Hemibonding of hydroxyl radical and halide anion in aqueous solution

Molecular geometries and properties of the possible reaction products between the hydroxyl radical and the halide anions in aqueous solution were investigated. The formation of two-center three-electron bonding (hemibonding) between the hydroxyl radical and halide anions (Cl, Br, I) was examined by density functional theory (DFT) calculation with a range-separated hybrid (RSH) exchange-correlation functional. The long-range corrected hybrid functional (LC-ωPBE), which have given quantitatively satisfactory results for odd electron systems and excited states, was examined by test calculations for dihalogen radical anions (X(2)(-); X = Cl, Br, I) and hydroxyl radical-water clusters. Equilibrium geometries with hemibonding between the hydroxyl radical and halide anions were located by including four hydrogen-bonded water molecules. Excitation energies and oscillator strengths of σ-σ* transitions calculated by the time-dependent DFT method showed good agreement with observed values. Calculated values of the free energy of reaction on the formation of hydroxyl halide radical anion from the hydroxyl radical and halide anion were endothermic for chloride but exothermic for bromide and iodide, which is consistent with experimental values of equilibrium constants.     

ESR studies on the preparation of the sulphite radical anion,SO3−., in aqueous solution

The sulphite radical anion, SO₃^?., was generated from the redox reactions between the bisulphite ion, HSO₃^?, and transition-metal i     

Self- and cross-termination rate constants for the isopropylol radical and its anion in aqueous solution

Rate constants for the self- and cross-termination of the isopropylol radical [(CH₃)₂?OH] and its anion [(CH₃)₂?O^?] in aqueous solution are determined by kinetic electron spin resonance. Whereas the self-termination of the neutral radical occurs close to the diffusion-controlled limit, the cross- and self-terminations involving the anion are slower and reflect effects of charge repulsion and steric constraints by solvation.     

Oxidation of caffeine by phosphate radical anion in aqueous solution under anoxic conditions

The photooxidation of caffeine in presence of peroxydiphosphate (PDP) in aqueous solution at natural pH (∼7.5) has been carried out in a quantum yield reactor using a high-pressure mercury lamp. The reactions were followed spectrophotometrically by measuring the absorbance of caffeine at λ_max (272 nm). The rates of reaction were calculated under different experimental conditions. The quantum yields were calculated from the rates of oxidation of caffeine and the intensity of light at 254 nm which was measured by using peroxydisulphate solution as a standard chemical actinometer. The reaction rates of oxidation of caffeine by PDP increase with increase in [PDP] as well as with increase in light intensity, while they are independent of [caffeine]. The quantum yields of oxidation of caffeine by PDP are independent of [PDP] as well as light intensity. However, quantum yields of oxidation of caffeine by PDP increase with increase in caffeine concentration. On the basis of these experimental results and product analysis, a probable mechanism has been suggested in which PDP is activated to phosphate radical anions (PO₄ ^·2−) by direct photolysis of PDP and also by the sensitizing effect of caffeine. The phosphate radical anions thus produced react with caffeine by electron transfer reaction, resulting in the formation of caffeine radical cation, which deprotonates in a fast step to produce C8-OH adduct radicals. These radicals might react with PDP to give final product 1,3,7-trimethyluric acid and PO₄ ^·2− radicals, the latter propagates the chain reaction.     

Kinetics of recombination,dismutation, and disproportionation reactions involving neutral ketyl radicals and radical anions

