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Photodecomposition of europium(III) acetate and formate in aqueous solutions
Abstract
The products obtained in the photodecomposition of europium(III) acetate and formate were determined quantitatively under various pH conditions. The two photoprocesses and the subsequent radical reactions have been found to proceed stoichiometrically with a cyclic redox reaction of the Eu/sup 3 +//Eu/sup 2 +/ couple being the key process. In the acetate system, (1) redox reactions of methyl radical with Eu/sup 2 +/ and Eu/sup 3 +/ aquo ions are supported to occur with formation of methane and methanol, respectively; (2) carboxymethyl radical produced through hydrogen abstraction from acetic acid by the methyl radical probably regenerates acetic acid via the intermediate formed through the radical capture reaction by Eu/sup 2 +/ ion, followed by acidolysis reaction. In the formate system, (1) it was confirmed that the photoprocess is followed by a chain reaction in which carboxyl radicals generated in the primary process and hydrogen atoms produced through reduction by illuminated Eu/sup 2 +/ ion behave as chain carriers; (2) evolution of carbon dioxide gas is mostly ascribed to the disproportionation of carboxyl radical and the reduction of Eu/sup 3 +/ ion by carboxyl radical. In both systems, minor products are formed by redox and disproportionation radical reactions.
... Formation constants of REE(III) (REE = La, Ce, Pm, and Eu) glycinatocomplexes in water are extremely small (log β < 1) [22], whereas the electrostatic effects of protonated glycine at a negatively charged chitin interface prevail in the enhanced adsorption of La or Eu [10]. Photochemistry mediated by Eu(III) and product patterns for simple carboxylates and the corresponding acids (pH range 1.5-6) were studied by [23], and Mn-mediated oxidation pathways were studied by [24], whereas Eu organic ligand complexes should be of minor importance at the ligand concentrations observed in open waters [25,26]. ...
... Inappropriately chosen buffers would either compromise adsorption studies (cp. [10]), or the buffers themselves would experience photooxidation by Eu(III), like acetate [23], or even do both (oxalate/Hox − ). ...
There is strong adsorption of metal ions and their complexes to chitin, which depends on both the oxidation and complexation states of many of the said elements (whereas others display chemical reactions detectable via electrochemical methods while being retained by chitin); thus, ad- and desorption at ambient water concentrations (often in the nMol/L range) are controlled by the presence and photochemical properties (concerning Eu and probably U and Ag) of mainly biogenic organic matter (both DOC and POC, and DON). With chitin forming the outer hull of mobile organisms (animals), this biopolymer is expected to take part in metal distribution in aquatic (limnetic and riverine) ecosystems. Having studied the attachment of many different elements to both crayfish and grafted (marine shrimp) chitin, with the highest accumulations observed in Bi, V, Ni, and LREEs, one should consider secondary biochemical transformations which take place at different water and sediment levels. After chitin had been embedded into sediment, methanogenesis (which requires Ni), Bi, and Sb biomethylations and photodesorption in the illuminated water column will occur if there are appropriate organics, causing the vertical separation of Eu from other REEs, at least during the daytime. Eutrophication will enhance both the production and especially the photooxidation rates of organics in water because phosphorylated sugars and lipids are formed quantitatively within min P, which enter water and undergo Eu-mediated photooxidation much more readily. Another biopolymer, gelatin, acts as an inert matrix-enhancing organic photooxidation product via Eu, producing chemical waves, indicating autocatalysis upon light impact. From the redox-related photodesorption of metal analytes from chitin, both sensors and devices for (light-assisted) electrochemical energy conversion are being developed by our workgroup. The electrochemical determination of adsorption thermodynamics on chitin is thus directly linked to its applications in environmental monitoring and technology.
