Showing papers on "Solvent effects published in 1969"

Solvent effects on cis‐trans azobenzene isomerization: a detailed application of a theory of solvent effects on molecular association*

Abstract: The free energy of a reaction-enthalpy and entropy as well-is modified by solvents. In a physical or chemical association of two molecular groups, or in a conformational change, solvent effects are usually significant. Recently, a quantitative and general theory for contribution of solvent to the forces between molecular groups in solution (solvophobic forces) has been formulated and outlined.' In the present work, the theory is applied in detail to a specific case: the effect of various solvents on the cis-trans isomerization reaction of azobenzene, which involves molecular groups more or less similar to those that occur in biopolymers and whose association influences the conformation. The calculation procedure is presented in some detail and should be useful in indicating the relative magnitudes of the various effects involved, and how the theory may be applied in other contexts.

Solvent Effect in Radical Copolymerization of Methyl Methacrylate with Styrene

Abstract: A study of the copolymerization of methyl methacrylate and styrene in monosubstituted benzenes such as benzene, benzonitrile, benzyl alcohol, and phenol was made at 60°C. It was found that the copolymerization parameters changed significantly with the polarity of solvents, and the relative reactivity of methyl methacrylate toward the polystyryl radical was correlated with the ET values of solvents. These results and spectroscopic data suggest that in the transition state of this copolymerization the polarized structure of methyl methacrylate monomer became important, enhancing its reactivity, as the solvents became more protic.

Solvent Effects in Proton Magnetic Resonance Spectroscopy

Abstract: Publisher Summary This chapter discusses the effects of solvents in proton magnetic resonance spectroscopy. The bulk susceptibility term, van der waals term, anisotropy term, and reaction field term are briefly described in the chapter. Bulk susceptibility effects cause paramagnetic shifts in all cases, where an external reference is used and a correction must be made. An internal reference removes, for all practical purposes, the necessity for such a correction. Van der waals effects are responsible for paramagnetic shifts only. Anisotropy effects cause paramagnetic or diamagnetic shifts, depending on the orientation of the axis of largest diamagnetic susceptibility in the solvent molecule. The presence of steric hindrance to the approach of bulk solvent to a solute proton reverses the trends in the observed shifts for anisotropic and van der waals effects. The studies of steric effects exhibit that the so-called inert solvents, carbon tetrachloride, and chloroform are unsuitable for the determination of the functional group anisotropies, if the introduction of the functional group modifies the steric environment of the neighboring protons.

Electron paramagnetic resonance spectra of flavonoid anion-radicals

Abstract: Flavonoid anion-radicals, obtained in aqueous dimethyl sulphoxide by the aerial, alkaline oxidation of flavonoid compounds, have been studied by means of e.p.r. The spectra of these radicals indicate oxidation of the isolated or conjugated B-ring to give a para- or ortho-benzosemiquinone or a pyrogallol-type anion-radical. Solvent effects result in variations in hyperfine coupling constants and radical stability.

Nuclear magnetic resonance study of the structure and interactions of actinomycin D: temperature and solvent effects on the N--H and NH2 groups.

Abstract: THE structure and interactions of actinomycin D (ACD) have recently been much studied (refs. 1, 2 and Fig. 1). We have made an extensive nuclear magnetic resonance study of ACD and in this communication we report the assignment and the effects of solvent and temperature on the N—H, NH2 and actinocin ring proton signals.

Solvent Effects of Charge-transfer Complexes

Abstract: The solvent effects of charge-transfer complexes were examined in various solvents. During the course of investigations pertaining to aromatic-TCNE complexes, Brownstein’s and McRae’s formulae were found to be valid. The gas-phase spectra of these charge-transfer complexes may be regarded as the origin of the solvent effects. The anomalous behaviour of carbonyl solvent molecules was also discussed.

