Showing papers on "Solvent effects published in 1980"

On the quantum mechanical treatment of solvent effects

Abstract: A new formalism is proposed for incorporating solvent effects into the quantum mechanical description of molecular electronic states. In contrast to existing methods, it does not lead to a non-linear effective hamiltonian, while both the solvent/solvent and the solute/solvent interactions are treated self-consistently. It also accounts more accurately for the solute's electrical field than the usual dipole approximation. Although formally treated on the Hartree-Fock level, the method incorporates dispersion interactions between solute and solvent.

Hydrogenation of carbon monoxide to methanol and ethylene glycol by homogeneous ruthenium catalysts

Abstract: Hydrogenation studies of carbon monoxide to methanol and ethylene glycol by homogeneous ruthenium catalysts in several types of solvents are reported. In contrast to previous reports of catalysis under extreme pressures, these studies were conducted at pressures of 340 atm and below. Overall rates of carbon monoxide hydrogenation in these catalytic reactions (in several types of solvents) are nearly equal to those recently reported for a similar ruthenium system operated at much higher pressure - for example, a rate of 8.3 x 10/sup -3/ turnovers s/sup -1/ was observed, as compared with a reported rate (to methanol and methyl formate) of 1.05 x 10/sup -2/ s/sup -1/ at 270/sup 0/C under 1300 atm in THF solvent. This comparison exhibits the importance of solvent effects in homogeneous catalysis, even when the catalyst is presumably uncharged and mononuclear; rate improvements obtainable by large increases in pressure may also be achieved by appropriate choice of solvents. An even more important role of reactive, carboxylic acid solvents in this system is demonstrated by the discovery that they cause formation of a two-carbon product by a catalyst which otherwise produces only methanol. The function of this unique solvent/promoter is apparently to intercept a catalytic intermediatemore » and change the course of its reaction. Further research based on these results is in progress.« less

Stochastic trajectory simulation of iodine recombination in liquids

Abstract: A stochastic trajectory simulation of iodine recombination in dense liquid solvents is presented. The calculations utilize a mean force potential which contains direct I–I interactions as well as solvent structure effects. Dynamical solvent effects are accounted for by a random force and friction coefficient. The time dependent probability of reaction for two initially separated radicals is determined. The choice of initial separations and atomic velocity distributions is appropriate for secondary recombination. The results of this study show the importance of including the strong direct chemical forces between the I atoms; the validity of simple diffusion equation approaches can thus be assessed. Effects due to solvent structure are quantitatively examined and are interpreted in terms of ’’caging’’ in dense fluids. The computer simulation results are also compared with the solution of the Smoluchowski equation for this problem and effects due to friction coefficient variation are discussed.

Transition-state structure in the yeast alcohol dehydrogenase reaction: the magnitude of solvent and alpha-secondary hydrogen isotope effects

Abstract: Solvent and alpha-secondary isotope effects have been measured in the yeast alcohol dehydrogenase reaction, under conditions of a rate-limiting transfer of hydrogen between coenzyme and substrate. Determination of catalytic constants (at saturating concentrations of substrate and coenzyme) in H2O and D2O as a function of pH(D) has allowed the separation of solvent effects on pKa from kcat: delta pKa = pKD--pKH = 0.02--0.21, kH2O/kD2O = 1.20 +/- 0.09 in the direction of p-methoxybenzyl alcohol oxidation, and kH2O/kD2O = 0.50 +/- 0.05 and 0.58 +/- 0.06 for p-methoxybenzaldehyde reducation by NADH and [4-2H]NADH. The small effect of D2O on pKa, which contrasts with the common observation that delta pKa congruent to 0.4--0.6, is tentatively assigned to ionization of an active-site ZnOH2. The near absence of an isotope effect on kcat in the direction of alcohol oxidation rules out a mechanism involving concerted catalysis by an active-site base of hydride transfer. In the direction of aldehyde reduction, the observation of inverse isotope effects on kcat is concluded to reflect displacement of zinc-bound water by substrate to form an inner-sphere complex, subsequent to the E.S complex. Equilibrium alpha-secondary isotope effects, measured as a frame of reference for kinetic values, indicate KH/KT = 1.33 +/- 0.05 and 1.34 +/- 0.09 for the oxidation of [1(S)-3H]benzyl alcohol and p-methoxy[1(S)-3H]benzyl alcohol, respectively. Kinetic alpha-secondary isotope effects are within experimental error of equilibrium values, kH/kT = 1.34 +/- 0.07 and 1.38 +/- 0.02 for [1(S)-3H]benzyl alcohol and p-methoxy[1(S)-3H]benzyl alcohol oxidation, respectively. The near identity of kinetic and equilibrium alpha-secondary isotope effects in the direction of alcohol oxidation implicates a transition-state structure which resembles aldehyde with regard to bond hybridization properties. This result contrasts sharply with previously reported structure--reactivity correlations, which implicate a transition-state structure resembling alcohol with regard to charge properties. The significance of these findings to the mechanism of NAD(P)H-dependent redox reactions is discussed.

