Showing papers on "Solvent effects published in 1981"

The SN2-SN1 spectrum. 3. Solvolyses of secondary and tertiary alkyl sulfonates in fluorinated alcohols. Further evidence for the SN2 (intermediate) mechanism

Abstract: Kinetic data are reported for solvolyses of secondary and tertiary alkyl tosylates in trifluoroethanol and hexa- fluoroisopropyl alcohol and also for 1-adamantylmethylcarbinyl (VI) and l-bicyclo(2.2.2)octy1 (VII) tosylates in a wide range of solvents. The relative solvolysis rates of 2-adamantyl (I), 1-adamantylmethylcarbinyl (VI), and l-bicyclo(2.2.2)octyl (VII) tosylates are essentially independent of solvent (in ethanol, methanol, water, trifluoroethanol, hexafluoroisopropyl alcohol, acetic acid, formic acid, and trifluoroacetic acid). Thus these three substrates are good models for sN1 (k, or limiting) mechanistic behavior; they respond almost identically to changes in solvent ionizing power and are insensitive to changes in solvent nucleophilicity. In contrast relative solvolysis rates of 2-adamantyl and 2-propyl tosylates vary with solvent 105-fold from 134 in hexafluoroisopropyl alcohol to 0.001 1 in ethanol. For straight-chain secondary alkyl tosylates, logarithms of solvolysis rate constants in hexafluoroisopropyl alcohol correlate with u* and give a large negative p* value (-9.1). Other solvents give less negative p* values (e.g., CF3C02H, p* = -7.3; CF3CH20H, p* = -5.2; H20, p* = -4.3) and smaller 2-adamantyl/2-propyl (2-AdOTs/Z-PrOTs) rate ratios. Increasing amounts of nucleophilic solvent assistance in the more nucleophilic solvents leads to decreased electron demand by the cationic center (i.e., less negative p*), and solvolyses of 2-propyl become more rapid than 2-adamantyl. Solvent effects on the relative reactivity of secondary alkyl tosylates (ROTS) are correlated accurately by using the linear free energy relationship: log (k/kO)ROTs = Q'log (k/k&.~d~~~ (k/ko)~.~d~~~ + (1 - e') log (k/k0)2.~~~ where k refers to any solvent, ko refers to 80% ethanol/water (v/v), and Q'is an adjustable blending parameter. The high precision of correlations using this equation for 2-butyl, 2-pentyl, 3-pentyl, 4-heptyl, cyclopentyl, cyclohexyl, and cycloheptyl tosylates provides evidence for a gradual change of mechanism from SN2 (one-stage) through sN2 (intermediate) to SN1 mechanisms. Solvolyses of 3-methyl-2-butyl tosylate show significant sensitivity to nucleophilic solvent assistance (Q' = 0.42). Solvolyses of pinacolyl (11), 2-exo-norbornyl (111), 2-endo-norbornyl (IV), menthyl (V), and cyclooctyl tosylates are either k, or kA (not k, as proposed by others), since they respond less to changes in solvent ionizing power than the k, mechanistic models (I, VI, VII).

Spectral evidence for a rapidly formed structural intermediate in the refolding kinetics of hen egg-white lysozyme.

Abstract: For investigation of the conformation of the unfolded species and its role in the refolding kinetics, refolding kinetic measurements were made on hen egg-white lysozyme by using the stopped-flow method at 25 degrees C in the four sets of initial and final folding condition: (1) 4 M guanidinium chloride (GdmCl) and 0.5 M GdmCl; (2) 40% acetic acid (HOAc) and 5% HOAc; (3) 4 M GdmCl and 0.5 M GdmCl-5% HOAc; (4) 40% HOAc and 0.5 M GdmCl-5% HOAc. The kinetic results as measured by absorbance at three wavelengths, 301, 292, 250 nm, agreed with each other and indicated strict biphasic behavior without exception. The kinetic parameters were determined only by the final refolding conditions. The spectral properties of the unfolded species at the end of stopped-flow mixing were investigated by comparing the total kinetic amplitude with the difference between the static absorbance of the native molecule in the final refolding conditions and that of the unfolded molecule in the initial unfolding conditions. The solvent effect was considered in the comparison. It was concluded that the unfolded species assumed a new transient conformation in the mixing process and that the transformation was completed within the mixing time.