The influence of irradiance on the photochemical reduction of europium(iii) to europium(ii) is studied. It is known that europium removal from various rare-earth mixtures is feasible by means of photochemical reduction followed by precipitation, but the effect of the nature of the light source has not been investigated in detail yet. It is shown that irradiance rather than electrical power is a key parameter to characterize light sources used for photochemical experiments and to compare experiments with set-ups using different light sources. The irradiance of a light source, expressed in Watts per unit of area (mW cm⁻²), has a crucial impact on the photochemical reduction of europium(iii) in aqueous media, in particular on the illumination time needed for europium removal from the solution. The influence of the irradiance on the induction time (a period with no or little europium removal in the first illumination hours), the europium removal rate after the induction time, the overall removal time and the time needed to remove 50% of the initial europium content from the solution (t50%) is studied. It is observed that higher irradiances result in shorter induction times, faster europium removal rates, lower t50% and hence shorter overall illumination times. The threshold irradiance to obtain 50% europium removal in 36 hours was found to be 2.7 mW cm⁻², with an induction time of 28 hours. The residual europium content in all samples was lower than 2.5%, resulting in removal efficiencies of over 97.5%. This residual europium content seems to be increasing when the relative irradiance related to the back-oxidation increases, although this correlation cannot be fully explained yet. This work gives new insights that can be used for the photochemical recycling of europium from end-of-life red phosphors in compact fluorescent lamps (CFLs). This journal is
This chapter deals with the experimental time-resolved emission spectroscopy (TRES) results, and the theoretical approaches that are relevant to the question of lanthanides and actinides solution chemistry. The basic principles of solution chemistry of lanthanides and actinides are discussed in the chapter. Most of them are built from individual devices (laser, detection system) hence they are not of a “plug and press” type, so that their exact characteristics may vary from one installation to the other. The experimental data on the spectroscopic changes observed upon modifications of the medium surrounding the luminescent probe is presented in this chapter. The term luminescent probe refers to a complex, for which the lifetime changes induced by modifications of the surrounding medium are investigated. Various models and empirical approaches are used to interpret the data. The use of TRES for the study of a reaction rate constant is discussed in the chapter. TRES has been demonstrated to be of great interest for the study of U(VI) dissolved in supercritical CO2.
The review considers non-classical mechanisms of chain reactions, which were proposed in recent years. In contrary to classical radical chain mechanisms such mechanisms include electron transfer, heterolytic and other non-heterolytic elementary propagation steps and a variety of transients (ions, ion-radicals, biradicals etc.). Most of the known non-classical chain reactions include electron transfer propagation step. A lot of new speculative chain mechanisms was proposed in literature, but further experimental investigations of short living transient and kinetics of these reactions is necessary to prove their mechanisms. Such investigations will open new possibilities to use chain reactions in chemical and other processes.
The first spectroscopic study of metal ion-THFTCA (tetrahydrofuran-1, 2,3,4-tetracarboxylic acid) coordination is reported. Luminescence studies provide evidence for predominance of a complex containing 1 Eu(+3) ion and 2 THFTCA molecules in aqueous processing nuclear waste. Evidence that THFTCA coordinates, at least in part, via carboxylate groups was found in the Eu(3+) âµDâ state fluorescence decay rates. Direct photochemical destruction of THFTCA was demonstrated in the study of the influence of uv photolysis on THFTCA solutions; photolysis of THFTCA is promising for minimizing waste during solvent extraction of nuclear waste. Laser-induced fluoresence can provide near-real time monitoring of metal ion coordination in THFTCA solutions and photolysis of THFTCA and its metal ion complexes.
In the case of a rapid photochemical process, a new theoretical result relating time-resolved emission spectroscopy data to three physical parameters of the chemical system has been recently proposed. This previous work, based on a simulation study, is experimentally evidenced in the present paper, using europium/acetate as a model system. The comparison of the emission spectra obtained upon direct excitation of europium (394 nm) and by use of the "antenna effect" (266 nm) evidences the occurring of a back-dissociation of excited europium complexes to form solvated excited free europium ions.
Matrix isolation ESR study showed that the ligated HCCO− ion was decomposed into H+ and ·COO− radical anion through CTTM process at λ = 254 nm, by contrast, ·CH3 radical and CO2 were produced from CH3COO− ligand. In order to explain the photo- and related reactions in the liquid solution, a proposal is made for a cyclic scheme conjugated with the photo-decomposition of the complex. The cycle consists of three steps; photo-reduction of H+ by Eu2+, radical alternation from ·H to ·COO−, and oxidation of ·COO− by Eu3+.
The rates of the photochemical and thermal decarboxylation of aliphatic acids by lead (IV) tetraacete have been examined by a competition method. Photochemically, pivalic (tertiary) acid decarboxylates at 30° approximately 100 times, and isobutyric (secondary) acid 20 times, faster than n-butyric (primary) acid. The competitive decarboxylations of acids by lead(IV) carried out thermally at 80–100° show a smaller spread in reactivity. The thermal procedure is complicated by further oxidation of the olefinic products by PbIV. Rates of decarboxylation are related to the multibond cleavage of the Pb-carboxylate linkages directly into alkyl radicals and carbon dioxide. The mechanism of the reduction of PbIV is compared to the homolytic decarboxylations of the analogous CoIII, MnIII and CeIV carboxylates.
