Hydroxyl radical reactivity with nitrobenzene in subcritical and supercritical water

The bimolecular rate constants of the addition reaction between hydroxyl radical (*OH) and nitrobenzene (C(6)H(5)NO(2)) were measured in subcritical and supercritical water (SCW) at temperatures between ambient and 390 degrees C. The measured bimolecular rate constants showed distinctly non-Arrhenius behavior (i.e., essentially no increase with temperature) from ambient to 350 degrees C, but increased in the slightly subcritical and supercritical region between 350 and 390 degrees C. These data were modeled reasonably well over the entire temperature range with a three-step reaction mechanism, originally proposed by Ashton et al.(1) This model includes the formation of a pi-complex intermediate as the precursor of the nitrohydroxycyclohexadienyl radical.     

The ESR study of reactivity of perfluoroacetyldiisopropylmethyl radical

The kinetics of hydrogen abstraction from hydrocarbons by the air-stable perfluoroacetyldiisopropylmethyl radical was studied by ESR, and a reaction mechanism was proposed. The degree of delocalization of the unpaired electron in the model C(5)F₃−C(1)·[C(2)(O(4))C(3)F₃]C(6)F₃ radical was calculated by the MNDO/PM3 method in the UHF approximation. For the conformation in which the CO group lies in the plane passing through the C(1), C(2), and C(5) atoms, the electron density on the O atom is 0.22. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1954–1957, November, 1997.     

The ESR spectrum of the cis-stilbene radical anion

The ESR spectrum of the cis-stilbene radical anion has been observed under conditions where its conversion into the trans isomer is relatively slow. Assignment of the proton coupling constants has been based on studies on the 4,4′- and 7,7′- dideuterio derivatives, as well as on comparison with structurally related species. The values for the radical anions of cis- and trans-stilbene are very similar, except those (2.68 and 4.49 G, respectively) arising from protons in the 7,7′-positions. The difference is attributed to strong distortions of the π-system in the C1C7′C1′ fragment of the cis isomer.     

The onset of electron localization in subcritical water vapor

Excess electron mobilities in subcritical water vapor are reported for the density range 6.9 X 10¹⁷ <n < 6.0 x 10²⁰ molecules/cm³ which show the transition from the quasi-free to the localized electron state. Onset of electron localization obeys Mott's criterion for quantum localization. We determined the energy levels in water vapor populated by electrons during the photoinjection process.     

Phenanthrene degradation in subcritical water

Subcritical water (<374 °C and <221 bar) has unique characteristics such as dramatically decreased dielectric constant, surface tension, and viscosity with increasing temperature, allowing for dissolution and reaction of organics in high-temperature water to occur. Additionally, the dissociation constant of water at temperatures of 200-300 °C is three orders of magnitude greater than that of ambient water, which may also contribute to the reactivity of subcritical water with certain organic compounds. In this study, the degradation and oxidation of phenanthrene in subcritical water were investigated. Both deionized water and water with 3% hydrogen peroxide were used in the degradation and oxidation studies. The effect of temperature on degradation efficiency has been determined with a temperature range of 100-350 °C. When the temperature was increased from 150 to 350 °C, the amount of phenanthrene degraded varied from 6 to 243 μg in each milliliter of deionized water. However, these quantities were increased to 195 μg at 150 °C and 3680 μg at 350 °C in each milliliter of water with 3% hydrogen peroxide. Several degradation products including phenol, benzoic acid, and ketones were identified by using gas chromatography/mass spectrometry (GC/MS).     

The ESR spectra of p-benzosemiquinone radical anion included in cyclodextrins

The ESR spectra of p-benzosemiquinone radical anion included in cyclodextrins (CyDxs) in aqueous solutions were observed and analyzed. Remarkable differences in the line shape were found between the spectrum in α-CyDx aqueous solution and that in 95% ethanol. They are explained in terms of the suppressed molecular rotation of the radical included in the α-CyDx cavity. The ESR spectra of the radicals included in β- and γ-CyDx in aqueous solutions are identical with that in 95% ethanol, hence in the β- and γ-CyDx cavities the radical rotates as freely as in 95% ethanol.     

The triethylenediamine cation radical: An ESR study

Compared with normal and bridge-head aminium cations, the triethylenediamine cation is relatively stable. Its ESR spectrum shows that the two nitrogen atoms and all the protons are magnetically equivalent even at 77 K. This favours a symmetrical ground state rather than a dynamic equilibrium between classical, asymmetric structures.     

