OH Conversion Research Articles

On the stability of symmetric dimers of dehydroascorbic acid: A study of the esters in the crystalline and the solute state

Acetylated and benzoylated dimeric dehydroascorbic acid have the same molecular structure as the crystalline parent compound. X-Ray analysis of the tetra-acetate reveals only moderate deviation from two-fold symmetry, caused, presumably, by the packing requirements of the acetate groups. The central dioxane ring is stabilised by the conversion of OH into OAc or OBz. In methyl sulfoxide or N,N-dimethylformamide solutions, no anomerisation occurs as is found with dehydroascorbic acid.

Formation of organic nitro-compounds in flowing H2O2+ NO2+ N2+organic vapour systems. Part 1.—Surface initiation efficiency and reactions of ethane

When H2O2 vapour is added to an excess of NO2 in a flow system with ethane as the carrier gas in the presence of a boric-acid-coated surface at temperatures in the range 298–363 K, the organic products are nitroethane, ethyl nitrate and smaller amounts of ethyl nitrite. On the basis of the initiating steps [graphic omitted] the total yield of nitro-compounds, extrapolated to [NO2]= 0, provides a direct measure of the H2O2 to OH conversion efficiency in step (1). This is shown to correspond to that indicated by the previously applied indirect method involving measurement of the variation of CO2 yields with the ratio [S]/[CO] in H2O2+NO2+CO systems containing diethyl ether (S) as the internal kinetic calibrant. This efficiency does not change significantly throughout the above temperature range.The mechanism of formation of the nitrocompounds involves the following competitive steps: [graphic omitted] Arguments are presented that step (8) may proceed by way of partial dissociation of chemically activated C2H5ONO molecules rather than by a bimolecular O-abstraction route.A rate constant of k4=(1.3±0.2)× 1010 dm3 mol–1 s–1 is obtained for the reaction OH + NO2(+ M)→ HNO3(+ M)(4) at ambient temperature for M = C2H6 at a pressure of 40 kPa.

Effects of pressure and surface initiation efficiency on the flowing H2O2+ NO2+ CO + N2 chain reaction system

The yields of CO2 from the H2O2+ NO2+ CO reaction system in flowing carrier gas at 298 K have been measured at total pressures (P) of 13.3, 40.0 and 93.3 kPa. The variations of the yield as functions of [NO2] and [S](S = acetaldehyde or diethyl ether) have been shown to indicate that the efficiency of conversion of H2O2 to OH radicals in the heterogeneous initial step H2O2+ NO2→ OH + HNO3(1) is independent of total pressure for a particular surface activity. The results are consistent with the pressure dependence of k2 for the propagation step CO + OH → CO2+ H (2) indicated by other work. For the rate constant of the termination reaction OH + NO2(+ M)→ HNO3(+ M)(4) low apparent values (k′4) are indicated compared with literature values of k4. At P= 40.0 kPa, for three surfaces of apparent efficiencies of ca. 20%, ca. 53% and ca. 81% in converting H2O2 to OH in reaction (1), k′4/k4 values were ca. 0.45, ca. 0.65 and ca. 0.85, respectively. This trend is interpreted in terms of decreasing activity of these surfaces for adsorption of H2O2, producing decreasingly localized reaction zones: depletion of [NO2] in the reaction zone depresses k′4/k4 while increased surface concentrations of H2O2 decrease the apparent H2O2 to OH conversion efficiency by inducing an additional route for destruction of the active centre involved in reaction (1).

Submicrosecond pulse radiolysis conductivity measurements in aqueous solutions—II: Fast processes in the oxidation of some organic sulphides

Successful application of a submicrosecond pulse radiolysis conductivity technique with a time resolution of 50 ns in aqueous solutions is demonstrated for a number of fast processes which occur during the OH· radical induced oxidation of some organic sulphides. The reliability of the technique has been tested with well-known systems, i.e. the neutralization reaction H + aq + OH - aq→ H 2O and the conversion of the neutral OH· adduct to 1, 4-dithia-cyclohexane into the intramolecular radical cation complex ▪. New observations on the submicrosecond time scale include the identification of a molecular radical cation, R 2S +, from tetrahydrothiophene and diethylthioether, and a nonionic species in the reaction of OH· with di-tert-butylthioether, probably a (R 2S) 2(OH)· complex. The new results lead to a modified, more general overall mechanism for the OH· induced oxidation of organic sulphides.

