Overlooked Formation of Carbonate Radical Anions in the Oxidation of Iron(II) by Oxygen in the Presence of Bicarbonate

Iron(II), (Fe(H₂O)₆²⁺, (Fe^II) participates in many reactions of natural and biological importance. It is critically important to understand the rates and the mechanism of Fe^II oxidation by dissolved molecular oxygen, O₂, under environmental conditions containing bicarbonate (HCO₃⁻), which exists up to millimolar concentrations. In the absence and presence of HCO₃⁻, the formation of reactive oxygen species (O₂⋅⁻, H₂O₂, and HO⋅) in Fe^II oxidation by O₂ has been suggested. In contrast, our study demonstrates for the first time the rapid generation of carbonate radical anions (CO₃⋅⁻) in the oxidation of Fe^II by O₂ in the presence of bicarbonate, HCO₃⁻. The rate of the formation of CO₃⋅⁻ may be expressed as d[CO₃⋅⁻]/dt=[Fe^II[[O₂][HCO₃⁻]². The formation of reactive species was investigated using ¹H nuclear magnetic resonance (¹H NMR) and gas chromatographic techniques. The study presented herein provides new insights into the reaction mechanism of Fe^II oxidation by O₂ in the presence of bicarbonate and highlights the importance of considering the formation of CO₃⋅⁻ in the geochemical cycling of iron and carbon.     

Formation of oxygen atoms in the reaction of chlorine atoms with ozone

Oxygen atoms are detected by NO + O + M chemiluminescence as a secondary product of the reaction between Cl and O₃. The mechanism Cl + O₃ → ClO + O₂(¹Σ⁺_g), O₂(¹Σ⁺_g) + O₃ → O₂ + O₂ + O is proposed to account for the oxygen atom formation. The branching ratio to the O₂(¹Σ⁺_g) product in the reaction of Cl with O₃ is estimated to be in the range (0.1–0.5) x 10^?2.     

Activation of molecular oxygen in trifluoroacetic acid

The interaction of molecular oxygen with aqueous trifluoroacetic acid (TFA) led to an increase in pH. This effect was explained by a decrease in the concentration of the protonated CF₃CO₂H₂⁺ and H₃O⁺ species after oxygen was fed in the reactor. Quantum-chemical calculations show that a radical pair can be formed in an activation-free exothermal reaction involving the radical residue of the acid, the CF₃CO₂H₂⁺...³O₂...CF₃CO₂⁻ peroxide radical, and the acid molecule in the CF₃C₂^·...HOO^· collision complex. It was assumed that the activation of molecular oxygen in aqueous TFA solutions, providing the activity of the system in oxidations of various organic and inorganic substrates, is related to the formation of peroxide radicals in them.     

Primary processes in the thiacyanine dimer-photosensitized reduction of p-nitroacetophenone in water by

Primary processes in the reduction of p-nitroacetophenone (p-NAP) by ascorbic acid (AA) in water photosensitized by thiacyanine dimers M ₂ ^2? have been considered. For M ₂ ^2? , the quantum yields of fluorescence and intersystem crossing to the triplet state (M ₂ ^2? )_T increases in comparison to the monomers M^?. The dimers (M ₂ ^2? )_T enter into the reactions of both one-electron photoreduction by ascorbic acid to give AA^+· and M ₂ ^3? and one-electron photooxidation by p-nitroacetophenone to give p-NAP^?· and the dimeric radical anion M ₂ ^? which dissociates to M^? and M^· within 25–30 μs. The primary oxidative or reductive photosensitization in the ternary systems containing (M ₂ ^2? )_T, p-NAP, and AA affords p-NAP^?· and AA^+·.     

Effect of anions on the dissolution of Al in acid solutions

The effect of Cl^?, Br^?, I^?, ClO₄^?, NO₃^?, HSO₄^?, HCrO₄^? and H₂PO₄^? on the of Al in 2 M HCl is studied by the thermometric method. Three sets of experiments are carried out, which allow the variation of the concentration of the various species in a programmed manner. Dissolution promotion is noted in solutions to which HCl, HBr and H₂CrO₄ are added. The way of action of each of these anions is discussed. Additions of HI, HClO₄, H₂SO₄ and H₃PO₄, on the other hand, first retard and later enhance the dissolution of Al in 2 M HCl, as their concentration in solution is increased. This is related to anion adsorption, which is counterbalanced by increase in acidity. HNO₃ differs from the other tested acids in causing only dissolution retardation. Experiments in which LaCl₃ is added to the test solution indicate that the NO₃^? is adsorbed as such on Al₂O₃. The ability of the various anions to retard the dissolution of Al in 2 M HCl decreases in the succession: NO₃^? (strong)>I^?>HSO₄^?>H₂PO₄^?>Br^?, ClO₄^? (weak)     