The kinetics of fast elementary recombination of neutral ketyl radicals of benzophenone and its four derivatives (BPH?), the dismutation of benzophenone radical anions, the disproportionation between BPH? and stable nitroxyl radicals, ( ), and the electron transfer have been investigated in both individual solvents and binary mixtures of different viscosities. Reaction (1) for unsubstituted BPH in water, water glycerol, and n-hexane is controlled by diffusion with 2k₁ ? k_diff. In aliphatic alcohols and toluene, which form solvation complexes with BPH?, reaction (1) is diffusion-enhanced and activation-controlled, respectively, with 2k₁ < k_diff. In a viscous solvent such as 1-propanol–glycerol mixture (100 ? η ? 450 cP) reaction (1) is diffusion-controlled. Reaction (2) in alkaline 1-propanol and alkaline 1-propanol–glycerol mixture is activation controlled. The rates of reactions (3) and (4) for benzophenone radicals and nitroxyl radicals of the imidazoline series decrease as the viscosity of the water–glycerol and 1-propanol–glycerol mixtures is increased. The reactions are molecular mobility limited; nevertheless, the numerical values of k₃ (k₄) are 2–6 times as small as the corresponding k_diff values due to the low steric factor of the reactions (therefore called pseudodiffusion-controlled reactions). The theoretical estimates of k₃ (k₄) are in good agreement with the experimental results. The elimination of spin forbiddance in the process of radical recombination in viscous solvents is discussed.     

Generation of 2-lithio-2-(methylthio)-1,3-benzodithioles,new carboxyl carbanion equivalents,and their application to the syntheses of unsymmetrical hexathioorthooxalates

2-Lithio-2-(methylthio)-1,3-benzodithioles are readily generated either by thiophilic addition of methyllithium to o-phenylenetrithiocarbonates or by deprotonation of 2-(methyl-thio)-1,3-benzodithioles with n-butyllithium in THF; these anions are protonated (aqueous NH₄Cl) or methylated (MeI) as expected, but undergo an unexpected, overall carbophilic addition reaction with cyclic trithiocarbonates to yield, after alkylation, unsymmetrical hexathioorthooxalates, compounds which have been difficult to prepare by traditional methods.     

A laser flash photolysis kinetic study of reactions of the Cl2− radical anion with oxygenated hydrocarbons in aqueous solution

A laser photolysis–long path laser absorption (LP‐LPLA) experiment has been used to determine the rate constants for H‐atom abstraction reactions of the dichloride radical anion (Cl₂⁻) in aqueous solution. From direct measurements of the decay of Cl₂⁻ in the presence of different reactants at pH = 4 and I = 0.1 M the following rate constants at T = 298 K were derived: methanol, (5.1 ± 0.3)·10⁴ M⁻¹ s⁻¹; ethanol, (1.2 ± 0.2)·10⁵ M⁻¹ s⁻¹; 1‐propanol, (1.01 ± 0.07)·10⁵ M⁻¹ s⁻¹; 2‐propanol, (1.9 ± 0.3)·10⁵ M⁻¹ s⁻¹; tert.‐butanol, (2.6 ± 0.5)·10⁴ M⁻¹ s⁻¹; formaldehyde, (3.6 ± 0.5)·10⁴ M⁻¹ s⁻¹; diethylether, (4.0 ± 0.2)·10⁵ M⁻¹ s⁻¹; methyl‐tert.‐butylether, (7 ± 1)·10⁴ M⁻¹ s⁻¹; tetrahydrofuran, (4.8 ± 0.6)·10⁵ M⁻¹ s⁻¹; acetone, (1.41 ± 0.09)·10³ M⁻¹ s⁻¹. For the reactions of Cl₂⁻ with formic acid and acetic acid rate constants of (8.0 ± 1.4)·10⁴ M⁻¹ s⁻¹ (pH = 0, I = 1.1 M and T = 298 K) and (1.5 ± 0.8) · 10³ M⁻¹ s⁻¹ (pH = 0.42, I = 0.48 M and T = 298 K), respectively, were derived. A correlation between the rate constants at T = 298 K for all oxygenated hydrocarbons and the bond dissociation energy (BDE) of the weakest C‐H‐bond of log k_2nd = (32.9 ± 8.9) − (0.073 ± 0.022)·BDE/kJ mol⁻¹ is derived. From temperature‐dependent measurements the following Arrhenius expressions were derived: k (Cl₂⁻ + HCOOH) = (2.00 ± 0.05)·10¹⁰·exp(−(4500 ± 200) K/T) M⁻¹ s⁻¹, E_a = (37 ± 2) kJ mol⁻¹ k (Cl₂⁻ + CH₃COOH) = (2.7 ± 0.5)·10¹⁰·exp(−(4900 ± 1300) K/T) M⁻¹ s⁻¹, E_a = (41 ± 11) kJ mol⁻¹ k (Cl₂⁻ + CH₃OH) = (5.1 ± 0.9)·10¹²·exp(−(5500 ± 1500) K/T) M⁻¹ s⁻¹, E_a = (46 ± 13) kJ mol⁻¹ k (Cl₂⁻ + CH₂(OH)₂) = (7.9 ± 0.7)·10¹⁰·exp(−(4400 ± 700) K/T) M⁻¹ s⁻¹, E_a = (36 ± 5) kJ mol⁻¹ Finally, in measurements at different ionic strengths (I) a decrease of the rate constant with increasing I has been observed in the reactions of Cl₂⁻ with methanol and hydrated formaldehyde. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 169–181, 1999     

