The parametrization was carried out at both ab initio (6-31G*) and semiempirical (MNDO, AM1, PM3) levels. The MST/SCRF optimized versions provide accurate estimates of the free energy of solvation in CCl4 for the series of molecules studied. Furthermore, a precise description of the solvent effect on different chemical processes in CCl4 solution supports the reliability of the parametrization. © 1996 by John Wiley & Sons, Inc.

Reaction pathways in the photodetachment of an electron from aqueous chloride: A quantum molecular dynamics study

Abstract: Reaction and relaxation processes induced by photoexcitation of an aqueous chloride ion are studied with quantum molecular dynamics simulations. A predominant channel leading to a metastable hydrated electron‐chlorine pair is found. By means of theoretical transient and stationary absorption spectra, the solvent reorganization involved in the charge repartitioning is discussed. The dissipation of excess electron kinetic energy by surrounding water molecules plays an essential role in the equilibration of an electron‐atom pair. For this intermediate species, two competing reaction pathways are identified. One is the barrier‐impeded dissociation yielding a hydrated electron. Shape and height of the free energy barrier determined by quantum umbrella sampling point to a diffusion controlled electron photodetachment. The other channel is the geminate recombination via a nonadiabatic transition for which a self‐consistent and fully dynamical treatment of the solvent electronic polarization is found to be important. From the rate constants computed for the individual channels, a kinetic model is derived to explain time‐dependent spectral signatures and electron escape yields recently observed in photodetachment experiments on aqueous halides.

Solvent Effects in the Raman Spectra of the Triiodide Ion: Observation of Dynamic Symmetry Breaking and Solvent Degrees of Freedom

Abstract: Resonance Raman spectra, including absolute cross sections, have been measured for the triiodide ion in ethanol, ethyl acetate, and acetonitrile solvents using eight excitation wavelengths that span the two lowest absorption bands. Preresonance Raman spectra have also been obtained with 488 nm excitation. The apparent vibrational line widths observed on resonance are approximately twice the preresonant line widths in all three solvents. By quantitatively modeling the triiodide/solvent system using two intramolecular modes (the symmetric and antisymmetric stretches) and one low-frequency intermolecular or solvent mode, we are able to reproduce the vibrational broadening on resonance, which, within the context of our model, is due to unresolved combination bands between the intramolecular degrees of freedom and the intermolecular mode. The Raman spectra in ethanol clearly show one-quantum transitions in the antisymmetric stretching mode (ν3) which should be symmetry forbidden in the linear D∞h geometry, ind...

Phase diagram of a semiflexible polymer chain in a θ solvent: Application to protein folding

Abstract: We consider a lattice model of a semiflexible homopolymer chain in a bad solvent. Beside the temperature T, this model is described by (i) a curvature energy eh, representing the stiffness of the chain; (ii) a nearest‐neighbor attractive energy ev, representing the solvent; and (iii) the monomer density ρ=N/Ω, where N and Ω denote, respectively, the number of monomers and the number of lattice sites. This model is a simplified view of the protein folding problem, which encompasses the geometrical competition between secondary structures (the curvature term modelling helix formation) and the global compactness (modeled here by the attractive energy), but contains no side chain information. By allowing the monomer density ρ to depart from unity one has made a first (albeit naive) step to include the role of the water. In previous analytical studies, we considered only the (fully compact) case ρ=1, and found a first order freezing transition towards a crystalline ground state (also called the native state in...

Ultrafast Photoinduced Electron Transfer in a Chlorophyll-Based Triad: Vibrationally Hot Ion Pair Intermediates and Dynamic Solvent Effects

Abstract: We report ultrafast transient absorption studies of photoinduced electron transfer in the triad molecule zinc methyl 131-desoxopyropheophorbide a−pyromellitimide−1,8:4,5-naphthalenediimide (ZC−PI−NI) in solution. The absorption spectra of the radical anions of PI and NI possess narrow and well-separated absorption bands which permit the direct observation of both the intermediate and final charge-separated states. Selective optical excitation of the ZC donor results in the formation of the intermediate charge-separated state, ZC+−PI-−NI, in less than 2 ps in nonpolar solvents. The PI radical ion within the ZC+−PI-−NI intermediate is vibrationally excited as illustrated by time-dependent changes in the band shape of its transient absorption spectrum. The rate of the initial charge separation reaction forming ZC+−PI-−NI and the subsequent charge shift reaction to form the final state, ZC+−PI−NI-, as well as the appearance of the vibrationally excited intermediate are all highly solvent dependent even for so...