A study of photochemical reactions of uranyl ions in aqueous acid solutions was carried out by means of flash photolysis. Short-lived absorption (∼ 10−4 s) was observed on flash excitation of uranyl ion in solution and was assigned to singlet-singlet transitions. In the presence of certain organic substances a new transient absorption was detected due to production of Uv intermediate. Deactivation pathways of excited ion (UO2+2)* and decay kinetics of Uv were studied. Values of rate constants for some elementary processes were determined.
The deuterium content of the methane formed by radiolysis of deuterated
acetic acids, and the tritium content of methane formed from tritiated acetic
acids, both indicate that methane is forined from acetic acid almost cxclusively
by a radical process in which the intermediate methyl radicals abstract hydrogen
almost entirely from the methyl group of acetic acid. The isotope content of the
hydrogen produced in these irradiations, together with the rates of hydrogen
production (G(H/sub 2/)) values suggest, although less conclusively than for
methane, a radical process as the origin for hydrogen. The origin of methane by
a radical process in the irradiation of liquid acetic acid is compared with the
evidence for a similar radical process in the fragmentation of CH/sub 3/COOH by
electron impact in the highly dilute gas (mass spectrum). (auth)
Photolysis of deoxygenated solutions of ceric perchlorate and ceric ammonium nitrate in alcoholic, aqueous alcoholic and water + alcohol + acetonitrile solutions at 77 K by light of wavelength >300 nm leads to formation of radicals derived from the alcohol identified by electron spin resonance (e.s.r.) spectroscopy. Primary alcohols RCH2OH yield RĊHOH but secondary alcohols R2CHOH yield R· in addition to, and sometimes to the exclusion of R2ĊOH. Tertiary alcohols always give an alkyl fragment, e.g. C2H5· from tert-amyl alcohol. Allylic alcohols produce the corresponding hydroxyallyl radical and 1,2-diphenylethanol gives a spectrum of benzyl radical, indicating C—C cleavage. The mode of photo-oxidation by Ce(IV) clearly parallels that of the corresponding thermal oxidation.
The primary processes in the photo-oxidation of a large number of carboxylic acids RCO2H by Ce (IV) ions have been characterized by electron spin resonance spectroscopy, mostly at 77 K, utilizing an experimental procedure described previously. The most general pathway is that of oxidative decarboxylation to give the readily charactized R ˙, although alternative routes are found with lactic and acetic acids. Well-resolved spectra of R ˙ are recorded for R ˙= cyclopropyl, cyclobutyl, allyl, vinyl, ˙CH2F, ˙CO2H and ˙CH2NH+3, amongst others, and H ˙and D ˙ atoms display interaction with solvent protons. Secondary reactions are found in some cases, especially with two dicarboxylic acids which yield initially the spectra of ˙(CH2)nCO2H(n= 1 or 2); these decay on warming, but renewed photolysis then produces either CH3˙(from malonic acid) or C2H5˙(from succinic acid). Mechanisms of these processes are discussed.
The long-standing problem of the mechanism of photo-oxidation of organic and inorganic molecules by the UO2+2 ion has been explored in some detail by the matrix isolation method, the detection of trapped substrate-derived radicals being effected by electron spin resonance spectroscopy, mostly using organic matrices at 77 K. A variety of mechanisms is apparent even within a given homologous series, but the most common feature is abstraction of a hydrogen atom from a carbon atom adjacent to an activating group such as —CO—, —CN, —CO2H, —CHO, —OH, —CONH2, —CO2R or from a phosphorus atom; carbon-carbon cleavage is found (sometimes exclusively) with secondary and tertiary alcohols, ketones, some carboxylic acids and diethyl ether. These results are discussed in the light of recent steady-state studies involving comparison of quantum yield and fluorescence quenching data.