Electrochemical studies of complex fluorides in non-aqueous solvents

Electrochemical studies on hydrolytically unstable complex fluorides have been shown to be possible in highly purified acetonitrile. The techniques of study will be described briefly. A survey of the results obtained to date wille given.MoF₆ is a stronger oxidising agent than WF₆ by 1.06 volts. Neither |MoF₆|^? nor |WF₆|^? can be reduced below |MF₆|^? in acetonitrile. |UF₆|^? can be reduced to the transient species |UF₆|^2? which is unstable in solution. |OsF₆|^? undergoes successive reduction to |OsF₆|^2? and |OsF₆|^3?.|MF₆L|^? undergoes complex electrochemical behaviour involving reduction to |MF₆L|^2? with subsequent dissociation to |MF₆|^? and L^?. Reoxidation is in the presence of L^? and hence none of the redox steps are fu]ly reversible. The relation between the electrode potentials and the ligand L will be discussed.We thank NATO for a grant in support of this work.     

Mechanisms of molybdenum substitution in vanadium antimonate

The formation of a solid solution containing the three elements V, Sb and Mo, which are key-elements in the design of light alkane oxidation catalysts, has been studied by incorporating molybdenum into the pure VSbO₄ compound as obtained in air at 700°C (V³⁺_0.28V⁴⁺_0.64□_0.16Sb⁵⁺_0.92O₄). Monophasic compounds with a rutile-type structure have been obtained and characterized by X-ray diffraction, electron microscopy, Infrared Fourier transform, X-ray absorption and electron spin resonance spectroscopies. At low molybdenum content, Mo⁶⁺ substitute V⁴⁺ in the cationic-deficient structure. The charge balance is maintained by an increase of the cationic vacancy number. This leads to the formation of a solid solution corresponding to the formula V³⁺_0.28V⁴⁺_0.64−3xMo⁶⁺_2x□_0.16+xSb⁵⁺_0.92O₄ with 0<x<0.09. At higher molybdenum content, Mo⁵⁺ are stabilized and substitute Sb⁵⁺ in the rutile structure: V³⁺_0.28V⁴⁺_0.37Mo⁶⁺_0.18□_0.25Mo⁵⁺_ySb⁵⁺_0.9−yO₄ with 0<y<0.06. At higher molybdenum content the rutile phase is no longer stable and two new phases are formed: Sb₂O₄ and a new mixed vanadium molybdenum antimonate.     

Role of electronic symmetry of stark components in non-radiative relaxation in Eu3+ in Eu diglycolate

A time-resolved study of emission, following selective excitation of the ⁵D₁, ⁵D₂ and ⁵D₃ levels of Eu³⁺ in Na₃Eu(C₄H₄O₅)₃·2NaClO₄·6H₂O (EuDG) using a pulsed dye laser is reported. On excitation of ⁵D₂ (F) the emission from ⁵D₀ exhibits a fast and a slow build-up. This has been explained by involving two independent decay modes of ⁵D₂.     

Rapid determination of strontium-90 in seawater

It is shown that Y₂O₃, YF₃, LaF₃, and CeF₃ help concentrate ⁹⁰Y from seawater. To determine ⁹⁰Sr, YF₃ is the best reagent because the effect of interfering lead and thorium radioisotopes is minimum in this case. It is proposed to preconcentrate ²¹⁰Bi on PbS to eliminate its interference. To determine ⁹⁰Sr we measure the Cherenkov radiation of ⁹⁰Y concentrated on YF₃ without prior elution.     

