Showing papers on "Solvent effects published in 1988"

Intramolecular vibrational excitations accompanying solvent‐controlled electron transfer reactions

Abstract: The theory of dynamic solvent effects on outer‐sphere electron transfer (ET) was extended to incorporate the modification of the high‐frequency quantum modes, which is manifested by the reduction of the electronic coupling by nuclear Franck–Condon factors and by the change of the energy gap Explicit expressions for the ET rates were obtained in terms of a sum over parallel vibronic channels, each involving a distinct intramolecular vibrational excitation of the final state In the solvent‐controlled adiabatic limit, the effects of intramolecular vibrational excitation are exhibited by the modification of the (partial) activation energies, while the frequency factor is dominated by the longitudinal dielectric relaxation rate of the solvent

Structural and solvent effects evaluated from acidities measured in dimethyl sulfoxide and in the gas phase

Abstract: with the experimental log kobsd values. Application of the test to reactions with Ph2CHCl, which gives S N ~ kinetics and products with no evidence of radical-type products, gave linear Marcus-type plots and log ksET values corresponding to the experimental log kobsd values. It was concluded that the “SN2 reactions” of 9-R2N-Flions with Ph2CHCl are occurring by a radical pair mechanism. This approach promises to be of general use for elucidating the role of SET in reactions of families of anions with electrophiles. Concluding Remarks. The Me2S0 acidity scale, for which about 300 representative values are given in Table 11, furnishes (a) quantitative acidity data that can

Solvent effects in organic chemistry — recent developments

Abstract: Solvent effects on a number of different processes have been surveyed, and results of the application of multiple linear regression analysis are discussed. The processes examined include examples o...

Solvent-induced polymorphism in syndiotactic polystyrene

Abstract: Preparation de fibres par moulage de poudre, trempe rapide, etirage, gonflement dans differents solvants et sechage sous vide

A multiconfigurational self‐consistent reaction‐field method

Abstract: We present theory and implementation for a new approach for studying solvent effects: the multiconfigurational self‐consistent reaction‐field (MCSCRF) method. The atom, molecule, or supermolecule is assumed to be surrounded by a linear, homogeneous, continuous medium described by its macroscopic dielectric constant. The electronic structure of the compound is described by a multiconfigurational self‐consistent field (MCSCF) wave function. The wave function is fully optimized with respect to all variational parameters in the presence of the surrounding polarizable dielectric medium. We develop a second‐order convergent optimization procedure for the solvent states. The solvent integrals are evaluated by an efficient and general algorithm. The flexible description of the electronic structure allows us to accurately describe ground, excited, or ionized states of the solute. Deficiencies in the calculation can therefore be assigned to the cavity model rather than the description of the solute.

Urea as the most reactive and versatile nitrogen nucleophile for the palladium(2+)-catalyzed cyclization of unsaturated amines

Abstract: Des urees substituees en position 1 par un alcenyl: allyl, butene-3yl, pentene-4yl et hexene-5yl, subissent une aminocarbonylation pour donner les imidazolidones-2, pyrimidones-2, diaza-1,3 bicyclo [4.3.0] nonadiones-2,4, et diaza-1,3 bicyclo [4.4.0] decanediones-2,4, correspondantes

Electronic dephasing, vibrational relaxation, and solvent friction in molecular nonlinear optical line shapes

Abstract: The role of solvation dynamics in molecular nonlinear optical line shapes is analyzed using a reduced description based on the time evolution of the density matrix in Liouville space. Langevin equations are used to treat the coupling of the solvent to the molecular electronic and nuclear degrees of freedom. Electronic dephasing is calculated using a solvation coordinate which satisfies a generalized Fokker–Planck equation, and vibrational relaxation is related to the solvent viscosity and friction. The combined effect of both processes is incorporated into appropriate multitime correlation functions which are evaluated using a Liouville‐space generating function. The present eigenstate‐free approach is particularly suitable for calculating spectral line shapes as well as rate processes (isomerization, electron transfer) of large polyatomic molecules in condensed phases.