Thioether oxidation by hydrogen peroxide using titanium-containing zeolites as catalysts

Abstract: Titanium substituted ZSM-5 (TS-1) and BEA (Ti-beta) are investigated as catalysts for the selective oxidation of thioethers with diluted H2O2, in a protic solvent, at 303 K. The performances of TS-1 and Ti-beta samples are analyzed in relation with structure of thioethers, nature of solvent and shape selectivity effect of catalysts. A mechanism for organic sulfides oxidation on Ti-containing zeolites is proposed.

Energetics of charge transfer reactions in solvents of dipolar and higher order multipolar character. I. Theory

Abstract: We extend a recent molecular theory of solvation dynamics to accommodate static solvent effects on the energetics of charge transfer (CT) processes. Our theory is based on a simple renormalized linear response development which incorporates nonlinear aspects of equilibrium solvation. It can accommodate polarizable solvent molecules as well as the limiting case represented by electronically rigid interaction site model (ISM) solvent molecules. We focus on the diabatic free energy profiles governing CT processes in solute donor–acceptor systems of chemical interest. By studying CT in ISM solution models we naturally cover both the short range and long range solute‐solvent interactions, thereby enabling applications to CT in solvents of higher multipolar as well as dipolar character. We derive expressions for the key energetic parameters of a CT process; the solvent reorganization energy, the solvent contribution to the change in thermodynamic free energy, and the optical absorption and fluorescence frequencies.

Fluorescence loss mechanism due to large-amplitude motions in derivatives of 2,2′-bipyridyl exhibiting excited-state intramolecular proton transfer and perspectives of luminescence solar concentrators

Abstract: Fluorescence quantum yields and lifetimes as functions of temperature and solvent polarity are compared for two different hydroxy derivatives of 2,2′-bipyridyl. Both dyes show strongly red-shifted excited-state intramolecular proton transfer (ESIPT) fluorescence at all temperatures investigated. In protic solvents, the ESIPT emission is of a more allowed nature and somewhat blue shifted, owing to specific solvatation. A strong difference in behaviour can, however, be observed for the non-radiative losses. The symmetric dye 1 possesses large fluorescence quantum yields, whereas sizeable fluorescence quantum yields from 2 could only be obtained under high viscosity conditions. Increased solvent polarity leads to enhanced fluorescence losses. These facts and the comparison with quantum chemical calculations for planar and twisted structures of 1 and 2 are interpreted as evidence for a photochemically reached low-lying state with charge separation (twisted intramolecular charge transfer (TICT)), responsible for these fluorescence losses and especially active for 2. For 1 in aprotic solvents, this reaction is discussed within the quantum-chemical extension of electron transfer theory and shown to involve nuclear tunnelling. In addition to this viscosity-dependent decay channel, 2 also decays via a viscosity-independent (presumably n −π ∗ ) channel. The identification of the TICT and n −π ∗ non-radiative channels allows a new approach to the development of highly fluorescent ESIPT dyes with very large Stokes shift for use in fluorescence solar collectors or other devices utilizing the principle of fluorescence-based light-pipes.

Solvent Effects on Molecular and Ionic Spectra. 7. Modeling the Absorption and Electroabsorption Spectra of Pentaammine- ruthenium(II) Pyrazine and Its Conjugate Acid in Water

Abstract: This work brings to a focus a series of papers concerning the modeling of solvent shifts in systems in which specific solute−solvent interactions such as hydrogen bonding occur: we consider the in...