Oxidation of the [Rh(CO)2Cl2] anion in aqueous solution

The reaction of rhodium(I) carbonyl chloride, [Rh(CO)₂Cl]₂, with dichromate, cerium(IV) sulfate, hexachloroplatinic acid or p-benzoquinone in aqueous hydrochloric acid proceeds by consumption of 4 equivalents of oxidizing agent per mole or rhodium(I) in accordance with the equation Rh^I(CO)₂  4e + H₂O → Rh^III(CO) + 2H⁺ + CO₂A “cyclic” oxidation mechanism is suggested.     

Solvent and external pressure effects on the ratio of the cyanoisopropyl radical recombination and disproportionation rates

A GC-MS analysis of the azobisisobutyronitrile thermal decomposition products of in solutions at 80°C showed that the ratio of recombination and disproportionation rates of the cyanoisopropyl radical does not depend on the medium viscosity, but increases when the internal pressure of the solvent increases according to the log(k _dispr/k _rec) = ?1.25 + 0.096 P _int ^0.5 law. This means that the activation volume corresponding to recombination is larger than that corresponding to disproportionation. It follows from the relationship log(k _dispr/k _rec) = (ΔV _rec ^≠ ? Δv _dispr ^≠ )ΔP/RT that, for the decomposition of the substrate in benzene under a pressure of 0.5–4.0 kbar, the difference between the activation volumes is ΔV _rec ^≠ ? ΔV _dispr ^≠ = 8 cm³/mol.     

A supramolecular receptor of diatomic molecules (O2, CO, NO) in aqueous solution

A per-O-methylated beta-cyclodextrin dimer, Py2CD, was conveniently prepared via two steps: the Williamson reaction of 3,5-bis(bromomethyl)pyridine and beta-cyclodextrin (beta-CD) yielding 2A,2'A-O-[3,5-pyridinediylbis(methylene)bis-beta-cyclodextrin (bisCD) followed by the O-methylation of all the hydroxy groups of the bisCD. Py2CD formed a very stable 1:1 complex (Fe(III)PCD) with [5,10,15,20-tetrakis(p-sulfonatophenyl)porphinato]iron(III) (Fe(III)TPPS) in aqueous solution. Fe(III)PCD was reduced with Na2S2O4 to afford the Fe (II)TPPS/Py2CD complex (Fe(II)PCD). Dioxygen was bound to Fe(II)PCD, the P(1/2)(O2) values being 42.4 +/- 1.6 and 176 +/- 3 Torr at 3 and 25 degrees C, respectively. The k(on)(O2) and k(off)(O2) values for the dioxygen binding were determined to be 1.3 x 10(7) M(-1) s(-1) and 3.8 x 10(3) s(-1), respectively, at 25 degrees C. Although the dioxygen adduct was not very stable (K(O2) = k(on)(O2)/k(off)(O2) = 3.4 x 10(3) M(-1)), no autoxidation of the dioxygen adduct of Fe(II)PCD to Fe(III)PCD was observed. These results suggest that the encapsulation of Fe (II)TPPS by Py2CD strictly inhibits not only the extrusion of dioxygen from the cyclodextrin cage but also the penetration of a water molecule into the cage. The carbon monoxide affinity of Fe(II)PCD was much higher than the dioxygen affinity; the P(1/2)(CO), k(on)(CO), k(off)(CO), and K(CO) values being (1.6 +/- 0.2) x 10(-2) Torr, 2.4 x 10(6) M(-1) s(-1), 4.8 x 10(-2) s(-1), and 5.0 x 10(7) M(-1), respectively, at 25 degrees C. Fe(II)PCD also bound nitric oxide. The rate of the dissociation of NO from (NO)Fe(II)PCD ((5.58 +/- 0.42) x 10(-5) s(-1)) was in good agreement with the maximum rate ((5.12 +/- 0.18) x 10(-5) s(-1)) of the oxidation of (NO)Fe(II)PCD to Fe(III)PCD and NO3(-), suggesting that the autoxidation of (NO)Fe(II)PCD proceeds through the ligand exchange between NO and O2 followed by the rapid reaction of (O2)Fe(II)PCD with released NO, affording Fe(II)PCD and the NO3(-) anion inside the cyclodextrin cage.     