Photochemistry of tetraalkylammonium tetrachlorocuprates in low-temperature matrices

The product composition and the principles of photochemical transformations of tetrahexylammonium tetrachlorocuprate [(RH)₄N⁺]₂[Cu^IICl₄]²⁻ (RH = C₆H₁₃) in 2-chlorobutane at 77 K have been found out by ESR spectroscopy. It has been shown that the photolysis of [(RH)₄N⁺]₂ [Cu^IICl₄]²⁻ results in the formation of alkyl radicals (R^⊙), presumably, anions [Cu^ICl₃]²⁻ and organic copper(II) compounds {Cu^IIR}. A reduction in the quantum yield of primary photolysis products during the reaction, nonequivalence of the quantum yield of the buildup of paramagnetic photolysis products to that of [Cu^IICl₄]²⁻ consumption, and a decrease in the total number of paramagnetic particles in the system during the photolysis have been revealed. A photolysis mechanism involving both photochemical and thermal processes is proposed.     

Consecutive reactions of diethoxydimethylsilane and triethoxymethylsilane with methoxide ions in methanol solution

The consecutive reactions of (CH₃)₂Si(OC₂H₅)₂ and CH₃Si(OC₂H₅)₃ with methoxide ions were investigated in methanol solutions. The reverse transesterification reactions with ethoxide ions could be neglected in both cases since the concentration of ethoxide in methanol solution was assumed to be low due to the fast equilibrium reaction C₂H₅O^? + CH₃OH ? C₂H₅OH + CH₃O^?. The progress of the reactions was followed by monitoring the formation of ethanol with a Fourier-transform infrared spectrometer. All rate constants were determined at 295 K. The reactions between the dialkoxydimethylsilanes and methoxide ions were assumed to consist of two consecutive steps that can be represented by the net reaction; (CH₃)₂Si(OC₂H₅)₂ + 2CH₃O^? → (CH₃)₂Si(OCH₃)₂ + 2C₂H₅O^?. The two consecutive rate constants were established as 1.93 ± 0.12M^?1s^?1 and 1.00 ± 0.12M^?1s^?1, respectively. The consecutive rate constants for the reactions between the trialkoxymethylsilanes and methoxide ions can be written according to the total reaction; CH₃Si(OC₂H₅)₃ + 3CH₃O^? → CH₃Si(OCH₃)₃ + 3C₂H₅O^?. The three rate constants corresponding to each consecutive step were established as 1.12 ± 0.09 M^?1s^?1, 0.82 ± 0.10 M^?1s^?1, and 0.51 ± 0.06 M^?1s^?1, respectively. © 1995 John Wiley & Sons, Inc.     

Carbon skeletal rearrangements in 2-phenylthiophene and 2,5-diphenylthiophene under conditions of electron-impact

The mass spectra of ¹³C-labelled 2-phenylthiophenes and 2,5-diphenylthiophenes were studied. The label distributions for the [HCS]⁺, [C₂H₂S]^+·, [C₈H₆]^+·, [C₉H₇]⁺ and [C₇H₅]S⁺ ions from 2-phenylthiophene and the [HCS]⁺, [C₉H₇]⁺, [C₇H₅S]^+·, and [C₁₅H₁₁]⁺ ions from 2,5-diphenylthiophene were interpreted in terms of both carbon skeletal rearrangements in the thiophene ring and migration of the phenyl substituent. The degree of carbon scrambling in the thiophene ring appeared to be almost independent of the electron beam energy. The formation of some of the fragment ions studied seems to be so fast that no carbon scrambling could be detected at all; in neither case was complete scrambling of the carbon atoms of the thiophene ring observed.     

Absorption,fluorescence and phosphorescence spectra of the singlet and triplet states of s-tetrazine in the crystal and in mixed crystals at low temperatures

The electronic absorption, fluorescence and phosphorescence spectra of s-tetrazine at low temperatures (4.2-1.5 K) are reported and analyzed in the neat crystal and in several mixed crystals. The ³B_3u-¹A_g (nπ^*) origin is at 18414 ± 5 cm^?1 for neat tetrazine. In the mixed crystal several sites identified. The lowest energy origin is at 17453 cm^?1 for tetrazine in pyrazine; 17 701 cm^?1 in pyrimidine; and 17 676 cm^?1 in pyridazine. The ^eB_3u-¹A_g (nπ^*) origin is at 14 096 ± 2 cm^?1 for the neat crystal. The phosphorescence lifetime of neat tetrazine is measured to be 96.8 ± 2.1 μs at 4.2 and 1.8 K. All the spectra are predominately composed of members of progressions in a single totally symmetric mode (ν_6a) built upon site origins and vibrational fundamentals. The ν_6a interval is: 743 (¹A_g), 715 (³B_3u), and 709 cm^?1 (¹B_3u) in the neat tetrazine crystal; 732 (¹A_g) and 705 cm^?1 (¹B_3u in pyrazine host, 737 (¹A_g) and 701 cm^?1 (¹B_3u) in pyrimidine host, and 732 (¹A_g) and 703 cm^?1 (¹B_3u) in pyridazine host mixed crystals. All emission spectra may be analyzed by Oi → (ν″_6a)^o_n (i), i indicating the observed s     