ESR spectrum of the cyclopropenyl radical

The ESR spectrum of a radical produced by photolytically generated t-butoxy radicals and cyclopropene in solution is reported. Arguments are presented for an assignment of the spectrum to the cyclopropenyl radical (CH)₃ undergoing fast exchange between equivalent distorted r_e structures.     

CIDNP-detected ESR of short-lived radical pairs in solutions

CIDNP-detected ESR spectra of short-lived radical pairs in solution have been obtained for the first time in the example of the photolysis of benzoyl peroxide.     

Organic reactions in subcritical and supercritical water

This review describes applications to several important organic reactions in subcritical and supercritical water.

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Retention mechanisms in subcritical water reversed-phase chromatography

Differences in the properties of subcritical water and conventional water/acetonitrile and water/methanol mobile phases for reversed phase separations are explored. Using van’t Hoff plots enthalpies and entropies of transfer are compared among the mobile phases while linear solvation energy relationships are used to quantify contributions to retention based on a solute's polarizability, dipolarity, hydrogen bond donating ability, hydrogen bond accepting ability, and molecular size. Results suggest the presence of acetonitrile or methanol in the mobile phase may decrease dispersive interactions of the solute with the stationary phase compared to subcritical water, thereby lowering enthalpic contributions to retention. Enthalpic contributions are found to drive the retention of a methylene group in all systems studied.     

Thermal stability of fatty acids in subcritical water

In this work, hydrolytic reaction conditions of various temperatures (300–370 °C) and times (0–30 min) at a constant pressure of 20 MPa were applied to the thermal decomposition of three kinds of fatty acids (FAs), stearic acid, oleic acid, and linoleic acid, in subcritical water. The degradation characteristics were investigated from the derived data, and the thermal stability of FAs in subcritical water was estimated. The primary reactions we observed were isomerization and pyrolysis of FAs. The main pathway of degradation was deduced by analyzing the contents of pyrolyzed products. We found that more saturated FAs have greater thermal stability in subcritical water. All FAs remained stable at 300 °C or below. Based on these results, we recommend that hydrolysis of vegetable oils and fats using subcritical water should be carried out below 300 °C (at 20 MPa) and for less than 30 min to obtain high-yield FA production.     

ESR of radicals in γ-irradiated mono and polycyclic olefin-I Cyclopentadiene: identification of a new radical species

In addition to cyclopentenyl radicals, the γ-irradiation of solid cyclopentadiene at 77°K results in the formation of polymeric radicals whose ESR h.f. structure has been isolated by progressive thermal annealing of glassy samples. Ultra-violet irradiation experiments suggest that the polymeric radicals have an ionic origin. Experimental evidence is also given regarding the absence of cyclopentenyl radicals arising from hydrogen atom addition to the double bonds. A reaction mechanism is proposed.     

The radical cation of hexafluoro-1,3-butadiene: An ESR study

Two distinct radical cations have been observed by ESR in a γ-irradiated solid solution of hexafluoro-1,3-butadiene in freon. INDO calculations of ES     

The swelling and dissolution of cellulose crystallites in subcritical and supercritical water

The swelling and dissolution phenomena of microcrystalline cellulose (MCC) were investigated in subcritical and supercritical water. Commercial MCC was treated in water at temperatures of 250–380 °C and a pressure of 250 bar for 0.25–0.75 s. As reaction products, undissolved but depolymerised cellulose residue, short-chain cellulose precipitate, water-soluble cello-oligosaccharides and monosaccharides, as well as their degradation products, were detected. The highest yield of the cellulose II precipitate was obtained after a reaction time of 0.25 s at 360 °C. Our hypothesis was that if the crystallites were swollen, the depolymerization pattern would be that of homogeneous reaction and the cellulose Iβ to cellulose II transformation would be observed. The changes in the structure of the undissolved cellulose residue were characterised by size exclusion chromatography, wide-angle X-ray scattering and ¹³C solid-state NMR techniques. In many cases, the cellulose residue samples contained cellulose II; however, due to experimental limitations, it remains unclear whether it was formed through the swelling of crystallites or the partial readsorption of the dissolved cellulose fraction. The molar mass distributions of untreated MCC and after low intensity treatments showed a bimodal shape. After high intensity treatments the high molar mass chains disappeared which indicated a complete swelling or dissolution of the crystallites.