Radiation-induced dimerization of 1,3-cyclohexadiene. Solvent effects and the formation of the Diels-Alder dimers by a cationic chain mechanism

Abstract: : The radiation-induced dimerization of 1,3-cyclohexadiene occurred by two simultaneous mechanisms: mechanism 1 produced mainly the endo and exo Diels-Alder products of 1,4,1',2' addition; mechanism 2 produced mainly the cis-anti-cis and cis-syn-cis isomeric products of 1,2,1',2' addition. Both mechanisms were sensitized by the aprotic solvents benzene, n-hexane, cyclohexane, and di-n-propyl ether, and were inhibited by the protic solvent ethanol. Mechanism 1 involved a cationic chain reaction in benzene, and probably also in the other aprotic solvents. In all the aprotic solvents the yield of the Diels-Alder products went through a maximum as the 1,3-cyclohexadiene (CHD) concentration was increased. There was no evidence of a chain in mechanism 2 and the yields of the corresponding dimers were relatively small. It appears that triplet-state CHD molecules were the immediate precursors of the dimers from mechanism 2, and that roughly half of the triplet CHD molecules resulted directly or indirectly from neutralization reactions. (Author)

The Effects of Organic Additives on the Solubilities and CMC’s of Potassium Alkyl Sulfates in Water. II. Effects of Some Nonhydroxy Compounds

Abstract: The solubilities and critical micelle concentrations (CMC) of potassium dodecyl sulfate (KDS) and hexadecyl sulfate (KHS) were measured in methanol-, ethanol-, 1-propanol-, ethylene glycol-, 1,2-propanediol-, and glycerol-water mixed solvents over the temperature range from 0 to 40°C. It was found that the higher the hydroxy group density in the solvent, the less the effects on both the solubility and the CMC, and that the presence of an alkyl group in the solvent causes a greater temperature-dependence in the solubility. The solvent effects on the CMC and on the single-ion dispersion were discussed separately. An effect of a net-work structure formed between the hydroxy compound and water on the solubility is suggested. Solvent effects on the alkyl group and the ionic group of a surfactant molecule were separated by considering the difference between the standard free-energy change for the solution process of KHS and that of KDS. The effects on the Krafft point were also discussed in term of the above su...

Ionic Equilibria in Mixed Solvents. IV. Solvent Effect on Dissociation Constants of Acids

Abstract: Equilibrium of hydronium ions with alkoxonium ions in water-alcohol mixtures being taken into account, solvent effects on dissociation constants of acids were quantitatively interpreted by the following: free energy changes on transferring ions from an aqueous solution to a mixed solvent and difference in standard states in different solvent compositions. Choosing suitable values for effective ionic radii for ions involved, satisfactory agreements were found between experimental and calculated values of ΔpK, the difference of pK in water and in an aqueous mixture, for positively charged acids (e.g., ammonium, anilinium, etc.) as well as neutral acids yielding a negatively charged conjugate base (e. g., acetic acid, benzoic acid, p-nitrophenol, etc.).

Fluorescence Lifetime of Acridine in Water-Organic Solvent Mixtures

Abstract: Fluorescence lifetime of acridine has been measured in water-ethanol and water-glycerol mixtures of different composition at 25°C. Remarkable decrease in lifetime value was found as the amount of organic component increases. The lifetime of 10.3 ns was obtained for the aqueous solution. The lifetimes in glycerol and in ethanol were determined as 3.2 and 0.72 ns, respectively. The observed ratio of the lifetime in the mixture to the lifetime in water, τ⁄τ0, has the same functional dependence on solvent composition as the ratio of fluorescence yield, Φ⁄Φ0. These quenching phenomena by organic molecules have been interpreted as due to the solvent effects upon the radiationless transition rate. Both static and dynamic mechanisms of quenching are not applicable in the present cases of solvent quenching.

Peresters XIII. Solvent effects in the decomposition of t-butylperoxy α-phenylisobutyrate with special reference to the cage effect

Abstract: t-Butylperoxy α-phenylisobutyrate (I) decomposes thermally by concerted formation of carbon dioxide, t-butoxy, and cumyl radicals. Radical pair return in the solvent cage therefore does not affect the observed rate of decomposition, but is readily determined by means of galvinoxyl and other scavengers. In a series of 15 solvents the rate constant varies over a 2.8 fold range, being fastest in aromatic solvents. In the same solvent series the relative rates of diffusion and combination of radicals, measured by the cage effect, change by tenfold and are largely determined by the viscosity of the solvent. In all solvents of η > 8 mP, the reciprocal of the cage effect is a linear function of (T1/2/η), as recently observed for trifluoromethyl and methyl radicals [16]. This property of the cage effect provides a test by which it can be distinguished from other processes that reduce the efficiency of free-radical production from an initiator.