ESR study of the reversible dimerization of phosphonylfullerenyl radicals

A. N. Nesmeyanov Institute of Organometallic Compounds, Russian Academy of Sciences, 117813 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 8, pp. 1936–1937, August, 1992.     

Steric effects on the reversible dimerization of short-chain oligothiophene cation radicals

The reversible dimerization of cation radicals of a series of end-capped 2,5″-dihexylterthiophenes with the median thiophene ring either unsubstituted (1) or mono- and di-substituted by methyl and hexyl groups (2–5) has been investigated. Concentration-dependent cyclic voltammetry, UV-Vis-NIR spectroelectrochemistry and temperature-dependent ESR spectroscopy lead to consistent results showing that, whereas the cation radical of compound 1 already dimerizes at room temperature, substitution of the median thiophene ring leads to a dramatic decrease in the propensity of the corresponding cation radical to dimerize. These results provide conclusive evidence for a steric control of the reversible dimerization of short-chain oligothiophene cation radicals.     

Destruction of silicon-containing phenoxy radicals

Conclusions It was shown that the destruction of silicon-containing phenoxy radicals in benzene is second order by means of ESR and spectrophotometric methods.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 572–574, March, 1972.We thank G. A. Abakumov for obtaining the ESR spectra and I. A. Chuev for his assistance in carrying out the spectrophotometric studies.     

Effect of the solvent on reduction of azomethine dyes with ketyl radicals and recombination of ketyl radicals

The rate constants of transfer of a hydrogen atom from ketyl radicals to azomethine dyes (k_H) and the rate constants of recombination of ketyl radicals (2k_r) in different solvents were measured by the method of pulsed photolysis. The dependences of k_H and 2k_r on the polarity of the solvent are V-shaped: k_H and 2k_r are maximum in weakly solvating solvents and in water and are minimum in nucleophilic solvents of medium polarity. This is due to the fact that nucleophilic solvation decreases and electrophilic solvation increases the reactivity of the ketyl radicals.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1745–1749, August, 1989.     

Effect of solvent nature on intrinsic viscosity of an ethylene-propylene-dicyclopentadiene copolymer

The hydrodynamic behavior of solutions of the ethyleneópropylene-dicyclopentadiene copolymer in hydrocarbon solvents has been studied. It has been shown that the effects of temperature on the thermodynamic affinities of aromatic and aliphatic solvents for copolymer macromolecules are substantially different. The thermodynamic quality of aromatic solvents relative to the copolymer and the hydrodynamic volume of macromolecular coils increase with temperature; the aliphatic solvents show quite a reverse behavior.     

Effect of solvent nature on the photoreduction kinetics of substituted benzoquinones

Effect of the solvent nature on the kinetics of photoreduction of substituted benzoquinones in the presence of hydrogen donors has been studied. It has been found that the effective photoreduction rate constant (k_H) for quinones decreases with an increase in solvent polarity. For the 3,6-di-tert-butylbenzoquinone–1,2-N,N-dimethylaniline pair, the dependence of ln k_H on the difference of the reciprocals of optical and static solvent permitivities (1/ε _∞–1/ε₀) is stepwise with a break point corresponding to CH₂Cl₂. A similar relationship lnk_H = f(1/ε _∞–1/ε₀) is observed for the p-chloranil–mesitylene pair. In the study of the photoreduction kinetics for a series of seven o-benzoquinones in the presence of p-derivatives of N,N-dimethylaniline in CH₂Cl₂, it has been found that the dependence of k_H on the free energy of electron transfer (ΔG_e) has a maximum for the 3,6-di-tert-butylquinone-1,2–N,N-dimethylaniline pair at ΔG_e = 0.11 eV.     

Generation and reversible dimerization of the perfluoro-tert-hexyl radical

Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, p. 2426, October, 1988.     

Effect of solvent quality on kinetics of tethered layer formation

We conducted a study of the effect of solvent quality on the kinetics of formation of a layer of polymer chains tethered to a solid substrate. In these experiments, tethering was accomplished by means of chemical bond formation between reactive sites on the surface and the end‐functional groups of the polymer chains in solution. All experimental variables were held constant except for the χ‐parameter between the polymer and solvent. Variation in the χ‐parameter was achieved by use of a series of nonpolar, organic solvents. The distinct three‐regime kinetics, typical of tethering reactions run in a good solvent and in the absence of segmental adsorption, was observed over the range of values for the χ‐parameter. As expected, an increase in the χ‐parameter (a decrease in solvent quality) did result in increased tethering density, but, contrary to expectation, no increase in tethering rate was observed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5530–5537, 2004     

Effect of solvent on catalytic performance of anhydrous catalyst in acetylene dimerization to monovinylacetylene

The production of monovinylacetylene (MVA) through Cu(I)-catalyzed acetylene dimerization reaction was performed in different reaction media. Based on the analyses of crystals precipitated from the catalyst solution and UV-Vis spectra of the catalysts, the reaction mechanism and solvent dependence were studied. The highest yield of MVA can be obtained when dimethylformamide is used as solvent because of its strong coordination ability to Cu(I). The activation of C≡C bond is presumed to be improved when the catalytic metal ion is coordinated by a solvent with less steric hindrance and electron-rich coordination atom. The results of the present study provide a possible way to accelerate the metal-catalyzed homogeneous reaction of alkyne substrates through careful selection of a solvent.     

