Radiolytic degradation of chlorobenzene in aerated and deoxygenated aqueous solutions.

Radiation-induced degradation of chlorobenzene was conducted at 0.1, 0.4, 0.5, 0.7, and 1.0 mmol/dm3 concentrations in aerated environment and at 1.0 mmol/dm3 in oxygen-free and N2O-saturated solutions. The results demonstrated that the elimination of chloride is important when the solution is oxygen free, because the [Formula: see text] attacks at the ipso position of the chloro group produces hydrochloric acid. The degradation was affected to a large extent by the concentration and to a lesser extent by the presence or absence of oxygen in the solutions which were irradiated. Thereby, the degradation occurred faster in the solutions with air and without oxygen and more slowly in the saturated solution with N2O. Some by-products were identified using an HPLC-UV-mass system. In addition, it was found that there is a linear correlation between the ln C/C0 and the dose, indicating that the radiolytic degradation followed pseudo-first-order reaction kinetics. The radiolytic oxidation was followed by the chemical oxygen demand (COD) test. The COD decreases when the solute concentration increases. The COD results were for a 0.47 mmol/dm3 of 5.94 mg O2 dm-3 kGy-1 and for 0.09 mmol/dm3 of 7.45 mg O2 dm-3 kGy-1.

Radiolytic oxidation and degradation of 2,4-dichlorophenol in aqueous solutions.

Radiolytic oxidation of 2,4-dichlorophenol (2,4-DClP) in aqueous solutions demonstrated that ·OH predominantly adds to the unsubstituted positions of the aromatic ring and that elimination of chloride at the 4 position is important because the -OH group enhances the electron density at this position, which is favorable for the electrophilic reactions. The total yield obtained was 0.540 µmol/J. Radiation-induced degradation of 2,4-DClP was conducted in oxygen-free aqueous solutions (0.1, 0.25, 0.50, and 0.7 mmol/dm3), saturated with N2O, and aerated and under irradiation at low and high doses. The results demonstrate that the largest degradation occurred in oxygen-free solutions due to oxidation (·OH) and reduction reactions (H· and [Formula: see text]) and attack of the [Formula: see text] at the ipso position of -Cl, producing HCl. The degradation was affected to a large extent by the concentration and to a lesser extent by the presence or absence of oxygen in which the 2,4-DClP solution was irradiated. At concentrations less than 1 mmol/dm3, 2,4-DClP was degraded in the solution at an absorbed dose level of 1 kGy. At higher doses, the product concentrations increased to up to 30% of the dose required for the total degradation of 2,4-DClP; then, they decreased. A graph plotting the logarithm of the relative concentration as a function of the dose shows a linear correlation, which indicates that the radiolytic degradation followed pseudo-first-order reaction kinetics. The oxidation was followed by the chemical oxygen demand (COD). COD decreases when the solute concentration increases. This fact has a dependence on the presence or absence of oxygen too.

OH Radical as a Probe of the Spin Polarizability in 1- and 2-Naphthol.

The relative yields for addition of the OH radical at the various positions of 1- and 2-naphthol provide a measure of the spin polarizability in the naphthols. The observed yields show that addition occurs predominantly at the naphthol positions that are conjugated with the OH substituent. They also show that the electronic structures of the naphthols are significantly affected by a concerted interaction between the OH substituent and the unsubstituted ring and that this effect is somewhat greater when the OH substituent is adjacent to the naphthol bridge. The yields for addition at the different naphthol positions correlate with the local spin polarizabilities at reactive carbons in the naphthol. The spin polarizability may be a general property governing the reactivity of closed-shell molecules with radicals.