Some quantum chemical aspects of solvent effects on NMR parameters

Abstract: The non-bonded interactions which occur between solute and solvent molecules are discussed and the various models which describe the interactions are presented. The models are employed, within a quantum chemical framework, to estimate the extent of medium effects on chemical shifts and spin-spin couplings. Comparison between the calculated and available experimental data is provided for some first row nuclei.

Saddle point model for atom transfer reactions in solution

Abstract: A simple classical model for a collinear atom transfer (exchange) reaction in solution is investigated. The reaction is analyzed in terms of reactive and nonreactive modes in a saddle point region. Dynamical solvent effects are treated at the generalized Langevin equation level. The reaction rate constant k is determined and compared to the transition state theory prediction kTST. It is found that, for typical reactions governed by sharp potential barriers, the solvent is not very effective in the saddle region in reducing k much below kTST. In addition, solvent‐induced coupling of the reactive mode to nonreactive modes of motion typically has only a small influence on k. Some limitations of the model are discussed.

Direct evidence for solvent coordination in migratory carbon monoxide insertion

Abstract: Two theories have been advanced to explain the large solvent effects on the stereochemistry of CO migratory insertion in organotransition metallic compounds: 1) generalized stabilization of the migratory insertion transition state by solvation and 2) direct attack of solvent at the metal center. A technique is described here that was applied to the study of the reaction of CpMo(CO)/sub 3/CH/sub 3/ where Cp = eta/sup 5/ -C/sub 5/H/sub 5/ with varying but at least 5-fold molar excess concentrations of diphenylmethyl phosphine in tetrahydrofuran (THF) at 60/sup 0/C. The formation of reaction product accompanies the first-order disappearance of starting materials. The same rate law and similar rate constants were noted in 3-methyl- and 2-methyltetrahydrofuran. These solvent effects can be explained only in the context of direct attack of the solvents at the metal center as the alkyl migration is occurring. (BLM)

Solvent effects on the kinetics and thermodynamics of stacking in poly(cytidylic acid).

Abstract: The Raman laser temperature-jump technique has been used to measure the kinetics of the coil to helix reaction of poly(cytidylic acid) [poly(C)] in aqueous cosolvent mixtures. The rate of helix formation has a low activation energy and is proportional to reciprocal solvent viscosity. The observations suggest helix formation is rotationally diffusion controlled. The rate of coil formation in poly(C) has an activation energy of approximately 11 kcal/mol, presumably reflecting the electronic stacking interactions which stabilize the helix. Viscous cosolvents, glycerol or sucrose, slow down the rate of coil formation; acetonitrile and formamide at 5 mol % increase the rate relative to that in water. The polar cosolvents may specifically attack a cytosine stack. The absorbance vs. temperature profiles for poly(C) are analyzed with the one-dimensional Ising model. When only optical data are used, the cooperativity parameter, sigma, and the enthalpy, delta H, cannot be uniquely determined. A method is proposed that allows determination of sigma by combining spectroscopic and calorimetric data. The values of sigma derived for poly(C) are between 0.8 and 1.0, and delta H is about -9 kcal/mol of stack. An alternative method using integration of the excess heat capacity curve and extrapolation to fully stacked and random coil species yields a delta H of -7 kcal/mol of stack.

Solvent effects on the thermodynamics of double-helix formation in (dG-dC)3

Abstract: The thermodynamics of double-helix formation by (dG-dC)3 have been measured in aqueous solvent mixtures containing 10 mol % methanol, ethanol, 1-propanol, formamide, N,N-dimethylformamide, or urea and 20 mol % ethanol. Optical activity measurements indicate the conformation of the double helix at 3 degree C is the same in all the solvent mixtures except 20 mol % ethanol. All the cosolvents destabilize the helix relative to water. With 10 mol % alcohol cosolvents, this destabilization is associated with a more unfavorable entropy change averaging approximately 8% and a more favorable enthalpy change averaging approximately 5%. This is consistent with a small contribution of hydrophobic bonding to stability. In contrast, the destabilization by formamide, N,N-dimethylformamide, and urea is associated with a more unfavorable enthalpy change averaging approximately 23% and a more favorable entropy change averaging approximately 21%. Since all three of these cosolvents have dipole moments larger than water, this is consistent with increased competition for dipolar interactions between the nucleic acid bases. None of the results correlate with any one bulk solvent parameter such as surface tension, viscosity, or dielectric constant. With 20 mol % ethanol, optical activity measurements are consistent with a partial B to C form transition. This is associated with a 27% less favorable enthalpy and 25% more favorable entropy for helix formation relative to water. Since the B to C transition is associated with helix dehydration, this may imply a significant contribution of bound water to stability.

