Showing papers on "Solvent effects published in 1972"

The Influence of the Solvent on Organic Reactivity

Abstract: There can be no general theory of organic reactivity without consideration of solvent effects. Solvent effects on rate and equilibrium constants, and on spectral characteristics, for example, are no less in magnitude than structural effects. It is generally agreed that the problem of solvent-solute interaction is no less complicated than that of structural effects. Therefore all the various approaches in this field, as reflected in the correspondingly vast literature, cannot be reviewed in this chapter, even briefly. Hence we have restricted ourselves to certain definite aspects of the problem of solvent effects, viz. the results one can hope to obtain by the use of the LFER principle.1,2,3

Preferential and absolute interactions of solvent components with proteins in mixed solvent systems

Abstract: An equation is derived relating total and preferential interactions of solvent components with macromolecules in a three-component system. Application of this equation to literature data shows that binding of non-polar solvents to proteins parallels the unfolding of the latter. This observation is discussed in terms of local inter-residue and residue-solvent interactions.

Dipalmitoyl-lecithin: assignment of the 1H and 13C nuclear magnetic resonance spectra, and conformational studies

Abstract: The 13C resonances of the glycerol and choline groups of dipalmitoyl-lecithin in CD3OD, CDCl3, and D2O have been assigned from the 14N–13C and 31P–13C coupling constants, and proton decoupling studies on the assigned 1H spectra. Well-defined solvent effects are observed, but the chemical shifts for dipalmitoyl-lecithin in bilayers in D2O are very similar to the shifts of the corresponding resonances in biological membranes. From an analysis of the 1H spectrum of the CH2 multiplet in the CH–CH2OCOR fragment, the two vicinal coupling constants were extracted and used to calculate rotamer populations from a Karplus treatment. In a high proportion of the molecules of dipalmitoyl-lecithin in CD3OD and in CDCl3 the fatty acid chains are arranged gauche to each other, suggesting the presence of strong hydrophobic interactions. From an AA′BB′ analysis of the choline –CH2·CH2–fragment in dipalmitoyl-lecithin observed vicinal coupling constants indicate a similar conformation to that found in choline derivatives where an electrostatic interaction between the N+ and the O atoms leads to the conformation being exclusively in the gauche+N/O form.

Some studies of benzenoid-quinonoid resonance. Part 2.—The effect of solvent polarity on the structure and properties of merocyanine dyes

Abstract: The SCF π-electron theory with bond length optimization, described in part 1, has been used to calculate the effects of solvent polarity on the structure and spectroscopic properties of a simple merocyanine dye. The calculations predict a minimum in energy and oscillator strength of the first absorption band over the range of solvent parameters used, but these extrema should not occur at the same solvent compositions. The measured u.-v. spectra can be interpreted from these results, and the n.m.r. chemical shifts, which change with solvent, can be correlated with calculated net atom charges.

Nuclear magnetic double-resonance studies of organo-selenium compounds

Abstract: 1 H–{77Se} Double-resonance experiments have been used to determine selenium chemical shifts in 80 organoselenium compounds containing 77Se in natural abundance. The shifts cover a range of over 1500 p.p.m. and are relatively insensitive to solvent effects. Electronegative substituents give shifts to low field, and in general the shifts parallel those found in similar phosphorus compounds, but are several times larger for a given electronic change. Correlations are found with the extent of α-chain branching in alkyl derivatives, and with Hammett σ-constants in substituted aryl derivatives. The chemical shifts show greater promise as diagnostic tools than do coupling constants involving selenium.

Substitution at saturated carbon. Part XIV. Solvent effects on the free energies of ions, ion-pairs, non-electrolytes, and transition states in some SN and SE reactions

Abstract: Standard free energies of transfer from methanol to various other solvents are reported for dissociated species R4N++ X–(20–30 solvents), ion pairs R4NX (20–30 solvents), t-butyl chloride (20 solvents), t-butyl bromide (10 solvents), tetramethyltin (15 solvents), tetraethyltin (15 solvents), and tetraethyl-lead (10 solvents). A dissection on solvent influences on ΔG‡ into initial-state and transition-state contributions has been accomplished for the decomposition of t-butyl chloride (18 solvents), the decomposition of t-butyl bromide (10 solvents), the Menschutkin reaction of NN-dimethylaniline with methyl iodide (3 solvents), and the iododemetallation of tetra-alkyl-leads (8 solvents).A comparison of solvent effects on the free energies of ion pairs with solvent effects on the free energies of transition states suggests that transition states carry the following units of charge separation: [ButCl]‡ 0·70 (polar solvents), [graphic omitted] 0·5 (non-polar solvents); [ButBr]‡ 0·68 (polar solvents); [p-NO2·C6H4·CH2Cl–Me3N]‡ 0·35. Charge separation in the [R4Pb–l2]‡ transition states is probably rather higher than in the transition state of the Menschutkin reaction.