Influence of concentration on the radiolytic decomposition of thiamine, riboflavin, and pyridoxine in aqueous solution / Influencia de la concentración en la descomposición radiolítica de tiamina, riboflavina y piridoxina en solución acuosa / Influência da concentração na decomposição radiolítica de tiamina, riboflavina e piridoxina em solução aquosa

Vitamin loss during irradiation has been claimed as a critical area in food irradiation technology, especially that of thiamine (B1), which has been considered as the most sensitive to radiation. Although it has been suggested that no vitamin deficiency could result from consuming irradiated food, a long debate on the loss of vitamins and other nutrients during food irradiation has been maintained by the lack of experimental studies monitoring decomposition rates at different concentrations and doses. Since thiamine, riboflavin, and pyridoxine are labile vitamins, this study has focused on their radiolytic decomposition in dilute aqueous solutions in the presence of air. The decomposition process was followed by HPLC and UV-spectroscopy. The results obtained in aqueous solutions showed a dependence of the decomposition as a nonlinear function of the dose. Of these three compounds, the decomposition was higher for thiamine than for riboflavin and even less in pyridoxine.

La pérdida de vitaminas durante procesos de irradiación ha sido considerada como un área crítica en la tecnología de irradiación de alimentos, especialmente la tiamina (B1), que ha sido considerada como la más sensible a la radiación ionizante. La deficiencia de vitaminas en humanos no es producida por el consumo de alimentos irradiados, sin embargo, existen debates sobre la pérdida de vitaminas y otros nutrientes provocada por la irradiación de alimentos, esta discusión sigue latente debido a que hay pocos estudios experimentales de la descomposición de vitaminas a diferentes dosis y concentraciones. Esta investigación se centró en el estudio de la descomposición radiolítica de tiamina, riboflavina y piridoxina en soluciones acuosas y en presencia de aire. El proceso de descomposición fue seguido por cromatografía líquida con detección UV. Los resultados obtenidos en soluciones acuosas mostraron una dependencia no lineal entre la descomposición en función de la dosis. De estos tres compuestos, la descomposición fue mayor en tiamina que en riboflavina y menor en la piridoxina.

A perda de vitaminas durante processos de irradiação tem sido considerada uma área crítica na tecnologia de irradiação de alimentos, especialmente no caso da tiamina (B1), que tem sido considerada como a mais sensível à radiação ionizante. Embora a deficiência de vitaminas em seres humanos não seja produzida pelo consumo de alimentos irradiados, longos debates sobre as perdas de vitaminas e outros nutrientes causadas pela irradiação de alimentos tem sido mantidos devido aos estudos experimentais limitados monitorando a proporção da decomposição em diferentes concentrações de vitaminas e doses de radiação aplicadas. Considerando que a tiamina, riboflavina e piridoxina são vitaminas instáveis, o presente estudo focalizou a decomposição radiolítica dessas vitaminas em soluções aquosas diluídas e na presença de ar. O processo de decomposição foi analizado por cromatografia líquida com detecção UV. Os resultados obtidos em soluções aquosas mostraram uma dependência da decomposição como função não linear da dose. Destes três compostos, a descomposição foi mais alta para tiamina que na riboflavina e menor para piridoxina.

Absorption spectrum, mass spectrometric properties, and electronic structure of 1,2-benzoquinone.

Absorption spectrophotometric and mass spectrometric properties of 1,2-benzoquinone, prepared in aqueous solution by the hexachloroiridate(IV) oxidation of catechol and isolated by HPLC, are reported. Its absorption spectrum has a broad moderately intense band in the near UV with an extinction coefficient of 1370 M(-1)cm(-1) at its 389 nm maximum. The oscillator strength of this band contrasts with those of the order-of-magnitude stronger approximately 250 nm bands of most 1,4-benzoquinones. Gaussian analysis of its absorption spectrum indicates that it also has modestly intense higher energy bands in the 250-320 nm region. In atmospheric pressure mass spectrometric studies 1,2-benzoquinone exhibits very strong positive and negative mass 109 signals that result from the addition of protons and hydride ions in APCI and ESI ion sources. It is suggested that the hydride adduct is formed as the result of the highly polar character of ortho-quinone. On energetic collision the hydride adduct loses an H atom to produce the 1,2-benzosemiquinone radical anion. The present studies also show that atmospheric pressure mass spectral patterns observed for catechol are dominated by signals of 1,2-benzoquinone resulting from oxidation of catechol in the ion sources. Computational studies of the electronic structures of 1,2-benzoquinone, its proton and hydride ion adducts, and 1,2-benzosemiquinone radical anion are reported. These computational studies show that the structures of the proton and hydride adducts are similar and indicate that the hydride adduct is the proton adduct of a doubly negatively charged 1,2-benzoquinone. The contrast between the properties of 1,2- and 1,4-benzoquinone provides the basis for considerations on the effects of conjugation in aromatic systems.