Solvent effects on the visible absorption spectrum of crystal violet

Abstract: Addition of polar solvents to non-polar solutions of crystal violet results in dramatic changes in the visible absorption spectra. The observed changes are explained in terms of crystal violet exis...

The separation of polar and steric effects. Part 14. Kinetics of the reactions of benzoic acid and of ortho-substituted benzoic acids with diazodiphenylmethane in various alcohols

Abstract: The main results now discussed are rate coefficients for the reactions of benzoic acid and 32 ortho-substituted benzoic acids with diazodiphenylmethane (DDM) at 30.0 °C in 11 alcohols including 2-methoxyethanol. The reaction involves a rate-determining proton transfer. The results have been subjected to correlation analysis in two ways. (a) For the reactions of a given acid in the various alcohols, the log k values are correlated through multiple regression on appropriate solvent parameters. (b) For reactions in a given alcohol, the log k values for the various acids are correlated by using the appropriate form of the extended Hammett equation involving inductive, resonance, and steric parameters. Analyses of type (a), as in earlier work, involve the σ* value of the group R in the alcohol ROH, the Kirkwood function of dielectric constant, f(Iµ)=(Iµ– 1)/(2Iµ+ 1) and nγH, the number of hydrogen atoms in the γ-position in the alcohol. These correlations yield information on the influence of ortho-substituents on the relative importance of specific and non-specific solvent effects. In analyses of type (b) the correlation equations are established with a limited set of substituents (up to 18), whose characteristic parameters σI, σR, and v(determined by Charton) are well established, and are unlikely to show effects due to hydrogen-bonding or substituent conformation. The results for other substituents are then interpreted by comparing log k(calc). from a correlation expression with log k(obs.).

Solvent effects of flavin electron transfer reactions.

Abstract: The effects of solvent environment on the rates of several flavin redox reactions have been studied by using laser flash photolysis. These include electron transfer to the flavin triplet state (a measure of oxidized flavin electrophilicity) and the oxidation of flavin semiquinone by oxidized flavin radical, oxidized phenol radical, and quinone. The rate constant for triplet quenching by 2,6-dimethylphenol was found to be proportional to the inverse of solvent viscosity, as would be expected for a diffusional process. The flavin semiquinone yield due to flavin reduction during the quenching reaction was linearly dependent on the solvent dielectric constant. This implies the existence of a polar or charged intermediate along the reaction pathway. A similar effect of solvent dielectric was found for the self-quenching reaction, which produces semiquinone and an oxidized flavin radical. The rate constants for all three of the semiquinone oxidation reactions studied were found to exhibit a biphasic dependence on solvent dielectric, being virtually independent of dielectric at low values and sharply increasing at high values. This is interpreted in terms of a change in mechanism with solvent polarity. Specifically, we propose a neutral transition state and hydrogen atom transfer in low dielectric media and a dipolar transition state and electron transfer in high dielectric media. No specific effects of hydrogen-bonding interaction between flavin and solvent were observed for any of the processes studied. The mechanistic implications of these results for flavoenzyme catalysts are discussed.

Solvent effects on the nitrogen screening of some nitroalkanes

Abstract: High precision 14N screening measurements are presented for some nitroalkanes. The screening increases as the dielectric constant of the solvent decreases in all cases considered. The β effect on the nitrogen screening is found to be largely independent of solvent, and is thus an intrinsic property of the nitroalkanes. Good agreement is obtained between the observed solvent effects on nitrogen screening and those calculated by the SOS procedure using INDO/S parameters. It is concluded that the measured nitrogen screening changes monitor the electronic changes which occur in the nitroalkanes as the medium is changed.