Influence of solvent perturbation on the radiative transition probability from the 1B1u state of pyrene

Abstract: Solvent effects on the radiative transition probability ( k F ) of pyrene from S 1 have been examined using an integrating sphere fluorimeter. The fraction of the total k F arising from the 0 0 band has been measured. The solvent effect on this quantity exactly parallels the extinction coefficient change for the 0 0 band with a rate of change in agreement with theory. The remainder of the transition probability from S 1 also shows an increase with increased solvent polarity which is not predicted from spectral observations. However, the overlaps of the S 0 → S 1 and S 0 → S 2 transitions may mask the effect. The Ham-band intensity was observed to decrease with increasing temperature, an effect which correlated with the decrease in dielectric constant with temperature. Evidence is presented to support the hypothesis that some type of specific solvent—solute interaction is involved in the Ham-band phenomenon in alcohols and aromatic solvents.

Solvent effect on the protonation of some alkylamines

Abstract: Thermodynamic data were obtained at 25 °C for the solution and protonation of six bases B: ammonia, three methylamines, and monoethyl and n-propylamines in dimethylsulfoxide (Me2SO) and propylene carbonate (PC). Vapour pressure, potentiometric, and calorimetric determinations were carried out. The solvent effect on the basicity order and protonation of the amines is analysed by using gas phase data and comparing the solution thermodynamics of gaseous B and BH+. The heat of solution of B in both solvents is close to its heat of condensation. The solution data for BH+ are discussed in terms of two recent hydration data interpretations.

The solvophobic theory for the prediction of molecular conformations and biopolymer bindings in solutions with recent direct experimental tests

Abstract: Molecules exposing considerable microsurface areas to the surrounding solvent such as amino acids, nucleotide bases in biopolymers, or various drug molecules, antigens, and substrates, tend to be pulled apart or pushed together as the case may be by the surrounding solvent medium. These solvophobic forces and their quantitative theory were introduced sometime earlier [O. Sinanoglu, in Molecular Associations in Biology, B. Pullman, Ed. (Academic, New York, 1968), pp. 427–445; and references therein]. The forces involve both enthalpy and entropy effects. The case in water had been shown to involve particularly strong forces but not differing in kind from other solvents. There are still somewhat contradictory views on the aqueous case, referred to in the literature as “hydrophobic bonding” treated as a phenomena unique to water and previously thought to involve only entropic effects. The temperature dependence and volumetric effects on the earlier type of “hydrophobic bonding” are presently not always reconcilable with recent experimental evidence. By contrast, the solvophobic force theory allows the calculation of the full solvent effect on various association or isomerization equilibria using its quite rigorous relations and basic data derived with it from liquids and solutions. [cf., e.g., for an early application to cis-trans azobenzene isomerization, T. Halcǐoǧlu and O. Sinanoglu, Ann. N.Y. Acad. Sci., 158, 308 (1969)]. The solvophobic force theory had also introduced as a measurable new quantity, “the thermodynamic microsurface area change of a reaction,” which is now finding considerable use in protein chemistry [cf., e.g., F.M. Richards and T. Richmond, in CIBA Symposium Proceedings on Molecular Interaction and Activity in Proteins (CIBA, New York, 1977)]. Solvophobic force theory has been tested and applied recently in detail in high pressure liquid chromatography (HPLC) by C. Horvath, W. Melander, and I. Molnar [J. Chromatograph. 125, 123 (1976)] who verified the predicted temperature, molecular surface area, and salt concentration dependence. It worked well when applied in detail to the complicated methanol–water mixtures as well. The theory also gave the first a priori derivation of the experimentally well-known protein salting-in–salting-out curve [W. Melander and C. Horvath, Arch. Biochem. Biophys. 183, 200 (1977)]. We have now extended the theory further with new basic derivations which eliminate the need for the direct calculation of some cumbersome effects. The theory has also been recently applied to a number of new areas including drug-receptor interactions, aqueous amino acid interactions, and to multicomponent phase equilibria; the latter also of chemical engineering interest. Solvophobic interactions, although part statistical thermodynamical in nature, were shown to be usable as if they were an ordinary U(R) potential added on to quantum-chemical intrinsic in vacuo potentials for prediction of conformations in solution [O. Sinanoglu, in The World of Quantum Chemistry, R. Daudel and B. Pullman, Eds. (Reidel, Dordrecht, 1974)]. There have also been some recent applications that illustrate the convenience of our solvophobic theory in correcting for solvent effects once quantum-mechanical molecular electronic structures and potential-energy surfaces are computed.