Solution Properties of Synthetic Polypeptides. XI. Solvent Effect on Helix–Coil Transition in Polypeptides

Abstract: A number of synthetic polypeptides are known to undergo “inverse” thermal helix–coil transitions in solvent mixtures containing an “active” solvent such as dichloroacetic acid (DCA). A statistical mechanical theory of the effects of the active component on the transition is developed here on the assumption that the active solvent exists in the form of a dimer and that the dimerized molecules react with pairs of CO and NH groups of randomly coiled peptide units. Expressions are derived for the equilibrium constant, u, of helix formation and the cooperativity parameter, σ, and they are used to analyze data obtained for poly(γ-benzyl-L-glutamate) and poly(β-benzyl-L-aspartate) in mixtures of DCA and ethylene dichloride as functions of temperature and solvent composition. The values of the transition parameters so determined make it possible to understand not only why and how the observed values of transition enthalphy ΔH and cooperativity parameter σ depend on temperature and solvent composition, but also the general features of inverse transitions in quantitative terms. An important conclusion is that σ is profoundly affected by the binding of the active solvent on the polypeptide chain.

Nuclear magnetic resonance studies of rate processes and conformations. Part XX. Nitrogen inversion in the gas phase

Abstract: The barrriers to nitrogen inversion in N-methyl-, N-methyl-2,2-dimethyl-, and N-isopropyl-2,2-dimethyl-aziridine have been studied in the gas phase and in a variety of solvents. The barrier is found to be almost the same in the gas phase and in C6D12 solution. Solvent effects are discussed. Some small coupling constants have been determined by double irradiation.

Temperature and solvent effects on the fluorescence of some simple aromatic molecules

Abstract: The fluorescence yields and lifetimes of α-methylnaphthalene, o-xylene and tetralin in ethanol and methylcyclohexane, toluene in methylcyclohexane, benzene in cyclohexane, methylcyclohexane and ethanol have been measured as a function of temperature, and also for benzene in methanol and acetonitrile at room temperature. In every case, the measured fluorescence yield and lifetime decrease with temperature and are dependent on the solvent used. Expressing the radiative lifetime, τr, as τr=τ°r/n2, where n is the refractive index of the medium, it has been found that τ°r is constant with temperature for α-methylnaphthalene, o-xylene, tetralin, and toluene. It decreases slightly with increasing temperature for benzene. In every compound the value for τ°r, at room temperature, is independent of the solvent used. Possible reasons for the variation of the fluorescence yields with solvent and temperature are discussed. In benzene, solvent perturbations can increase non-radiative disappearance of the S1 state.

Solvent and Substituent Effects on the Ultraviolet-Visible Absorption Spectra of Azo Dyes

Abstract: The effects of solvents, including polymeric substrates, and substituentson the absorption spectra of 4,4'-donor acceptor- substituted azo dyes were studied. The position of the intense, visible ch...

2,4-Diazapentadienes. II. A Carbanion Cyclization

Abstract: The mechanism of the cyclization and 1,3-proton shift of 1,3,5-triaryl-2,4-diaza-1,3-pentadienes (1) catalyzed by phenyllithium and by potassium methoxide–methanol has been studied. On the basis of substituent effects, hydrogen–deuterium exchange, isotope effects, and solvent effects, it was deduced that both the cyclization and prototropy involve a common W-shaped carbanion which rapidly cyclizes. A kinetic deuterium isotope effect of 2 was calculated for protonation of this intermediate carbanion in methanol.

Influence of crystallization conditions on single crystal formation

Abstract: The effects of solvent, molecular weight of polymer and its distribution, and crystallization temperature on the formation and structure of single crystals from solution are discussed. With respect to the solvent effect, the intrinsic viscosity and lamellar thickness of crystals formed were investigated with various solvents as functions of temperature. It is pointed out that the solvent effect primarily reflects the equilibrium dissolution temperature and. moreover, affects the surface roughness or the conformation of fold portion which relates to the numerical value of the surface free energy. Concerning the effects of molecular weight and its distribution, measurements were carried out on the lamellar thickness. thickness of fold-containing surface, density. etc.. with single crystals formed and taken out at constant temperatures. It is pointed out that, with sharp fractions of low molecular weights and at lower supercooling, we can obtain assembly of single crystals with higher density and larger thickness. INTRODUCTION As is well known. there are two theoretical approaches to the formation of single crystals from solution. One. based on thermodynamic considerations developed by Peterlin. Fischer and Reinhold'. leads to the fold length of single crystals being directly related to the crystallization temperature 7. The other theory. proposed independently by Price2. Lauritzen and Hoffman3, and Frank and Tosi4, is based on a kinetic approach. and predicts that the fold length of solution-grown single crystals depends on the degree of super-cooling. T, — T. in which TI5 denotes the melting temperature of single crystals with infinite fold length. and is alternatively called the equilibrium dissolution temperature. Investigations into the effect of solvent on single crystal formation have been expected to help to differentiate between the kinetic and equilibrium theories. For several years. we have studied5'2 the formation of single crystals from various solvents under different conditions. and pointed out that the fold length and the structure of fold surface are dependent on the crystallization condition. Among the various crystallization conditions for solution-grown single crystals. nature of solvent is one of major importance. because thermodynamic values of the solvent, such as molar volume and solvent—polymer interactions, affect not only the dissolution temperature. but also the chain