Positive Ion Scavengers Research Articles

Leptospermum petersonii as a Potential Natural Food Preservative.

Leptospermum petersonii (family Myrtaceae) is often cultivated for ornamental purposes but also serves as a rich source of bioactive essential oils. While several studies focused on the activities of the essential oils, this study analysed the potential of spent L. petersonii leaves as a natural food preservative. Method: We investigated the in vitro antioxidant and antimicrobial activities of crude L. petersonii extracts against activities of the purified isolated flavonoid, 6-methyltectochrysin, which was characterized using spectroscopic methods. The antioxidant assays followed ORAC, FRAP and TEAC tests. The antimicrobial activities of the extract and purified flavonoid were analysed against six multi-drug resistant microbial strains in broth dilution assays. Result: The results revealed that both the crude extracts and isolated 6-methyltectochrysin exhibited positive radical ion scavenging antioxidant potential, however the crude extract was about 6-fold more potent antioxidant than the purified 6-methyltectochrysin. The crude extract also showed strong antimicrobial activities against Bacillus cereus, and even more potent antimicrobial agent than the reference ampicillin antibiotic against Klebsiella pneumoniae subsp. pneumoniae. A higher resistance was observed for the tested Gram-negative strains than for the Gram-positive ones. 6-methyltectochrysin was generally inactive in the antimicrobial assays. Conclusion: The crude methanolic extract showed significant bioactivity which validates the medicinal relevance of the plant. The observed biological activities, especially against a notorious strain of B. cereus, suggest that L. petersonii could be a promising natural source of food preservatives.

Pulse radiolytic studies of supercritical CO 2

The species produced by ionizing radiation in supercritical CO 2 have been measured at a reduced temperature of 1.03 from 78 to 117 bar. It has been shown that C 2O 4 + is formed. This then reacts with positive ion scavengers. Its rate constant with dimethylaniline is 1.2×10 11 l mol −1 s −1 at 104 bar. The amount of C 2O 4 + formed is not proportional to the CO 2 density. This is attributed to an increased escape of the geminate ions at lower densities. We have also measured the production of ozone, which implies the production of O atoms and a production of oxygen molecules.

Effects of scavengers on short-lived ortho-Ps

Positron lifetime spectra are usually analyzed with three exponentially decaying components, para-Ps(positronium), free positron and ortho-Ps. However, a fourth component was obtained in many liquids. We reported the first results of the inhibition of the fourth component by CCl 4 in hexane and proposed a model. We show that the effects of several electron and positive ion (hole) scavengers, such as CS 2 , C 6 F 6 , SF 6 and pyridine on the fourth component are successfully explained with this model

Influence of ionization density on radiolysis of cyclohexane-benzene mixtures

Cyclohexane-benzene mixtures were irradiated by 210Po-α-radiation (LET = 200 eV/nm) with the effective energy of 3 MeV and the results were compared with those obtained by 1.25 MeV λ-irradiation of 60Co (LET = 0.2 eV/nm). While during λ-irradiation—similarly to the data known from the literature—the G(c-C 6H 10) and the G((c-C 6H 11) 2) values were decreased even by a small amount of benzene, the yields of both products obtained by α-irradiation were in the same range practically constant, and began to decrease only above the benzene concentration of 0.3 mol dm -3 (about 3 electron %). The “protection” in the case of λ-irradiation was interpreted by positive ion scavenging, S 1 excited cyclohexane molecule quenching and thermal H atoms scavenging. The low efficiency of all these three components of “protection” in α-radiolysis is attributed principally to the very high concentration of intermediates in the track/core compared to the relatively low concentration of benzene. The conclusions were also supported by comparison with the data obtained in cyclohexane-iodine systems.

Multiphoton-induced chemistry of phenol in hexane at 266 nm

The technique of multiphoton-induced chemistry (MPIC) has been employed to initiate ion-molecule chemistry of phenol in n-hexane solution. The technique involves the use of liquid phase multiphoton ionization (MPI) to prepare organic cations, which then react with the solvent in ion-molecule processes. Products of the ion-molecule chemistry, detected by gas chromatography/mass spectroscopy, are phenoxyphenol and hexyloxybenzene. These products depend upon the square of the laser intensity. It is explicitly shown by pulsed ionization-current measurements, that ionic species are produced as precursors to the observed products. Moreover, positive ion scavengers inhibit the reactivity. Estimates of the rates and quantum yields for the title process are given, as are rate data for the inhibition of the reaction by the ion scavenger, decalin. A brief discussion of the mechanism is also presented.