The isoimide–imide rearrangement

Abstract: When imidoyl chlorides (8) are solvolysed in the pH range 3–13 in aqueous dioxan in the presence of acetate or benzoate ions, the isoimides (10) are formed in situ These isoimides undergo acid (pH 115) catalysed acyl transfer to the solvent, giving the amides (12) But at intermediate pH (6–11) rearrangement to the N-acyl form (11)(the Mumm rearrangement) alone occurs The specific rate of this O → N acyl group migration is pH independent and shows a low solvent effect (m 0175) Substituents attached to carbon or nitrogen have the same effect (ρ–084) When the migrating group R is varied a non-linear Hammett plot is observed, with ρ+060 for electron-withdrawing and +165 for electron-donating substituents The changeover point is a function of the migrating terminus varying from R = Ph (when Y = H) to R =p-tolyl (when Y =p-NO2) Rate-determining O → N acyl transfer is suggested in all cases, but this is preceded by an equilibrium which favours the Z form (10a) when R is electron donating

Solvent Effect on the Adsorption of 1-Octadecanol at Oil/Water Interface

Abstract: In order to study the effect of solvent on the adsorption at interfaces, interfacial tension was measured as a function of temperature and compositon for the systems of water and solutions of 1-octadecanol in cyclohexane, cyclohexene, mesitylene, toluene, and benzene. The interfacial density of 1-octadecanol and the entropy and energy of interface formation were calculated thermodynamically. It was found that cyclohexane causes a significant adsorption of 1-octadecanol and marked decreases in the entropy and energy changes, giving rise to phase transition from expanded to condensed film, while benzene, toluene, and mesitylene cause only a slight adsorption and small decrease in thermodynamic quantities. Cyclohexene was found to give rise to an intermediate adsorptive behavior of 1-octadecanol. The results were interpreted in terms of the mutual solubility between water and solvents and that of 1-octadecanol in the solvents.

Normal‐mode vibrational frequencies and thermodynamic properties of uracil and 4‐hydroxyuracil

Abstract: Normal-mode vibrational frequencies were obtained for uracil and 4-hydroxyuracil with the MINDO/3 SCF method. MINDO/3 vibrational frequencies and the moments of inertia for STO-3G optimized geometries yielded a ΔS for U ⇄ U* of 1.06 e.u./mol by standard statistical thermodynamic equations. ΔG is then 6.29 kcal/mol in the gas phase for U ⇄ U*. The reaction field continuum model of solvent effect lowers ΔG to 5.3 kcal/mol in aqueous solution. The tautomeric equilibrium constant in solution is then 1.3 × 10−4, in good agreement with experimental values.

Solvent effect on the rates of uncatalyzed isomerization and ligand substitution at a square-planar platinum(II) complex