The absorption spectrum and decay kinetics of intermediates formed by the reaction of titanium(III) ions with H atoms, hydrated electrons, and carboxyl radicals have been studied in aqueous solution using the pulse-radiolysis technique. The product of the reaction with H atoms in acid solution is a Ti3+–H hydride intermediate which decomposes by a first-order process with a half-life of ca. 3 s. Titanium(II) is formed by reaction with hydrated electrons and ˙CO2H radicals. The absorption spectrum of titanium(II) and the kinetics of its reactions are reported and discussed. The formation of molecular hydrogen by reaction of Ti2+ with water is suppressed by the other solutes in the solutions. Titanium(III) reacts with ˙CO2H, ˙CH2CO2H, and ˙CH(CO2H)2 radicals to give titanium–radical complexes.
Oxidation of titanium(III) to titanium(IV) by hydroxyl radicals is observed on pulse radiolysis of titanium(III) solutions at pH 1·4 in the absence of formic acid. In the presence of 1·0M-formic acid the formate radical CO2H (pKa= 1·4) is produced first, and titanium(III) is subsequently reduced to titanium(II), k=ca. 5 × 106 l mol–1 s–1 at room temperature, pH 1·4. Under the same conditions this radical also reduces europium(III), but not ytterbium(III), chromium(III), or scandium(III), Reduction of scandium(III) is not observed at pH 4·2–4·9 in the absence of formic acid.
ABS> Anhydrous acetates of Y, La, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu ; were prepared by desolvation of the acetates in vacuum at l50 deg C after ; dissolution of the oxides in 50% acetic acid and evaporation to dryness. ; Acetates of Pr and Nd did not reach constant composition before decomposition ; occurrcd. A hemihydrate of Sm acctate was formed which decomposed at ; temperatures above l50 deg C without converting to the anhydrous acetate. ; Hemihydrates of Tb and Ho were fornred at about 75 deg C. (auth);
Quantum yields of the photodissociation of some acyl radicals in solid glass matrices have been measured at −196°C. The effectiveness of the photodissociation process was found to be satisfactorily descried in terms of the free volume model.
Pulse radiolysis of aqueous solutions of formic acid and formate ion in the pH range 0–13 shows (i) that the absorption spectrum of the carboxyl radical is constant between pH 2.5 and 13, but decreases in intensity between pH 2.5 and 0; (ii) that the effect of ionic strength on the rate of reaction of the radical with itself is the same at pH 2.7 and 7 and is consistent with the radical having unit negative charge at pH 2.7. The data presented indicate that the pK of the radical is 1.4.
A survey is presented of the primary processes in the photo-oxidation by Ce(IV) ions at 77 K of a variety of organic molecules classified by functional group; these processes have been determined by characterization of the resulting organic free radical by electron spin resonance spectroscopy. Aldehydes RCHO and aldehyde hydrates RCH(OH)2 are oxidized to RĊO and RĊC(OH)2 respectively, although C2H5ĊO disappears at 120 K, producing instead CH3ĊHCHO. Ketones produce either the alkyl radical R from RCOR′ or the radical derived by abstraction of a hydrogen atom from the methylene or methine groups adjacent to the carbonyl group; in some cases both types of radical are discernible. Esters also undergo C—H or C—C fission processes depending on the degree of substitution of the R fragment of RCO2R′, whilst formamide yields a spectrum of ·CONH2. Methyl methacrylate yields a vinylic monomer radical at 77 K which induces polymerization at 175 K to give the polymethyl methacrylate radical.
Matrix isolation studies on CH3COOH, CH3COOD and CD3COOD, using e.s.r. to identify trapped radicals, suggest a mechanism for the γ-radiolysis of acetic acid at low temperatures. It is suggested that the primary anion, CH3ĊOOH–, which is the only species observed at –196°C, dissociates into CH3CO–2 and H, the latter reacting further to give CH3CO and H2O. The molecular cation CH3ĊOOH+ is not observed, but the CH3 radicals observed in acetic acid/water matrices are thought to be decomposition products, together with CO2. The mechanism proposed for radiolysis of the solid is also compatible with product yields obtained from radiolysis of the pure liquid.
The absorption spectrum of Eu/sup 2 +//sub aq/ was determined in the range lambda 200 to 400 nm. Aqueous solutions of Eu/sup 3 +/ containing organic H and OH scavengers, formate or 2-propanol, undergo on illumination with lambda 254 nm photocatalytic free radical redox reactions, in which Eu/sup 2 +/ is formed and becomes the main light absorbing species. Steady state phi/sub Hâ/ ..-->.. 9% is obtained. Once Eu/sup 2 +/ is present, light of lambda 300 to 400 nm also becomes photoactive and steady state phi/sub Hâ/ ..-->.. 5% obtained at 365 nm. 3 figures.