Determination of the spectroscopic properties and chromatographic sensitivities of substituted quinones by hexachlorate(IV) oxidation of hydroquinones.

HPLC studies of the oxidation of substituted hydroquinones show that the corresponding quinones can, in most cases, be produced quantitatively by two electron transfer to hexachloroiridate(IV). As a consequence the chromatographic and spectroscopic properties of substituted quinones that are not readily available can be determined without the necessity of preparation of an isolatable sample. Absorption spectra and extinction coefficients of bromoquinone, chloroquinone, hydroxyquinone and carboxyquinone anion at pH 7 are reported as illustrative examples. All have intense absorption bands at approximately 250nm that are characteristic of quinones. Oxidation of carboxyhydroquinone at low pH is, however, anomalous in that a hydroxylated carboxyquinone is produced as the result of four electron transfer to hexachloroiridate.

Hydroxyl radical as a probe of the charge distribution in aromatics: phenol.

Initial radiation chemical yields of 1.48 (2), 0.24 (2), and 2.01 molecules per 100 eV of absorbed energy are reported for addition of *OH radical to each of the ortho, meta, and para positions of phenol. These yields represent 91% of the yield of 5.96 expected for *OH addition to 5 mM phenol and are in general agreement with other previous measurements. Pulse radiolysis experiments show that phenoxy radical is produced in a yield of approximately 0.42 as a result of addition of *OH at phenol's ipso position. The total of these yields (5.84) accounts for the addition of virtually all of the expected *OH radicals. The relative yields for addition to the ortho, meta, and para positions provide a measure of the charge distribution in phenol that correlates quite well with the unpaired spin distribution in phenoxyl radical. This correlation indicates that the OH substituent similarly affects the charge distribution on the aromatic ring of phenol and the unpaired spin distribution in the phenoxyl radical.

Concerted effects of substituents in the reaction of .OH radicals with aromatics: the cresols.

The concerted effects of hydroxyl and methyl substituents in controlling the site of .OH radical attack on aromatics in aqueous solutions are explored using the cresols as typical examples. The distributions of dihydroxytoluenes produced in the radiolysis of aqueous solutions of the cresols containing ferricyanide as a radical oxidant were examined by capillary electrophoretic and liquid chromatographic methods. Because .OH is a strong electrophile, it adds preferentially at the electron-rich sites of an aromatic ring. As a result, the observed distributions of dihydroxytoluenes reflect the charge distributions in the cresols. It is shown that in the case of m-cresol the hydroxyl substituent has a dominant ortho-para directing effect similar to that observed for phenol. In o- and p-cresol, this effect is modified, indicating that the methyl substituent has a significant effect on the electronic structure of those cresols. Correlation of the charge distribution in the cresols indicated by the observed distribution of dihydroxytoluenes with the unpaired spin distribution in the corresponding methylphenoxyl radicals demonstrates that the electronic structures of o- and p-cresol and their corresponding phenoxyl radicals are similarly affected by hydroxyl and methyl substitution. Addition of .OH at the methyl-substituted positions of o- and p-cresol to produce o- and p-dienone is also reported. The observation of these dienones demonstrates that addition of .OH at the ipso positions of alkylated aromatics can be of considerable importance. Mass spectrometric studies show that these dienones have relatively higher proton affinities than their isomeric analogues.