Evidence for the essential role of hydrogen bonding in promoting amphiphilic self-assembly: measurements in 3-methylsydnone

Abstract: Mesure de la conductivite electrique et de la tension superficielle d'agents de surface cationiques en solution dans la methyl-3 sydnone

The dynamics of polar solvation: Inhomogeneous dielectric continuum models

Abstract: The influence of an inhomogeneous dielectric response on the dynamics of solvation of ions and dipoles is investigated. Solvent models considered include discrete shell models as well as models in which the solvent dielectric constant varies continuously as a function of distance from a spherical solute. The effect of such dielectric inhomogeneity is to introduce additional, slower relaxation times into the solvation response when compared to the homogeneous case. For all models studied, the deviation of the average relaxation time from that predicted for a homogeneous continuum solvent increases as the dielectric constant and the length parameter, which specifies the rapidity of approach to bulk behavior, increase. For a given solvent model the solvation response to a change in a point dipole moment is slower than the response to a charge jump. The continuum results are compared to a recent molecular model based on the mean spherical approximation. The comparison suggests that deviations from homogeneous continuum behavior in the molecular model can be accounted for by inhomogeneity of the solvent dielectric constant extending only over the first solvation shell. Predictions of inhomogeneous continuum models are also compared to experimental data. Both the observed dependence of average relaxation time on dielectric constant, and the detailed time dependence of the relaxation in high dielectric constant solvents can be rationalized on the basis of such models.

The importance of vibrational motion and solvent diffusional motion in excited state intramolecular electron transfer reactions

Abstract: Time resolved emission spectroscopy has been used to examine the dynamics of intramolecular charge transfer in dimethylaminobenzonitrile (DMABN) and diethylaminobenzonitrile (DEABN) in linear alcohol solutions as a function of temperature. For both DMABN and DEABN in methanol and DMABN in ethanol solutions, the population decay of the local excited (LE) state can be fit by a single exponential function. However, over the temperature range examined, 0 to −50 °C, the population decay of the local excited state in longer chain alcohol solutions (ethanol, propanol, butanol, pentanol, and hexanol) cannot be fit by a single exponential. The average survival probability of the LE state Q(t) is obtained by fitting the population decay to a multiexponential function. In all of the alcohol solvents studied, the average lifetime of Q(t) is faster than the solvent fluctuation rate gauged by the longitudinal relaxation time of the solvent τL(τDe∞/es) corresponding to the slow collective hydrogen bonding dynamics. Comp...

Solvent Dynamical Effects in Electron Transfer: Comparisons of Self-Exchange Kinetics for Cobaltocenium-Cobaltocene and Related Redox Couples with Theoretical Predictions.

Abstract: : Rate constants, kex, and activation parameters for the self exchange of cobaltocenium-cobaltocene, Cp2Co, and the decamethyl derivative, Cp2Co, in thirteen organic solvents have been evaluated using the proton nmr line-broadening technique with the objective of probing the influence of solvent dynamics upon the electron-transfer kinetics. Together with some, corresponding measurements reported earlier for ferrocenium ferrocene, Cp2Fe, additional measurements for the decamethyl derivative, Cp2Fe, and with corresponding data for Cp2Co electrochemical exchange, these results enable a systematic comparative examination to be made of the effects of solvent dielectric relaxation on the barrier crossing frequency for such simple outer-sphere reactions. For the facile Cp2Co couple, the solvent dependence of the 'observed' frequency factors, extracted from the kex values by correcting for the solvent dependent barrier height, is an approximate accordance with the relative frequency factors, predicted from the continuum model of overdamped solvent relaxation. Keywords: Self-exchange kinetics, Solvent-dynamical effects, Cobaltocenium-cobaltocene redox couples, Nuclear magnetic resonance.

Dependence of absorption spectra and solubility of poly(3-alkylthiophene) on molecular structure of solvent

Abstract: Solvatochromic shifts of spectra of solutions of poly(3-hexylthiophene) and poly(3-docosylthiophene) have been analyzed in terms of Taft linear solvation energy relationships. Color changes of solutions of the former at 25°C are primarily due to changes in normal solute-solvent interactions. On the other hand, color changes of solutions of poly(3-docosylthiophene) appear to be due to configurational changes of the polymer chain. Solubility of the polymer is affected by solvation of the alkyl groups, electron donating or Lewis basicity properties of the solvent, polarity, and molecular volume of solvent molecules.

Effects of the Nature of the Solvent and Solutes on the Response of a Light-Scattering Detector

Abstract: The effects of the nature of the solvent and solutes on the response of an evaporative light scattering detector (ELSD) are discussed. Data have been measured for six solutes with seven different solvents using concentric and cross-flow nebulizers. The experimental results suggest that the influence of the solvent on the properties of a dried aerosol are not properly accounted for by the equation of Nukiyama and Tanassawa. A modification of the droplet size distribution of the aerosol takes place, probably during the vaporization of the solvent. The amount of scattered light depends strongly on the molar absortivity of the solute at the light beam wavelength. Accordingly, the detector response depends on the properties of the solvent and the solute. The ELSD is not a mass detector and calibration is required for quantitative analysis.