The photolysis of the uranyl formic acid/formate system has been studied in the acidity range 1<- pH≤ 4, for [uranyl] to [total formate] ratios from 1 to 0.5 up to 1 to 30 at 25 °C. Qualitative and quantitative information was obtained by using spectrophotometric, polarographic and radiochemical techniques. Evidence is presented for the participation of an uranyl formate complex in the pH range where formate ions are present in appreciable quantities. The photolysis proceeds via uranyl sensitized decomposition of formic acid and formate ion, and via direct decomposition of the mentioned complex. U(IV) and CO2 are the only detectable photolysis products. The results are discussed in reference to earlier work on the uranyl oxalate system and data reported in the literature.
Polarographic and. chronopotentiornetric evidence for the formation of the Eu formate complexes, Eu(HCOO)2+ and Eu(HCOO2+, is given. Values for the corresponding step stability constants were estimated to be 20 and 120, respectively. The effect of such formate complexes on the original determination of the formal potential of the Eu(III)Ẽu(II) couple by McCoY (1963) is discussed. The conclusion is that McCoY's result is in error by approximately 70 mV and that the value of−0.35 V vs. N.H.E. is more realistic for the non-complexed system.
Electron spin resonance (ESR) spectra of the formyl radical (HCO) and
deuterated radical (DCO) were observed in solid carbon monoxide over the
temperature range 4.2 to 30 deg K. The observed line shapes were temperature
dependent, but the cause of the temperature dependence was not definitely
determined. The radical was produced by the reaction of CO with hydrogen atoms
produced by the photolytic decomposition of HI. The radical was also produced by
the photolytic decomposition of formaldehyde in solid argon, and it was iound to
be one of the products of the photolysis of methyl alcohol in solid argon. The
most striking feature of the ESR spectrum of HCO is the very large proton
hyperfine splitting (137 gauss). Also the lines were broad and unsymmetrical
with the linewidth varying from one hyperfine component to the next. It was
shown that the line shapes were due to the combination of a rather pronounced
anisotropy in the electronic g factor with a smaller anisotropy in the proton
hyperfine interaction. These characteristics of the radical gave a good deal of
information about its structure which supported the conclusion of others that the
formyl radical is not a pi -electron radical. (auth)
The photochemical and thermal reduction of CeIV carboxylates to CeIII proceed by decarboxylation and liberation of alkyl radicals. Alkanes are subsequently formed by hydrogen transfer, and alkenes, together with esters, result from oxidation of alkyl radicals by CeIV. n-Propyl and isopropyl radicals primarily afford propane, whereas t-butyl radicals are oxidized to isobutylene and t-butyl esters. The mechanism of the oxidation of alkyl radicals by CeIV is discussed. Quantum yield measurements show that photochemical homolysis of CeIV carboxylates is an efficient process. The thermal and photochemical reactions are otherwise equivalent. Alkyl radicals can be trapped with oxygen, chloroform, or CuII. If excess oxygen is employed a catalytic decarboxylation of pivalic acid occurs. Strong acids accelerate both the thermal and photochemical reduction of CeIV. Cationic carboxylatocerium(IV) species which are labile to homolysis and readily reduced by alkyl radicals are postulated as the reactive intermediates in the presence of acid.
A careful study was made of the chemical effects associated with the photon-initiated reduction of ceric ammonium nitrate in glacial acetic acid using light in the wavelength region from about 2800 to 4050 Å over the temperature range from 24 to 50°. The original cerium(IV) nitrate complex is reduced to a Ce(III) state with an experimental quantum yield (φ) which increases slightly from 0.36 at 30° to 0.38 at 50°. However, the true quantum yield of this reaction is always equal to φ CO2 which increases from 0.36 at 30° to 0.46 at 50°. The major volatile decomposition products produced during steady-state irradiation were found to be carbon dioxide, nitromethane, nitric acid, and methanol, but significant amounts of methyl nitrate, methane, and methyl acetate were also obtained. Comparison of the quantum yields from flash photolysis with the steady-state results revealed the surprising fact that no nitromethane was produced using pulsed light at flash intensities. All important experimental facts, including a quantitative material balance of the products, are explained by a surprisingly simple reaction mechanism in which the major product-forming steps all proceed by the ligand-sensitized decomposition of acetic acid ligands activated by either an absorbed photon and/or the oxidation-reduction energy latent in the Ce(IV) to Ce(III) conversion.
