Nitrous oxide electron scavenging

Steady photoemission currents can be realized when acceptors (scavengers) for the solvated electrons are present in the solution. A good scavenger should be nonelectroactive at the potenhal of interest, should react quickly with solvated electrons, and the reaction products should be either nonelectroactive or reducible. A reachon with acceptors implies that the current of reoxidation of the solvated electrons becomes lower, and thus a steady photoemission current appears. The acceptors most often used are nitrous oxide, N2O, and hydroxonium ions, HjO. In the former case, OH radical is produced in the scavenging process, which undergoes further reduction on the electrode, thus doubling the photocurrent ... 

Figure 6 Effect of scavenging capacity for e q on the number of electrons scavenged in aqueous solutions of nitrous oxide, nitrate, and selenate. Experimental data nitrate—( ) [77], ( ) [78] ...

Irradiation Procedure. 30% w/w solutions of poloxamer were prepared in distilled water by the cold process and saturated with nitrous oxide. This agent is a known scavenger of hydrated electrons and is known to enhance crosslinking of poly(oxyethylene) chains (9). Vials containing these solutions were irradiated at ambient temperature in a 2000 Ci ° Co source at a dose rate of 0.5 Mrad h- Physicochemical measurements were performed on solutions prepared by dilution of these irradiated samples. 

Nitrous Oxide, N2O, was the ubiquitous electron scavenger used by radiation chemists in the 1960s, and an understanding of the process of electron attachment and the subsequent reactions leading to the formation of nitrogen was of primary concern. Hence, one of the first applications of PR-TRMC, by one of the present authors (JMW), was to the study of electron attachment in pure NjO gas. The results indicated that direct dissociative attachment leading to O formation. 

Trichloroacetate rapidly reacts with the solvated electrons produced by laser flash photolysis of natural organic matter isolated from the Suwannee River, and thus quenches the absorption of the electrons at 720 nm. The ibsorption is also quenched by the addition of other good electron acceptors, including oxygen, protons, or nitrous oxide. In natural waters, halocarbon concentrations are typically very low, and the dominant scavenger of solvated electrons is oxygen. 

In very rare cases in which photoionization is the only photoprocess, the absorption observed in a nitrous oxide-saturated solution will be that of the cation radical. However, generally, other processes such as intersystem crossing also occur in parallel. In the presence of oxygen, the hydrated electron and the triplet will be scavenged at diffusion-controlled rates, and the absorption observed in the oxygenated solution will be due to the cation radical. Under these conditions, the cation radical spectrum is easily determined. The molar absorption coefficient of the cation radical can also be calculated using the hydrated electron as an internal standard. The molar absorption coefficient for sulphacetamide cation radical was determined in this manner (Land et al 1982) and later confirmed by pulse radiolysis (see Section 12.2.2.6). 

Similarly, the hydroxyl radicals and hydrogen atoms can be scavenged with t-butanol, leaving the hydrated electron to react with the compound to produce its anion radical. MQ anion radical was produced by pulsing a nitrogen-saturated aqueous solution of MQ in the presence of t-butanol (Navaratnam et al., 2000). Another one-electron reductant, C02" , is formed by pulsing a nitrous oxide-saturated solution of sodium formate ... 

The main-chain scission yield was recently compared at room and liquid nitrogen temperatures [415] in the presence of a large number of additives known as radical, cation or electron scavengers. The results are given in Table 31. The protection index, in this case defined as 100(7V0 — N)/N where N0 and N are the number of scissions per chain in the absence and in the presence of additive, respectively, is nearly independent of the irradiation temperature marked protection is observed for all the additives studied with the exception of nitrous oxide. It must therefore be concluded that the mechanism of main-chain scission is identical at room and liquid nitrogen temperatures and that ions and radicals are involved in the radiolysis. A detailed study of the effect of ethyl mercaptan on main-chain scission and volatile formation was then undertaken [395]. About 75% protection of main-chain scission was obtained at 313 and at 77°K when the polymer contained 1.49 wt. % of ethyl mercaptan the protection index increases to 90% for concentrations of the order of 10 wt. %. The yield of volatile products was, however, unaltered by the presence of 1.5 wt. % ethyl mercaptan. 

In the one experimental study on electron scavenging carried out with high LET radiations Burns and Reed (8) have examined the yields of N2 and H2 produced from nitrous oxide-cyclohexane solutions by 2 Mev. helium ions (LET — 20 e.v./A.). At particular N20 concentrations they find significantly lower N2 yields and smaller decreases in the H2 than observed in the y-ray experiments (at 0.1 M N20 G(N2) = 0.6 in experiments with helium ions vs. 3.8 for those with y-rays AG(H2) = 0.2 vs. 2.3). Here reaction between the electron and a positive ion other than its original partner apparently becomes significant, and the electronscavenging process cannot compete as efficiently as at low LET s. 

Electron Scavenging by Nitrous Oxide. Nitrous oxide is an effective electron scavenger in hydrocarbons (44), producing high yields of nitrogen. The possible reactions involved are... 

In the 1470-A. photolysis of cyclohexane-nitrous oxide solutions, nitrous oxide reacts with excited cyclohexane molecules to form nitrogen and oxygen atoms. The reaction of N20 with photoexcited 2,2,4-trimethylpentane molecules is much less efficient than with cyclohexane. In the radiolysis of these solutions, G(N2) is the same for different alkanes at low 5 mM) N20 concentrations. At higher concentrations, G(N2) from the radiolysis of cyclohexane is greater than G(N2) from the radiolysis of 2,2,4-trimethylpentane solutions. The N2 yields from 2,2,4-trimethylpentane are in excellent agreement with the theoretical yields of electrons expected to be scavenged by N20. The yield of N2 in the radiolysis of cyclohexane which is in excess of that formed from electrons is attributed to energy transfer from excited cyclohexane molecules to nitrous oxide. 

Kinetic studies of the competitive reactions of other electron scavengers support this hypothesis (18, 20). In the radiolysis of solutions of nitrous oxide in alkanes, reactions with other intermediates must be considered. Radicals, hydrogen atoms, and positive ions can be eliminated (5, 20), but a reaction with excited molecules is possible. It has been reported... 

Figure 3. Yields of nitrogen in molecules/100 e.v. as a function of nitrous oxide concentration in cyclohexane, O, and in 2,2,4-trimethylpentane. Solid line represents theoretical yields of electrons scavenged hy nitrous oxide (7)...

For cyclohexane solutions the yield of nitrogen exceeds that caused by electron scavenging (as given by theory or by the 2,2,4-trimethylpentane results). This excess yield can be reasonably accounted for if nitrous oxide reacts with some cyclohexane derived intermediate which... 

For studies of hydroxyl radical reactions, the hydrated electrons can be converted to hydroxyl radicals by saturating the reaction solution with nitrous oxide. N2O is substantially soluble in water with a Henry s law constant (mole fraction scale) at 25 °C of 0.182 X 10, it has a concentration of about 26 mmol dm at 25 °C. Solvated electrons are rapidly scavenged by the N2O (reaction... 

Under our experimental conditions the hydrated electron is effectively scavenged by nitrous oxide in competition with the reactions with hydrogen ion and with the aromatic molecule ... 

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