Positive ion scavenging by olefins in trans-decalin: TRMC and product analysis studies

The rate constants for scavenging of the mobile positive ion in trans-decalin have been measured using the time-resolved microwave conductivity (TRMC) pulse radiolysis technique for a series of olefins and cyclopropane. For the olefins the values vary from ⩽2x10 8dm 3/mol/s for ethylene to 1.6x10 11dm 3/mol/s for cyclohexene. Steady-state γ radiolysis of solutions of isobutene in trans-decalin results in the formation of isobutane and C 8 and C 14 products which display different dependences on solute concentration. The results are explained in terms of the formation of a complex between the solvent radical cation and isobutene which can dissociate, within a timescale of approx. 1 ns, to give isobutane or on neutralisation leads to the formation of C 14 condensation products. The C 8 products which are formed above 5 x 10 -2mol/dm 3 are attributed to reaction of the complex with isobutene to form the olefin dimer cation. Cyclopropane is unreactive, i.e. k<10 8dm 3/mol/s, towards the mobile positive ion in trans-decalin.

Positive-Ion Scavenging in the Positron Spur. Ortho-Ps Yields in Cyclopropane and Ammonia Solutions of Cyclohexane and Trans-Decalin

Positive-Ion Scavenging in the Positron Spur. Ortho-Ps Yields in Cyclopropane and Ammonia Solutions of Cyclohexane and Trans-Decalin

Positron spur reactions with excess electrons and anions in liquid organic mixtures of electron acceptors

By means of the positron lifetime technique we have measured positronium (Ps) yields in mixtures of nonpolar liquids with various electron scavengers which bind the electron fairly weakly (1–2 eV) in stable anions. The results are discussed with reference to recent excess electron works, and new experiments on anions and excess electrons are proposed. The minimum of the Ps yield versus CS2 concentration curves caused by partly delocalization of electrons on several scavenger molecules, which was observed previously in saturated aliphatic hydrocarbons occurred also in the saturated cyclic hydrocarbon cyclohexane, but did not appear in the aromatic benzene. This might be explained by the weak electron acceptor property of aromatics. In the Ps yield versus SF6 concentration curve in hexane a similar minimum appeared as in the CS2 case, probably by the same reason. By adding 0.8 M CS2 to the system the minimum was shifted towards the lower concentrations, which might be explained by some overlap of the energy levels on CS2 and SF6. Antiinhibition effect of C6F6 and CS2 was studied in 0.05 M CCl4/hexane solutions. In the CS2 case first a small minimum appeared in the Ps yield versus CS2 concentration curve. Alcohol clusters did not show antiinhibition in 0.05 M CCl4/neopentane system. Weak electron scavengers (C6F6, naphthalene, biphenyl, benzene) which generally act as antiinhibitors were added to pure non-polar solvents (hexane, isooctane, cyclohexane) and caused Ps enhancement. This can be attributed to “antirecombination” effect with a similar mechanism as that of the antiinhibition, but some contribution from positive ion scavenging cannot be excluded either. The increase of the Ps yield in hexane as a function of C6F6 or naphthalene concentration was much steeper if 0.8 M CS2 was added to the system, indicating that antirecombination and antiinhibition took place simultaneously. On addition of hexane, isooctane, CS2 or 0.8 M CS2/hexane to pure C6F6 the Ps yield drastically decreased. This correlates well with the drastic decrease of the electron mobility by adding similar additives to C6F6 observed recently.

Ionization in the track of a high-energy electron in hydrocarbon liquids; study of positive and negative ion scavenging using nanosecond pulse radiolysis

The lifetime distribution of the charged particles in the track of a high-energy electron in cyclohexane and isooctane has been considered. Results on the decay of biphenyl ions in solutions of biphenyl obtained by means of pulse radiolysis with nanosecond time resolution have been compared with lifetime distributions calculated from the concentration dependence of the steady state scavenging, using recently determined absolute rate constants for reaction of excess electrons and mobilities of the charged species. The discrepancy is relatively small for cyclohexane but large for isooctane. It is concluded that the scavengers considerably affect the initial spatial distribution of the charged species in the track. The kinetics of the reaction of the positive species in cyclohexane and isooctane with biphenyl, TMPD, and pyrene has been considered. It has been concluded that the mobile positive species in cyclohexane reacts with pyrene with a rate constant of 5×1011 mol−1⋅s−1, and has a lifetime of 50 ns. In isooctane no evidence for the existence of a mobile positive species has been obtained.

Effect of positive ion scavengers on recombination times: An attempt to use secondary electron transfer reactions as a probe for the existence of a high mobility hole in cyclohexane

In order to probe for the existence of a high mobility hole in cyclohexane, secondary electron transfer reactions between CH 3Cl − and other electron scavenging solutes have been studied in the absence and presence of positive ion scavengers. There is no evidence in these experiments for a change in the time scale for neutralization of the negative ions which can be attributed to trapping of the positive ions.

Pulse Radiolysis of Methane.