Pulse radiolysis study of protoferrihaem IX intercalated in sodium dodecyl sulphate micelles

Haemin intercalated in sodium dodecyl sulphate (SDS) micelles exists as monomer in an organic sphere of dimensions similar to that of cytochrome-c and thus provides a model which is closer to the biological molecule than any which has been studied hitherto. Hydrated electrons reduce intercalated haemin at pH 9.2 with k= 5.2 × 109 dm3 mol–1 s–1. CH3CHOH radicals reduce it at pH 9.2 with k= 5.6 × 108 dm3 mol–1 s–1 and at pH 4 with k= 1.6 × 109 dm3 mol–1 s–1. Reduction is accompanied by spectroscopic changes in the haemin molecule at both pH 9.2 and 4, but at pH 9.2 additional changes take place afterwards over periods of several milliseconds. At pH 4.0 there are no additional changes. The final spectrum attained in both cases corresponds to the formation of reduced haemin in 100 % yield.The additional changes taking place after reduction in mildly alkaline solution are found to have an activation energy of 66.6 kJ mol–1. They are independent of pH in the range 9.2–10.7, and of added electrolyte in the range 0.05–0.2 mol dm–3. They are attributed to conversion of OH– in reduced haemin into H2O, perhaps by protonation by water: [OH–FeIII(por)–H2O]°+ e–aq→[graphic omitted]. Corresponding changes are not seen at pH 4 because the haemin exists as [H2O–FeIII(por)–H2O]+ at that pH.

Hydrolysis of Lincomycin-2-phosphate and Clindamycin-2-Phosphate

The aqueous stabilities of lincomycin-2-phosphate and its 7-deoxy-7(S)-chloro analog, clindamycin-2-phosphate, were studied at a variety of temperatures and pH values. The predominant degradative routes of lincomycin-2-phosphate in the pH range 1-10 are thioglycoside and phosphate ester hydrolysis, and pH-rate studies show that it is most stable at pH 6-10. Clindamycin-2-phosphate degrades by three major routes from pH 1 to 10 with phosphate ester and thioglycoside hydrolysis predominating at pH less than 6 and scission of 7(S)-Cl to form the 7(R)-OH analog predominating at pH greater than 6. The rate of 7(S)-Cl to 7(R)-OH conversion was obtained using gas chromatography to measure the disappearance of clindamycin in reaction mixtures prepared under identical conditions as clindamycin-2-phosphate reaction mixtures. Rates of phosphate ester hydrolysis of the two compounds were measured by spectrophotometric determination of the amount of inorganic phosphate formed in reaction mixtures as a function of time. Apparent first-order rate constants of phosphate ester hydrolysis as well as activation energies (32.1 kcal./mole for lincomy-cin-2-phosphate and 32.9 kcal./mole for clindamycin-2-phosphate) agree favorably. The rate of lincomycin-2-phosphate hydrolysis in a formulated pediatric syrup at room temperature agrees favorably with extrapolated rates from high temperature hydrolysis in simple solutions.

Proton-induced hydroxyl formation on the lunar surface

The interaction of both the particle and photon component of the solar wind with the lunar surface material is expected to produce diverse chemical reactions. Experimental evidence for proton-induced OH formation was obtained by bombarding a glass, chemically similar in composition to common silicate minerals, with high-energy protons. The concentration of OH, before and after irradiation, was determined by infrared absorption measurements. The OH formation rate was greatest at the start of the bombardment and decreased with increasing dose. The maximum proton to OH conversion rate, at the start of the irradiation, is at least 5 or 10% and may be as high as 100%. Using this result, together with estimates of the lunar age and recent solar proton flux data, we were able to make very rough calculations of the minimum proton-induced OH content in the lunar surface. If mixing or churning is not important, the upper centimeter could contain 4×1016 OH per cm3. When protons below 40 Mev and the higher conversion rate are included in the computation, the estimated OH concentrations could increase by a factor of 10 or more. If surface mixing or churning has occurred, they should be divided by an average churning depth.

The Effect of Chloride Ions on Some Radiation Chemical Reactions in Aqueous Solution

It has been known for some time that chloride ions react with radiation-produced free radicals in aqueous solutions (1-5). For instance, Dewhurst (1) found that the radiation-induced chain oxidation of ferrous sulfate in aqueous solutions containing simple alcohols could be suppressed by the addition of sodium chloride. He suggested that this result is due to conversion of OH. free radicals to chlorine atoms by the reaction