Effect of the solvent on the kinetics of thermal decomposition of acetylcyclohexylsulfonyl peroxide

The kinetics of the decomposition of acetylcyclohexylsulfonyl peroxide (ACSP) in CCl₄, benzene, toluene, ethylbenzene, cumene, acetonitrile, ethanol, and 2-propanol in an atmosphere of O₂ were studied at 40–70 °C. The rate constants (k ₀) and activation parameters of the monomolecular decomposition of ACSP were determined. A linear dependence between logA ₀ and activation energyE ₀ (compensation effect) was established. The dependence ofk ₀ on the nature of a solvent is described by the four-parameter Koppel-Palm equation. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 935–937, May 1997.     

Effect of melt viscosity on the crystallization kinetics of pre-sheared metallocene polyethylene melts

This work aims to verify the impossibility mentioned in the literature of saturating the crystallization kinetics of pre-sheared metallocene polyethylene melts. Similarly to results reported for other materials, and contrary to other published works, an acceleration of crystallization kinetics with the increase of shear strain and its saturation at large strain values was found. Similar strain values, with the same temperature variation, were evaluated with independent experiments using different devices, which allowed us to identify the steady state as the melt state responsible for the saturation of crystallization kinetics. Since this is a partially disentangled melt state, with viscosity lower than that of fully entangled (relaxed) melts, we assign the acceleration of crystallization kinetics by application of shear, and its saturation, mainly to the facilitated diffusion of chain segments to the lamellae growth front. This conclusion is supported additionally with the experimental results of other authors.     

Role of phenoxy radicals in PCDD/F formation

In this work, the role of phenoxy radicals in polychlorinated dibenzo‐p‐dioxins and polychlorinated dibenzofurans (PCDD/F) formation was investigated by studying the slow oxidation of 2‐chlorophenol (2‐CP) and 2‐chloroanisole (2‐CA) at a gas‐phase concentration of 4 ppm (～2.1 × 10⁴ μg/m³) over a temperature range of 400–800°C. Residence times were maintained at 2.0 ± 0.10 s. PCDD/F reaction products were dibenzofuran, dibenzo‐p‐dioxin, 4‐chlorodibenzofuran, 1‐chlorodibenzo‐p‐dioxin, 4,6‐dichlorodibenzofuran, and 1,6‐dichlorodibenzo‐p‐dioxin (1,6‐DCDD). Major products observed in these experiments were 2,6‐dichlorophenol, 3‐phenyl‐2‐propenal, 1‐indanone, 1,3‐isobenzofurandione, and 3‐phenyl‐2‐propenoyl chloride. The 2‐CP and 2‐CA experiments, along with the variable concentration 2‐CA experiments, showed that the concentration of radicals present in the oxidation system has a significant effect on the PCDD/F product distribution and ultimately the PCDD/PCDF ratio. Also, the observation of dichlorinated phenoxy phenol and dichlorinated dihydroxybiphenyl, the proposed intermediate species in the radical–radical mechanism, suggests that radical–radical mechanism dominates gas‐phase PCDD/F formation. This information will be helpful in constructing a detailed kinetic mechanism of PCDD/F formation/destruction in combustor postcombustion zone. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 531–541, 2002     

Fluorescence polarization anisotropy and kinetics of malachite green measured as a function of solvent viscosity

The fluorescence kinetics and polarization anisotropy of the triphenylmethane dye malachite green were measured as a function of solvent viscosity. The relationshp between the relaxation kinetics and the solvent viscosity was investigated in order to obtain information on the effect of enviromnental changes on the orientational order of dye molecules in solution. It was found that the fluorescence lifetimes follow an η^{$\text{2}\text{3}$} dependence for 1 < η < 60 P, η^{$\text{1}\text{2}$} dependence for 60 P < η < 1000 P and approach a constant for η > 1000 P. The dependence of the fluorescence decay rates on the solvent viscosity was fit to k = 5 × 10¹⁰η^{?$\text{2}\text{3}$} + 5 × 10⁸ s^?1. The fluorescence polarization anisotropy term, R(0), was also measured as a function of solvent viscosity. A marked decrease in R(0) was observed at a viscosity of 1000 P. For η < 1000 P, R(0) was found to be close to the expected value for a random distribution of molecules, 0.4; and for η > 3000 P, R(0) was measured to be ≈0.11. This small value of R(0) may indicate a nonrand The observed change of the polarization anisotropy with increasing viscosity indicates that the dye molecules become ordered at higher viscosities. This may arise through the formation of a long range order due to lack of rotational deexcitation of the malachite green dye molecules at high viscosities.     

Photoinduced reversible change of fluid viscosity

We report a reversible photoinduced fluid viscosity change. A small amount of a "photoswitchable" azobenzene-modified cationic surfactant (4-butylazobenzene-4'-(oxyethyl)trimethylammonium bromide, AZTMA) was added to a wormlike micellar solution of cetyltrimethylammonium bromide (CTAB) containing sodium salicylate (NaSal). The trans-AZTMA solution had a remarkably high viscosity as a result of the entangled network of wormlike micelles. UV light irradiation on the trans-AZTMA solution remarkably decreased the viscosity of the solution because the bulky structure of cis-AZTMA is likely to disrupt the network structure of wormlike micelles. This photoinduced viscosity change is perfectly reversible between the trans- and cis-AZTMA solutions.