The pulse radiolysis of methane has been studied in the absence and presence of electron scavengers such as SF6 and CD3I and positive ion scavengers such as i-C4D10 in order to define the role of the intermediates H, C, CH, CH2, CH3, , and in product formation. The dose rate was varied from 0.68 to 15.2 × 1019 eV/g-s, the dose (number of pulses) was varied, and the duration of the pulse was changed from 3 ns to 100 ns. The variation of the yields of the ethylene and ethane products with dose is explained by the reaction of H-atoms with accumulated ethylene product. The fast reacting C, CH, and 1CH2 species insert into methane to form acetylene, ethylene, and ethane products, but all of the reactions of these species cannot be completely specified since they may originate in upper electronic states, whose reactions with methane are unknown. Product formation by the slow reacting 3CH2 and CH3 radicals is also examined; for instance, evidence is presented for the occurrence of the reaction: 3CH2 + CH3 → C2H4 + H. Results indicate that the ions and undergo neutralization mainly through the processes When i-C4D10 is added, a fraction of the and react with the additive rather than undergo neutralization. A calculation demonstrates that the fraction of ions undergoing reaction with a given concentration of i-C4D10 can be correctly predicted by assuming that the rate constant for neutralization of and is the same as that determined recently for the t-butyl ion.

A Positive-Ion Reaction Mechanism for the Radiation Induced Oxidation of Carbon Monoxide in the High Dose Rate Radiolysis of CO–O2 Mixtures

Carbon dioxide and ozone yields have been measured in the irradiation of CO–O2 mixtures with single pulses of electrons. The yields of CO2 at 2 × 1027 ev g−1 s−1 are large G(CO2) = 15 ± 1 from 20–60% O2. These decrease by ∼50% at 1028 eV g−1 s−1. Although the results are somewhat irreproducible the addition of c-C4F8, an electron scavenger, increases these CO2 yields at both dose rates by up to a factor of 15. They are reduced to G(CO2) &lt; 2 by the addition of positive ion scavengers. These results are consistent with a chain reaction similar to that proposed for low dose rate studies involving [Formula: see text], [Formula: see text], and [Formula: see text] as chain carriers.The ozone yields are consistent with a total oxygen atom yield of G(O) = 1.4 in pure CO from neutral processes.

Ion Scavenging by Neopentane in γ-Irradiated Liquid Alkanes

The γ-radiolysis of low concentrations of neopentane in liquid cyclo-C5H10 and C2H6 at 0.25 g ml−1 density leads to methane yields much greater than expected from the mixture compositions. In cyclo-C6H12, n-C6H14, cyclo-C6H11CH3, and 3-methylpentane the yields are much less than expected. The effect is explained in terms of positive ion scavenging which is enhanced by added electron scavengers. The efficiency of scavenging by C(CD3)4 in C2H6 is equal to that of ND3. The efficiency in cyclo-C5H10 is less than that in C2H6 but is greater than that observed previously in this solvent.

Effect of Pressure on the Rates of Reaction of Radiolysis Intermediates in Liquid n-Hexane

The yield of hydrogen from pure hexane, G(H2) = 5.3 ± 0.2 was independent of pressure in the range 1 bar to 4.6 kbars. The G values of H2, HD, and D2 obtained from 11 mol% C6D14 in C6H14 were independent of pressure and were respectively, 4.85, 0.34, and 0.11. The electron and hydrogen atom scavenging efficiencies of carbon tetrachloride and hexadiene-1,3, and the hydrogen atom scavenging efficiency of hexene-1 were independent of pressure. The positive ion scavenging efficiencies of benzene and aniline increased with increasing pressure. The volume of activation for the hydrogen atom reaction with n-hexane is essentially the same as that for the reaction with hexene-1. The pressure dependence of the charge scavenging reactions has been interpreted in terms of the kinetics of spur processes. The treatment is consistent with the fact that the efficiency of the electron scavenging reactions is independent of pressure. The increased efficiency of positive ion reaction under pressure implies that the ratio of the diffusion coefficients of the radiolytic positive ion and electron, D+/D−, increases with increasing pressure.

Effect of positive ion scavenging by benzene in the radiolysis of propane

Effect of positive ion scavenging by benzene in the radiolysis of propane

Kinetics of Electron and Ion Reactions during the γ Radiolysis of Liquid Neopentane; Dependence of Charge Scavenging Kinetics on Hydrocarbon Molecular Structure