Abstract: In a series of papers [1] we have suggested a dissociative asynchronous mechanism for the uncatalyzed cis-trans isomerization of the complexes cis -[Pt(PEt 3 ) 2 (R)X] (X = halide ions; R = alkyl or aryl groups). This mechanism contrasts with the well-known capability of the metal to undergo bimolecular attack and with the general mechanistic picture established for the nucleophilic displacement of ligands in square planar complexes. In this study we use solvent effect as a further criterion to distinguish between associative and dissociative reaction paths. The rates of isomerization, solvolysis and nucleophilic substitution of the complex cis -[Pt(PEt 3 ) 2 ( m -MeC 6 H 4 )Cl] have been measured in a series of alcohols and in acetonitrile. These processes represent three different ways for removing the chloride ion from the coordination sphere of the metal; the magnitude of the solvent effect is found to depen on the degree of charge separation occuring in the transition state. For all the solvents employed, the rates of solvolysis (path2) are of many order magnitude greater than the rates of isomerization. This large difference of reactivity, already observed and discussed in methanol [1c], argues strongly against a bimolecuar solvolysis being the initial step of the isomerization and rules out the possibility of a common intermediate for either solvolysis or isomerization. The rates of solvolysis as well as that of nucleophilic displacement by thiourea are little influenced by solvent changes, in keeping with the nature essentially associative of the activation processes, which do not require large changes of polarity or charge formation. By way of contrast, the spontaneous cis-trans isomerization of [Pt(PEt 3 ) 2 ( m -MeC 6 H 4 )Cl] is extremely sensitive to changes in the nature of the solvents employed, as shown by a decrease in rate of four orders or magnitude on going from methanol to 2-methyl-2-propanol. Fairly good straight lines correlate these rates and some empirical parameters which measure the relative electrophilic character of the solvents, such as α values of the Taft HBD scale or the Dimroth-Reichard's E T values. The role of the solvent is to promote the breakin of the PtCl bond in the rate determining step in leading to a cationic intermediate. Electrophilic solvation of the leaving chloride ion through nitrogen bonding is the major factor in determining the large differences in rates observed, the contribution of non specific solvent-solute interactions being negligible or constant.

Laser temperature jump study of solvent effects of poly(adenylic acid) stacking.

Abstract: The dynamics of the single-stranded, helix--coil transition of poly(adenylic acid) have been investigated by using the Raman laser temperature jump technique. The driving forces of this conformational transition have been probed by varying the cosolvent in mixed aqueous solutions. The rate of helix formation correlates well with the inverse of the solution viscosity. This correlation and the low activation barrier of approximately 4 kcal/mol for this process indicate a rotational diffusion controlled reaction. The rate of coil formation has a much higher activation barrier, presumably due to the strength of base stacking in the helix. This unstacking rate is virtually unaffected by alcohol cosolvents. A significant increase in this rate occurs when polar cosolvents such as urea, formamide, or acetonitrile are added. Absorbance changes with temperature suggest that urea specifically solvates the adenine base. The polar cosolvents appear to break up the stacked, helical conformation by interacting with the bases.

Donor-acceptor complex chromatography preparation of a chemically bonded acceptor-ligand and its chromatographic investigation

Abstract: The preparation of a TNF-phase for HPLC is described. From chemical and chromatographic evaluation it is concluded that chromatographic separation is the result of donor-acceptor complex formation. These findings are supported by the results of an investigation of solvent and temperature effect on retention. For solvent effects two models are discussed. According to the first model based on Fialkov and Borovikov a linear plot is obtained for log k′ against 1/∈ for 1/∈<0.4. In the second model it is shown that the Mass Law is appropriate to describe the solvent effect of binary mixtures of n-hexane and some polar component. Plots of k′ against 1/[S]0.5 yield straight lines in all cases and the stoichiometry of complex formation is discussed. The slopes of the lines could be shown to be a measure of solvent strength and are found to parallel Snyder's ∈∘ parameters. From the temperature dependence of log k′ using the van't Hoff relationship changes in ΔH∘ for complex formation are obtained.

Specific solvent effects on the thermal denaturation of ribonuclease. Effect of dimethyl sulfoxide and p-dioxane on thermo dynamics of denaturation.

Abstract: Differential scanning calorimetry has been used to study the thermal denaturation of ribonuclease in aqueous dimethyl sulfoxide and acqueous p-dioxane. A two-state reversible denaturation occurs in acqueous dimethyl sulfoxide. The denaturation is irreversible in aqueous p-dioxane. The transition temperatuire decreases in both organic solvents, with p-dioxane producing a considerably greater effect. The enthalpy of the denaturation initially increases with increasing dimethyl sulfoxide concentration and then decreases at high concentrations. Similar behavior is observed in the entropy in the dimethyl sulfoxide solutions. The enthalpy of the denaturation decreases markedly with low p-dioxane concentrations. Changes in pH due to presence of organic solvent cannot account for the changes in enthalpy of denaturation. Addition of organic solvent tends to increase the pH of the ribonuclease solution. Higher pH in aqueous solutions gives a high transition temperature, whereas addition of organic solvent results in the opposite bahavior. The relative changes in the two solvents systems suggest that specific solvent effects occur and that destablization of the native state relative to the denatured state is greater with p-dioxane than with dimethyl sulfoxide.