The redox potentials of some free radicals in aqueous solution have been determined using a new method. This method is described in detail and is based on the electron transfer properties and characteristics of the donor radical ·RH to a range of acceptor molecules A. The reaction ·RH + A ⇌ R + ·A - + H + occurs at an appreciable rate whenever the redox potential E° 1 (V, at pH 7.0, ∼25°) of A is higher than that of ·RH. The percentage efficiency of this electron transfer process is determined for a large number of acceptors by monitoring the formation of the ·A - radicals at the appropriate pH and wavelength. The fast reaction technique of pulse radiolysis and kinetic absorption spectrophotometry has been used to follow these reactions. Typical "titration" type curves are obtained when plotting the percentage efficiency vs. E° 1 of the acceptors. From the midpoint of such curves, the redox potentials of a number of organic free radicals have been determined. Various radicals derived from aliphatic alcohols, sugars, lactic acid, glycolic acid, lactamide, glycolamide, oxalacetic acid, glycine, and glycine anhydride have been produced and their potentials derived. These radicals are known to undergo acid-base reactions, and the variations of the E° 1 as a function of the acid or base forms of the radicals were studied. In all cases, the ionized (basic) forms of the radicals have significantly lower redox potentials than the acid forms of the radicals, making these radicals more powerful reducing agents. Some aspects of the mechanisms and kinetics of free radical reactions in the literature are discussed and can be rationalized on the basis of the determined redox potentials of these free radicals.
The 254-nm photolysis of (NH3)4Co(C2O4)+ and (NH3)5Co(C2O4)+ generates Co2+ and CO2 in a 1:1 ratio. The values of φCo2+ in deoxygenated acidic solution (pH 1) are 0.90 and 0.65, respectively; for the monodentate oxalate complex, φCo2+ is increased somewhat (0.84) when the free end of the ligand is deprotonated (pKa = 2.2). The presence of O2 reduces these φCo2+ values by a factor of 2. In the case of (en)2Co(C2O4)+, the yield of Co2+ is independent of the presence of O2 at pH 1, is somewhat dependent on O2 presence at pH 2.7-2.9, and is increased during a post-irradiation period. At pH 1, the post-irradiation formation of Co2+ is via first-order kinetics with k = 8.7 × 10-5 sec-1 at 25°. The CO2/Co2+ ratio also shows this post-irradiation behavior, decreasing from 2.2 to 1.5 over a 24-hr period following irradiation. In addition, a new product is obtained by ion-exchange chromatography which is identified as the C-bonded formato linkage isomer, (en)2(H2O)CoCO2H2+ (pKa = 2.6), and which spontaneously decays to Co2+ with an activation energy of about 21 kcal mol-1. Flash photolysis of these complexes reveals two transient species, the longer lived of which is assigned as the C-bonded formato linkage isomer of that complex arising from heterolytic C-C bond scission of the oxalate ligand and rotation of the resulting "carbene" within the coordination sphere of the complex. The excited state initially populated by the absorption of radiation is seen to undergo radiationless transition to at least two other electronic states: a charge-transfer excited state which produces the direct primary yield of Co2+ and the accompanying oxalate radical, and a ligand excited state which gives rise to the decomposition of the oxalate within the coordination sphere. In (en)2Co-(C2O4)+, the ratio of the formation of the C-bonded formato species to the direct, primary generation of Co2+ is ∼4. The observed photochemistry can be explained in terms of the radicals accompanying the direct primary generation of Co2+ and the intramolecular ligand-to-metal electron transfer in the thermal decay of the C-bonded formato species. The ammine-oxalate complexes are reduced by the protonated and deprotonated forms of C2O4- and CO2- radicals producing a secondary source of Co2+; O2 scavenges these radicals. The en complex is apparently inert to the protonated forms of these radicals but can be reduced by the basic forms. These reactivity differences are in accord with the redox potentials of the species involved.