Thermodynamics of ternary salts in dioxane—water mixtures from conductance measurements

Studies of electrolytic conductance in dioxane—water mixtures of varying dioxane content were initiated in 1978 [1]. In the present communication, an attempt has been made to evaluate the thermodynamic function ΔG⁰_t for the transfer of Mg(ClO₄)⁺, Mg(NO₃)⁺, BaCl⁺, BaBr⁺, Ba(ClO₄⁺, Ba(NO₃)⁺, SrCl⁺, Sr(NO₃)⁺, CaCl⁺ and Ca(NO₃)⁺ from water to dioxane—water media, which would give some information regarding ionic solvation.     

k 0-RNAA used in determination of 129I in a mixed resin sample

A k ₀-RNAA procedure was developed to determine ¹²⁹I in a mixed resin sample. CH₄ extraction and (NH₄)₂SO₃ back-extraction were used to separate ¹²⁹I in ashed samples. The ¹²⁹I target sample for irradiation in the reactor was prepared by heating the (NH₄)₂SO₃ back-extraction solution to reduce its volume and then to dry it in a quartz ampoule. No MgO and LiOH were needed during the target sample preparation. After irradiation, the nuclide ¹³⁰I was purified by combining hydrated antimony pentoxide column and CH₄ extraction separations. A k-factor was determined for the reaction of ¹²⁷I (n, 2n) ¹²⁶I and used for iodine chemical yield determination. The apparent ¹²⁹I concentrations of five nuclear reaction interferences were calculated. The relative standard deviation of three ¹²⁹I determinations was found to be 3.5 %. The ¹²⁹I content in the analyzed resin was found to be 1.36 × 10^?9 g/g (8.63 × 10^?3 Bq/g) with a relative uncertainty of 9.1 %. The detection limit of ¹²⁹I was calculated to be 7.4 × 10^?13 g (4.7 × 10^?6 Bq) in a k ₀-RNAA of a blank sample.     

Phase and dose effects of13N in pile-irradiated propionic-d2 acid

A sharp phase effect on the yields of¹³NH₄ ⁺ and¹³NO₂ ^– has been observed in propionic-d₂ acid irradiated in a reactor similarly to the other carboxylic acids previously reported. In the formation of¹³NH₄ ⁺,¹³NO₂ ^– and [¹³N]amide, there is no (H/D)-isotope effect between CH₃CD₂COOH and C₂D₅COOD systems unlike for protiated and deuterated benzene and cyclohexane. With a rise in irradiation dose from 100 to 400 MGy, the yield of¹³NH₄ ⁺ increased, but that of¹³NO₂ ^– decreased in proportion to the dose. The dose effect was clear more in liquid phase than in frozen one. The results are in harmony with the reaction mechanism proposed in our previous works.     

Chemical effect of13N formed in butyric-d7 acid irradiated in a pile

A distinct phase effect was observed on the formation of¹³NH₃, H¹³NO_x and [¹³N]-amide in pile-irradiated butyric-d₇ acid, although the magnitude of the effect was rather smaller than that in deuterated trifluoroacetic, acetic and propionic acids previously reported. In frozen butyric-d₇ acid, most of¹³N was found in the forms such as¹³NH₃ (54.8±0.7%), H¹³NO_x (26.8±0.6%). and [¹³N]amide (15.9±1.8%). The yields of HC¹³N and [¹³N]aminobutyric acid were only 0.6±0.3 and 1.2±0.3% even in the liquid, respectively. The scavenger effect of acetic anhydride-d₆ on the formation of¹³NH₃, H¹³NO_x and [¹³N]amide was examined. In liquid and frozen carboxylic acids, the yields of¹³NH₃ and H¹³NO_x were proportional and inversely proportional to the number of hydrogen atoms in a target molecule in the region of the number of hydrogen from unity to eight.     