Measurement of Solvent Shifts by Raman Difference Spectroscopy

Abstract: The application of Raman difference spectroscopy for the determination of the small frequency shifts arising from solvent effects has been demonstrated. Data are presented for benzene-carbon disulfide, benzene-pyridine, and acetone-chloroform mixtures. The majority of the solvent shifts measured are less than 1.0 cm−1 and these can generally be determined with better than 10% accuracy using a rotating two-compartment liquid cell. The smallest shift measured in this work was 0.06 cm−1. While statistical analysis indicates that uncertainties as small as ±0.01 cm−1 can be obtained in many cases, artifacts may cause apparent shifts up to ±0.03 cm−1.

Solvent‐Reactant‐Support interactions in liquid phase hydrogenation

Abstract: The effect of mixed solvents and catalyst support on the rate and selectivity of Ru-catalyzed liquid phase hydrogenations has been studied. Experiments in 1-propanol/water mixtures with Ru/C showed the adsorption of reactants on the catalyst and the reaction rate to vary with the composition of the solvent system. The influence of the support, in combination with the solvent surrounding the catalytic metal, on the selectivity in the competitive hydrogenation of cyclohexanone and cyclohexene has been studied using hydrophilic and hydrophobic supports in water/hydrocarbon mixtures. The results demonstrate the importance of both solvent-support interactions and partition of the reactant between bulk solvent and adsorbed solvent.

1H NMR studies of solvent effects on hydrogen bonding in some pyridine trifluoroacetates

Abstract: The chemical shifts of hydrogen bonded protons in complexes of 11 substituted pyridines with trifluoroacetic acid were examined, in five dry solvents of different activity, with respect to proton transfer and aggregation effects. The results were correlated with ΔpKa, the Kirkwood function and ET parameters. The solvent effect on the intersection point obtained from the plot of the chemical shift of the hydrogen bonded protons against ΔpKa can be used, similar to an isotopic effect, to differentiate strong hydrogen bonds. The aggregation of acid–base complexes can lead to downfield or upfield shifts; the variation of chemical shift with aggregation depends on the position of the proton in the hydrogen bridge.

Molecular structure of phosphaferrocene and charge-density distribution at low temperature

Abstract: While the chemical reversibility of the oxidations of (TPP)ZnL in EtCl, seen here is interesting, it is a t variance with previously reported work in acetonitrile. I t is surprising that no measurable quantity of the (8-py) derivative is formed and suggests that the solvent system plays an important role in the intermediate stablization of two possible oxidative pathways. Since the isolation of the (p-py) derivative was easily repeated and the [(TPP)ZnL]+ titration showed mixed species spectra, this solvent effect must indeed be operative. It should be noted, however, that even with the 500-ms time scale of our spectral acquisitions we cannot determine the exact nature of the intermediates in these pathways. Spectral changes were instantaneous with ligand addition.

1H NMR studies of solvent and substituent effects on strong intramolecular hydrogen bonds

Abstract: Chemical shifts of H-bonded protons in tetrabutylammonium hydrogen maleate and 14-substituted picolinic acid N-oxides have been measured in a number of dry solvents, of different activity, in order to distinguish between symmetrical single minimum and asymmetrical hydrogen bonds. In tetrabutylammonium hydrogen maleate the resonance was observed at 20.70 ppm and its was independent of the nature of the solvent used. The chemical shift value of picolinic acid N-oxide varies with the solvent. These observations suggest that the hydrogen bond is symmetrical in tetrabutylammonium hydrogen maleate but that it is asymmetrical in picolinic acid N-oxide. The chemical shifts of substituted picolinic acid N-oxides were correlated with σp, σm and ΔpKa. The substituent and solvent effects are compared and the position of the intramolecular H-bonded protons in picolinic acid N-oxides are estimated and discussed.

A hydrogen rearrangement of formamidine and the solvent effects thereupon

Abstract: The nature of the 1,3 hydrogen rearrangement of formamidine (H2N-CH=NH) and the solvent effects on that reaction are studied with ab initio molecular orbital calculations on the basis of the supermolecule model. The reaction path and the motion of the migrating hydrogen atom are traced by using the concept of the intrinsic reaction coordinate (IRC). Four types of orientation of one water molecule to formamidine at the transition state of reaction are examined and the results are discussed from the standpoint of the orbital interactions.