An investigation of the formation and kinetic stability of complexes containing a chromium-carbon bond in an aqueous medium is described. Pentaaquochromium(III) ions carbon bonded to small acyclic anions such as are derived from isopropyl alcohol and diethyl ether are formed by the reduction of organic radicals with Cr(II). The organic radicals are produced by hydrogen abstraction with OH radicals. Aquation rates of the Cr(III) complexes when the organic ligands contain hydroxyl groups (alcohols) are described by the differential rate law -d ln [(H 2O) 5CrL 2+]/dt = k 1 + k 2[H +]. The rate laws for complexes having organic ligands which do not have a hydroxyl group (e.g., methyl or the radical derived from diethyl ether) do not show the term independent of hydrogen ion. A mechanism is proposed in which the activated complex contains a solvent molecule in a position so as to take advantage of hydrogen bonding with the organic ligand.
Transient optical absorption spectra due to carboxyalkyl radicals have been observed on pulse radiolysis of aqueous solutions of some aliphatic acids (formic, acetic, propionic, n- and isobutyric, and trimethyl acetic acids). These radicals were produced as a result of dehydrogenation by H atoms and OH radicals at the α and/or β positions. The assignment of these radicals was supported by the results obtained from the reaction of eaq- with the corresponding monochloroaliphatic acids when dechlorination takes place. The α-carboxyalkyl radicals (C2-C4) studied have absorption maxima in the region 290-350 nm, while the β radicals (C3-C5) have maxima below 250 nm. The observed change with pH in the transient absorption spectra of these radicals was attributed to the dissociation of the carboxyl group. The pK values of these radicals were determined spectrophotometrically and were found to correspond in most cases to the pK values of the parent aliphatic acids. The extinction coefficients and decay rates were also found to be dependent upon pH. The reactivity of CH2OH, CH3CHOH, ·CH2C(CH3)2OH, ·CH2COO-, and ·CO2- radicals with ·CO2-and ·CH2COO- was investigated.
ABS>The reactivity of a number of organie compounds with hydrogen atoms ; was measured in n-hexaec- solution at room temperature. The following classes ; of compounds react by addition of hydrogen only: esters, methylbenzenes, ; disulfides, conjugated or condensed aromatic compounds, vinyl monomers, and ; stable free radicals. Reacting by both addition and hydrogen abstraction are: ; aliphatic acids, aldehydes, ketones, alkynes, and alkyl (other than methyl) ; benzenes. Alcohols, ethers, and mereaptans are relatively inert. The effect of ; temperature on these reactions was studied, but the results are not easily ; interpreted. Absolute values for the rate constants at room temperature were ; obtained from a comparison with values previously established. (auth);
Uranyl sensitized photodecomposition products of various organic acids ; are presented. Sensitized photodecomposition rates for malonic, succinic, and ; glutaric acids were determined in the pH range 0.5 to 3.0 by measuring the gas ; produced in a Warburg micromanometric apparatus. Theoretical analysis of the ; results showed that the photosensitive species is a 1 : 1 complex of diionized ; acid with uranyl ion for malonic acid and a mixture of 1 : 1 complex and 2 : 1 ; complex of monoionized acid with uranyl ion for succinic and glutaric acids. ; (auth);
The 254-nm photolysis of Co(NH3)5O2CH2+ in deoxygenated acidic aqueous solution generates Co2+, CO2, and H2 with quantum yields of 0.55, 0.43, and 0.15, respectively. Scavenging studies with O2, 2-propanol, and allyl alcohol show that Co2+ and CO2 arise from both scavengable and nonscavengable sources, while H2 is completely scavengable. Flash photolysis generates an intermediate which decays via first-order kinetics with a rate constant that is dependent upon pH with an activation energy of 23 kcal mol-1. This intermediate is identified as the C-bonded formato linkage isomer, Co(NH3)5CO2H2+, generated as a result of intraligand proton transfer and rotation of the ligand; the pKa of the ligand is 2.6. This nonscavengable species decays via ligand-to-metal intramolecular electron transfer releasing Co2+ and the · CO2H radical which disproportionates to CO2. Irradiation of Co(NH3)5O2CH2+ in its ligand-to-metal charge-transfer band generates an excited state which, by radiationless transition, degrades to a dissociative state yielding Co2+, H atoms, and CO2 and to an excited state which is largely ligand localized, yielding the C-bonded isomer.