Formation and reactivity of metal carbonyl anions in the gas phase

The reactions of metal carbonyl anions (M(CO)_n^?; M = Cr, Mn and Fe; n = 1–3) with n-heptane, water and methanol were studied with use of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer equipped with an external ion source. The M(CO)_n^? ions were formed in the FT-ICR cell by collision-induced dissociation of the most abundant primary ion generated by electron impact of the appropriate metal carbonyl compound present in the external ion source. The M(CO)_n^? ions were allowed subsequently to undergo non-reactive collisions with argon in order to remove possible excess internal/translational energy prior to the ion/molecule reaction. Only the Cr(CO)₃^?, Mn(CO)₃^? and Fe(CO)₂^? ions react with n-heptane. This reaction proceeds by loss of H₂ from the collision complex and the Cr(CO)₃^? and Fe(CO)₂^? ions react about three times more efficiently than the Mn(CO)₃^? ion. With water, Mn(CO)^? and Fe(CO)₃^? are unreactive, whereas the other ions react by loss of one or two CO molecules from the collision complex. The rate of the reaction with water decreases in the order Cr(CO)₃^?, Fe(CO)₂^?, Cr(CO)₂^?, Fe(CO)^?, Mn(CO)₃^? and Mn(CO)₂^?. With methanol, the Cr(CO)₂^? ion reacts by loss of two CO molecules from the collision complex, whereas loss of one CO molecule and elimination of CO + H₂ occur in the reaction with Cr(CO)₃^?. Competing loss of CO and one or two H₂ molecules occurs in the reactions of Mn(CO)₃^? and Fe(CO)₂^? with methanol. The rate of the reaction with methanol decreases in the order Cr(CO)₃^?, Fe(CO)₂^?, Cr(CO)₂^? and Mn(CO)₃^?.     

First-principles calculation of temperature-dependent electronic transitions mechanism in V or Nb substituted BiFeO3

Here, we present a simulation study of temperature-dependent electronic transitions in BiVO₃ (BVO) and BiNbO₃ (BNO) using density functional theory (DFT) together with generalized gradient approximation (GGA) and two-dimensional correlation analysis (2D-CA). The results indicate that heat accumulation can accelerate the degeneracy of V-3d orbital in BVO and the splitting of Nb-4d orbital in BNO at 750 K. We found changes in the type of d–p hybrid orbital as follows, for BVO: V-d_x²_+y² + d_Z²-O-2p_z → V-d_x²_+y²-O-2p_z; and for BNO: Nb-d_x²_+y²-O-2p_z → Nb-d_x²_+y² + d_Z²-O-2p_z. Furthermore, we found changes in the type of hybrid orbital leading to the following electron–electron interactions, for BVO: t_2g (V-d_Z²-O-2p_z) + e_g (V-d_x²_+y²-O-2p_z) → t_2g (V-d_x²_+y²-O-2p_z); and for BNO: t_2g + e_g (Nb-d_x²_+y² + d_Z²-O-2p_z) → t_2g (Nb-d_x²_+y²-O-2p_z) + e_g (Nb-d_z²-O-2p_z). The electronic transitions are determined by a charge-transfer from the occupied O-2p⁴ orbitals to the unoccupied V-3d³ (or Nb-4d³) and Bi-6p³ orbitals. Due to the temperature-dependent electronic structure closely related to these electronic transitions, this study provides a new perspective for the design and improvement of BFO-based temperature-sensitive devices.     

Reaction of silicon with alcohols in autoclave

A reaction of activated silicon with alcohols in an autoclave at 240—270 °C was studied. It was found that primary alcohols form tetraalkoxysilanes Si(OR)₄ with high selectivity (up to 97%), while the secondary PrⁱOH gave a mixture of compounds HSi(OPrⁱ)₃, Si(OPrⁱ)₄, HSi(OPrⁱ)₂OSi(OPrⁱ)₂H, HSi(OPrⁱ)₂OSi(OPrⁱ)₃, and Si(OPrⁱ)₃OSi(OPrⁱ)₃ with the predominance of trialkoxysilane (up to 67%). Carrying out the reaction under the indicated conditions has the advantage of experimental simplicity, reagent availability, high conversion of silicon, good isolated yields of products.