Solvent Effects on Homolytic Bond Dissociation Energies of Hydroxylic Acids

Abstract: The homolytic bond dissociation energies (BDEs) of the O−H bonds in DMSO solution for (a) phenol and a number of its derivatives, (b) three oximes, (c) three alcohols, (d) three hydroxylamines, and (e) two hydroxamic acids have been estimated by eq 1: BDEHA = 1.37pKHA + 23.1Eox(A-) + 73.3 kcal/mol. For most of these hydroxylic acids, the BDEs of the O−H bonds estimated by eq 1 are within ±2 kcal/mol of the literature values in nonpolar solvents or in the gas phase. There is no reason to believe, therefore, that these BDEs are “seriously in error because of failure to correct for solvent effects” as has been claimed on the basis that BDEs in highly polar solvents estimated for the O−H bond in phenol by photoacoustic calorimetry must be so corrected.

Aggregation of C70 in Solvent Mixtures

Abstract: The unusual solvatochromism of C70 is investigated in a variety of solvent mixtures by optical absorption and fluorescence techniques. Distinct reversible color change from pink to purple is seen in the solvent mixtures studied. Such changes are seen also for C60 solutions in some solvent mixtures. Formation of clusters is found to be responsible for the observed optical changes. Light scattering studies are carried out to confirm the presence of clusters which show that the particle size varies from ∼100 to ∼1000 nm depending on the concentration of the fullerene. It is found that the solubility of the fullerene in the solubilizing solvent and that in the solvent mixtures are the major factors governing the aggregation behavior of the fullerene. Polarity of the solvent plays a minor role in the formation of aggregates.

Introduction to Molecular Dynamics and Chemical Kinetics

Abstract: Interaction Potentials. Relative Motion. Collisional Approach. Partition Functions. Transition State Theory. Generalized Transition State Theory. Theory for Unimolecular Reactions. Classical Dynamics. Nonadiabatic Transitions. Surface Kinetics. Chemical Reactions in Solution. Energetic Aspects of Solvent Effects on Solutes. Models for Chemical Reactions in Solution. Kramers' Theory. The Classical Model of Electron Transfer Reactions in Solution. Appendices. Index.

Hydroxylation of phenol over Sn-silicalite-1 molecular sieve: solvent effects

Abstract: Tin-silicalite-1 (MFI) prepared by hydrothermal synthesis has been used as catalyst in the hydroxylation of phenol with aqueous H2O2. Isolated Sn4+ ions which are probably attached to the defect silanols are active in this reaction. At optimum conditions, a H2O2 efficiency of 70% and a ortho to para product ratio of 1.6 have been achieved. The solvent used has a strong influence on the activity. The UV-Vis spectral studies indicate that acetone and acetonitrile probably coordinate strongly with Sn4+ centers preventing the solvolysis of the SiOSn units and the formation of peroxo intermediate. Methanol probably causes the cleavage of SiOSn bond to form SiOH and SnOMe species. Water was found to be an efficient solvent. The formation of EPR active radical ion in presence of H2O2 and H2O and its attenuation in presence of acetone, acetonitrile and methanol support the above conclusion.

Solvent Effect on the NMR Chemical Shieldings in Water Calculated by a Combination of Molecular Dynamics and Density Functional Theory

Abstract: The solvent effect on the NMR chemical shielding in liquid water is calculated from a combination of molecular dynamics simulations and quantum chemical calculations for protons and 17O. The simulations are performed with three different potentials, ab initio as well as empirical ones, to study the influence of the force field. From the liquid configurations obtained in these simulations, molecules are randomly chosen together with neighbouring molecules to give clusters of water typical for the liquid at the selected temperature and density. Different cluster sizes are studied. The clusters are treated as supermolecules in quantum chemical calculations of chemical shifts by sum-over-states density functional perturbation theory with individual gauge for localised orbitals. The influence of the quantum chemical method is studied with an ab initio coupled Hartree-Fock gauge including atomic orbitals calculations with different basis sets for a selected cluster. An average over clusters yields the chemical shielding in the liquid at the selected temperature and density. The calculated values for the gas–liquid shift, which are in best agreement with experiment, are –3.2 ppm (exp. –4.26 ppm) for the proton and –37.6 ppm (exp. –36.1 ppm) for 17O, but the results depend strongly on the chosen interatomic potential.