Local Field Representation of Surrounding Medium Effects. From Liquid Solvent to Protein Core Effects

Abstract: An effective Schrodinger equation describing a small portion of a larger (macroscopic) system is derived from a general quantum/statistical mechanical approach. Only electrostatic interactions are allowed for between the subsystem. The effect of the environment (large position) on the solute (small position) is represented with a temperature dependent electrostatic potential. It is shown that, in general, this potential depends on the electronic wave function of the solute. This feature leads to a non-linear Schrodlnger equation. The non-linearity is being represented via the. moments of the. solute electronic density. In thu manner an adequate, framework is worked out which permits the. discussion of several local field theories of solvent effects that would otherwise Amain as singular models. A description of systems as digerent as diluted Liquid, solid or matrix solutions, defaults in crystalline environments, solvated electrons, and reactive subsystems in globular proteins can be given within this rather flexible theoretical approach.

Solvatons. II. Aqueous dissociation of hydrides in the MINDOS approximation

Abstract: A simple electrostatic model of solvation is presented which allows the interaction with solvent to be included systematically within semiempirical SCF calculations. Solvent effects are incorporated into the Hamiltonian for a solute molecule through a series of imaginary particles, solvatons, which represent the oriented solvent distribution around the solute.

The mechanism of oxidation of acetals by ozone. I. Stoichiometry, order of the reaction, solvent effects, and substituent effects

Abstract: A systematic investigation of the reaction between ozone and acetals to form acetal hydrotrioxides A has been undertaken. The stoichiometry of the reaction has been shown to be 1:1 in each reactant and the order of the reaction was also one in each reactant. Substituent effects measured in a variety of systems and under several conditions of temperature and solvent were found to be small (ρ = −1.10 to −1.58). Solvent polarity was also found to have little effect on the rate of the reaction. Mechanistically, these facts are interpreted in terms of a 1,3-dipolar insertion of ozone into the C—H bond of the acetal function.

Solvent effect on the complex formation of poly(methacrylic acid) with proton‐accepting polymers

Abstract: A pair of polymers may interact with each other through hydrogen bond and form a polymer complex. Such polymer complex formation is affected by several chemical and physical factors, especially by solvent properties. This solvent effect was studied in the complex formation of poly(methacrylic acid) with a series of proton-accepting polymers in aprotic solvents. The extraordinary behaviour of polymer complexes in dimethyl sulfoxide was well explained in terms of the strong interaction between proton-donating polymer and dimethyl sulfoxide. In addition, a stronger proton-accepting polymer, viz. poly(N,N-dimethyl-N′,N′,N″,N″-tetramethylenephosphoramide), was synthesized. This polymer could form a polymer complex with poly(methacrylic acid) through hydrogen bond even in dimethyl sulfoxide. Selective polymer complex formation was achieved by controlling such solvent properties.

The silica-gel surface and its interactions with solvent and solute in liquid chromatography

Abstract: The multilayer formation of water on silica gel is discussed, and experiments indicating the existence of three layers of water are described. The interaction of active silica gel with solvent is also considered. Activated silica gel as used in chromatography appears to contain one strongly hydrogen-bonded water molecule per silanol group which can dispersively interact with non-polar solvents forming a monolayer. In contact with a polar solvent, however, the hydrated silanol group can hydrogen-bond to the polar solvent and form a strongly held solvent layer on top of which a bilayer of polar solvent can form by polar interactions with the first layer. The mechanism of solute interaction with activated silica can be described as follows. Solutes interact with the multilayer surface in a chromatographic column in two ways. If the solvent layer is weakly held by dispersive forces as in the case of a non-polar solvent, the solute can displace the solvent layer and interact directly with the hydrated hydroxyl groups. If, on the other hand, the solvent is polar and is strongly held by hydrogen-bonding forces to the hydrated silanol group, solutes may associate directly with the polar solvent layer but not displace it unless the solute has a polarity similar to the solvent in which case it is consequently eluted at a high k′ value.At low concentrations of polar solvent only a small amount of the second layer of weakly held solvent is formed and thus the interaction of a solute with the surface will be with the primary layer of polar solvent. Under such circumstances, changes in retention resulting from changes in solvent composition will reflect changes in solute interactions with the mobile phase and not with the surface of the stationary phase. Such a system has been examined; it was shown that the probability of polar interactions in the mobile phase were directly related to the concentration of polar solvent, and this was substantiated by results obtained from the examination of liquid/liquid distribution systems. Evidence was also provided that indicated that the magnitude of polar interactions was related to the polarizability per cm3 of the interacting substances.