On the approximation of solvent effects on the conformation and dynamics of cyclosporin a by stochastic dynamics simulation techniques

Abstract: The molecular simulation technique of stochastic dynamics (SD) is tested by application to the immunosuppressive drug cyclosporin A (CPA). Two stochastic dynamics simulations are performed, one (SDCCl4 ) with atomic friction coefficients proportional to the viscosity of the nonpolar solvent CCl4, and one (SDH2O) with atomic friction coefficients corresponding to an aqueous solution. The atomic friction coefficients are also taken proportional to an approximate expression for the atomic accessible surface area. The properties of both stochastic dynamics simulations are compared to those of two full molecular dynamics (MD) simulations of cyclosporin A, one in a box with 591 CCl4 molecules, and one in a box with 632 H2O molecules. The properties of cyclosporin A as found in the molecular dynamics simulation in CCl4 are well reproduced by the SDCCl4 simulation. This indicates that the neglect of a mean force reresenting the average solvent effects on the solute is justified in the case of nonpolar so...

The molecular basis of temperature effects in supercritical extraction

Abstract: The behavior of solute partial molar enthalpies in dilute supercritical mixtures gives rise to the well-known phenomenon of retrograde solubility (equilibrium solubility decreasing with increasing temperature at constant pressure). A mechanistic interpretation of this phenomenon in terms of the formation of large clusters of solvent molecules around solute molecules is consistent with experimentally observed retrograde behavior. Cluster formation occurs as a consequence of the unbounded increase in the solvent's compressibility arbitrarily close to the latter's critical point. At infinite dilution, the solute's partial molar volume and enthalpy grow linearly with cluster size. This means that the negative divergence of these quantities is simply a macroscopic manifestation of a “condensation” in which an increasing number of solvent molecules cluster around solute molecules. Arbitrarily close to the solvent's critical point, scaling relationships show that the decay of both solvent-solvent and solute-solvent correlation functions exhibits the same radial dependence. This functionality is thus solute-independent, and is determined exclusively by the proximity to the solvent's critical point. The locus along which thermal effects associated with cluster formation are maximized is arbitrarliy close to the solvent's critical isochore as the latter's critical point is approached.

Photophysics of a cofacial porphyrin-quinone cage molecule and related compounds: fluorescence properties, flash transients, and electron-transfer reactions

Abstract: Fluorescence yields, fluorescence lifetimes, and flash photolysis transients have been measured for a tetrabridged cofacial porphyrin-quinone cage molecule and related derivatives. When the quinone is present and in the oxidized form, two fluorescence lifetimes are observed for the zinc complex, both shorter than the single lifetimes of control compounds. These are assigned to two conformers, with porphyrin to quinone interplanar separation of about 8.5 and 6.5 A, respectively. The fluorescence lifetime data lead to electron-transfer times for the two conformers of between 0.5 and 15 ns. The fast electron-transfer time is only weakly dependent on solvent and is independent of temperature between 290 and 124 K. Following flash photolysis of the zinc-porphyrin-quinone in polar solvents, a transient band at 415 nm grows in more slowly (150 ns) and decays much more rapidly (1.4 ..mu..s) than the triplet seen in control compounds and is assigned to a charge-separated ZnP/sup +/Q/sup -/ state. The reaction leading to this state exhibits an activation energy of only 6 kJ/mol. In contrast to the zinc chelate, the free base porphyrin-quinone shows no electron-transfer interactions. These results support a distance-sensitive nonadiabatic electron-tunneling mechanism for the transfer. Effects of solvent and protonation are interpreted in terms ofmore » conformational changes that modify the porphyrin-quinone distance.« less

Evidence for dynamical solvent effects on the photoisomerization of 4,4'-dimethoxystilbene

Abstract: The photoisomerization dynamics of 4,4'-dimethoxystilbene have been studied in n-alkanes and n-alkyl alcohols as a function of temperature at ambient pressure. By comparison with unsubstituted stilbene results the influence of the methoxy group, both intramolecular and intermolecular, on the isomerization process has been evaluated. The studies in n-alkane solvents reveal larger barrier heights and a weaker viscosity dependence for dimethoxystilbene compared to those of stilbene. This observation is in agreement with earlier ideas concerning frequency-dependent solvent effects but contrary to free volume models of solute-solvent friction. The studies in n-alkyl alcohol solvents demonstrate the inability of separating intramolecular from intermolecular effects. The potential energy surface associated with the reaction is strongly coupled with solvent dynamics, hence time dependent. These results demonstrate the importance of understanding microscopic friction and dielectric interactions on ultrashort time scales.