The large free ion yield in neopentane has been confirmed by chemical measurement: for nitrous oxide solutions at 23°C g(N2)fi = 0.95 and g(N2)max ≈ 4.7. The average number of nitrogen molecules formed per electron scavenged appears to be 1.1 ± 0.1 at all nitrous oxide concentrations. Thus gfi = 0.9 in neopentane, which is sevenfold larger than that in n-hexane. The median range of the secondary electrons in liquid neopentane is about fourfold greater than that in n-hexane. The greater range in neopentane correlates with the more sphere-like shape of the molecules. In spite of the larger median electron range in neopentane, the ease with which electrons and ions can be scavenged in spurs in that liquid is similar to that in hydrocarbons with distinctly nonspherical molecules. Furthermore, the ratio of efficiences of electron to positive ion scavenging (β− / β+) is 17 in neopentane, similar to the value 13 in the “nonspherical” methylcyclopentane. During the radiolysis of pure neopentane two types of ions are formed: G(PH+) = 1.7 and G(N+) = 3.0. PH+ yields hydrogen when neutralized by an electron, but not when neutralized by a massive negative ion. PH+ can also donate a proton to ammonia. N+ does not yield hydrogen upon neutralization, nor does it donate a proton to ammonia. In pure neopentane G(H2) = 2.5 ± 0.1 and G(CH4) = 3.9 ± 0.2 at a dose of 1.5 × 1018 eV/g. Scavenging the free ions by either N2O or ND3 reduces g(H2) by about 0.35, but has no effect on the methane yield.

The radiolysis of ethane. II. Liquid phase at 195°K; effect of electron and positive ion scavengers

H atoms and•C2H5 radicals in the radiolysis of liquid ethane have both ionic and non-ionic precursors. The use of SF6 as an electron scavenger leads to very low C2H4 yields which is consistent with the addition of•SF5 radicals to this product. H and D atoms produced by neutralization of ND3H+ can either add to C2H4 or abstract from C2H6 at 195°K.

Positive Ion Scavenging in the Radiolysis of Liquid Chloroform

The effects of the positive ion scavengers NH3 and n-C4H9OH, on the organic product yields in the radiolysis of liquid CHCl3 at 25°C were investigated. As the solute concentrations increase, the values for G(CH2Cl2), G(C2HCl5), and G(C2Cl6) increase, G(sym-C2H2Cl4) and G(C2Cl4) remain unchanged, and G(CCl4) decreases. These results are consistent with a free radical mechanism for all the radiolysis products except CCl4. The decrease in G(CCl4) indicates that CCl4 is formed by a secondary positive ion reaction. The concentration dependence of this decrease can be described by the expression G(CCl4)O−G(CCl4)S=G(CCl4)O((α[S])1/2/{1+(α[S])1/2}), where G(CCl4)O is the 100-eV yield in the absence of a scavenger, and α is an empirical parameter proportional to the reactivity of the solutes, and [S] is the concentration of the scavenger in moles per liter. Values of α were determined for each solute and compared to published values determined in cyclohexane. In chloroform, values for α were 4–19 times larger than in cyclohexane suggesting that the solutes are more efficient in the alkyl halide than in the hydrocarbon. One mechanism accounting for this efficiency increase is the formation of anions by the free electrons in the alkyl halide. This anion formation increases the average lifetimes of the ions in the alkyl halide and hence the efficiencies of the scavengers. The increase in the 100-eV yields of three of the products formed by the free radical mechanism indicates that the presence of a cation scavenger increases the radical yields. Thus, some of the charge neutralizations in pure CHCl3 do not produce radicals. This observation is in accord with the existence of the collisionally stabilized CHCl3− anion in the irradiated solutions. It is suggested that this anion may be an intermediate in the formation of CCl4.

Scintillation Efficiencies of Cyclohexane Solutions with Varying Solute Concentration

Increasing the concentration of scintillator solute in cyclohexane leads to an increase in the relative scintillation yield through the action of the scintillator molecules as electron and positive ion scavengers. In this work the increase was shown to be proportional to the logarithm of the cube root of the solute concentration. This dependency indicates that over the concentration range investigated, 10−3 to approximately 1 mole/liter, the scavenging process differs from the square-root dependence which has often been observed with other electron scavengers.

The Mechanism of the Scintillation from a Benzene Solution of p-Terphenyl

Abstract The luminescence from a benzene solution containing p-terphenyl induced by γ-ray irradiation has been compared with that induced by UV irradiation at 2500 Å in the presence of various quenchers. The quenching efficiencies of electron and positive ion scavengers (SF6, N2O, CO2 and NH3) were found to be greater for the γ-ray excitation than for the UV excitation, while 1,3-butadiene, which is known as a quencher for excited benzene, (1B2u), showed almost the same efficiency for both modes of excitation. These results suggest that, even in a benzene solution, a series of ionic reactions is partly responsible for the formation of the scintillating state of p-terphenyl, and that the apparent quenching effect of ion scavengers is mainly due to their interference in the formation of the excited state of the benzene which is produced by the recombination reaction between the benzene cation and an electron. It was estimated that more than 30% of the excited state of benzene is formed by the recombination reaction.