In situ radiolysis-esr experiments on acid solutions (0.8 N H2SO4) of malonic acid have established that the main reaction of OH is abstraction of a CH hydrogen to produce ĊH(CO2H)2 but that approximately 10% of the reaction leads to the radical ĊH2CO2H by decarboxylation. (In basic solution, decarboxylation of CH2(CO2-)2 by OH was not observed and, at most, can account for 0.5% of the total reaction.) The fact that the main reaction occurs by abstraction is in apparent disagreement with previous results of rapid-mixing experiments in which only the esr spectrum of ĊH2CO2H was found. Radiolysis experiments in which TiIII was added to the solution before irradiation showed that ĊH(CO2H)2 reacts with the metal ion while, at the concentrations used, ĊH2CO2H does not. This extra radical destruction reaction can explain, in part at least, the absence of detectable signals of the former radical in the rapid-mixing experiments. Radiation chemical experiments were carried out to further investigate the reaction of organic radicals with the reduced metal ions TiIII and FeII. Measurement of the yields of TiVI and FeIII in solutions containing various organic solutes showed that while a number of types of radicals such as those derived from alcohols and carboxylic acids as well as alkyl radicals can oxidize TiIII only those of type RR′ĊCO2H appear to be able to oxidize FeII. Methane was confirmed as the organic product of the reaction of ĊH3 with TiIII.
Methyl radicals, generated by flash irradiations of Co(NH3)5OCOCH32+, were intercepted with Cu2+(aq) and Cu+(aq) ions. Metastable species, formed in these reactions, were assigned as CuCH32+ (λmax ≤350 nm) and CuCH3+ (λmax 370 nm). A transient, assigned also as CuCH3+, was observed in irradiations of CuOCOCH3+. Product yields were measured in order to confirm the nature of the transients.
A kinetic study of the photochemical oxidation of europium(II) ions in aqueous perchloric acid solutions over the range of acid concentrations 8.7 × 10-3 to 3.74 M HClO4 showed the quantum yield for the reaction Eu2+ + H+ → Eu3+ + 1/2H2 to be of the form Φ = Φ0 + k[H+]1/2. The quantum yield for hydrogen production increases with decreasing wavelength varying from 0.053 at 405 nm to 0.20 at 254 nm in 1 M perchloric acid solutions at 296 K. Trapping experiments with the H atom scavenger 2-propanol indicated the presence of H atoms in the bulk solution. A mechanism is proposed which involves the scavenging of geminate (EuIIIH·) radical pairs by H+ preventing secondary recombination. The acid-independent yield may arise from water acting as a scavenger.
The specific rates of reaction of 16 aliphatic radicals with Cr(H2O)62+ were determined. The absorption spectra due to the chromium-carbon bonds in the products of these reactions were measured. The specific rates of aquation of these complexes as a function of pH were determined. The effects of the structure of the aliphatic radicals on these spectra and specific rates of reactions are discussed in detail. It is shown that chromium induces water elimination when a β-hydroxyl group is present.
The reduction of Eu(III) to Eu(II) was studied by voltammetry at constant current in sodium acetate and acetic acid solutions at constant ionic strength. The pH was kept constant at 4.5. Concentration vs. the square root of transition time plots at a current density of 0.0282 ma. per sq. cm. were linear in accordance with the Sand equation. A diffusion coefficient of 0.40 × 10-5 sq. cm. per second was evaluated for Eu(III) in 0.1M acetate solution at 25° C. The product, iT1/2/C, was constant over a range of current densities and europium(III) concentrations. The electrode reaction for the reduction of Eu(III) to Eu(II) in the acetate system was found to be reversible from current reversal studies and potential vs. time plots. Evidence is given for the presence of the acetate complexes, Eu(OAc)2+ and Eu(OAc)2+. The values of the corresponding stability constants were calculated to be 324 and 20.4.
Sensitive new analytical methods for water-soluble aliphatic aldehydes has been introduced. Versatile spot plate, paper, silica gel, and colorimetric modifications of the procedure are of value in the detection, estimation, and determination of these aldehydes. In the test procedure an intensely brilliant blue cationic dye is formed. Particularly advantageous is the simple sensitive silica gel procedure for the detection or estimation of aldehydes. The color of the dye obtained in the colorimetric procedure for formaldehyde is much more intense than any of the colors obtained for formaldehyde with well known procedures. In the determination of formaldehyde, Beer's law was obeyed from 5 to at least 125 /ig. per 100 ml. of solution. The silica gel and colorimetric methods can be applied to the analysis of auto exhaust fumes and polluted air. Two simple methods for the synthesis of the reagent, 3-methyl-2-benzothiazolone hydrazone, are described.