Formation and stability of proton-amine-inorganic anion complexes in aqueous solution

Interaction of some amines with inorganic ligands was studied potentiometrically in aqueous solutions at 25°C at different ionic strengths. The systems taken into account were: meta-PP, meta-TPP, en-PP, en-TPP, en-HPO ₄ ^2– , dap-SO ₄ ^2– , tetren-HPO ₄ ^2– , 6da-PP and 8da-TPP (meta=methylamine, en=ethylenediamine, tetren=tetraethylenepentamine, dap=1,2-diaminopropane, 6da=1,6-hexanediamine, 8da=1,8-octanediamine, PP=pyrophosphate, TPP=tripolyphosphate). Several ALH_r species are formed for all the studied mixed systems (A=amine, L =inorganic ligand), and in some cases A_pL_qH_r species were also found. The stability of mixed proton-ligand-ligand species is proportional to the charges involved in the complexation reaction and, to some extent, it depends on steric characteristic of the ligands. A simple relationship has been found between formation constants of mixed ligand species and charges involved in the complexation reaction. The ionic strength dependence of formation constants has been taken into account, and it was found that, also for these types of mixed complexes, a general equation independent of the nature of the reactants can be used.     

CO2 regulates phase transition of sodium oleate and 3-(diethylamino)-propylamine in aqueous solution

We reported that the phase conversion of the micelles in aqueous solution prepared by sodium oleate (NaOA) and 3-(diethylamino)-propylamine (DEAPA) in the presence of carbon dioxide. This micellar structure is very sensitive to CO₂ and the pH value of the solution is continuously reduced with the continuous introduction of carbon dioxide. When pH?>?10.2, the mixed solution of NaOA and DEAPA is mainly in the form of spherical micelles; when the 10.2?>?pH?>?9.6, the mixed solution mainly in the worm-like micelles (WLM), where the solution has a significant viscosity change; when the 9.6?>?pH?>?9.1, the NaOA and DEAPA mixed solution becomes the aqueous two-phase system (ATPS), which is composed of two layers, the upper layer is the vesicle structure and the lower layer is the WLM structure; when the pH between 9.1?>?pH?>?9.0, NaOA and DEAPA mixed solution form vesicles, thus realizing the regulation of CO₂ on the micellar structure transition of NaOA and DEAPA mixed solution. Such these microstructures transition could be confirmed by Rheology measurement, DLS and Cryo-TEM.     

UV and H2O2/UV degradation of a pharmaceutical intermediate in aqueous solution

The degradation of 5-methyl-1,3,4-thiadiazole-2-thiol (MMTD), a pharmaceutical intermediate found in some aquifers of Northern Italy, has been investigated by means of UV and UV/H2O2 treatments. The study has been carried out with a 17 W low pressure mercury lamp at room temperature, using a (100)/(1) (H2O2)/(MMTD) molar ratio. The results have demonstrated that: (i) with an initial MMTD concentration of 1 mg/l, 90% MMTD removal can be achieved within 1 hour or less than 5 minutes by UV or UV/H2O2 respectively; (ii) the sole UV irradiation does not cause any MMTD mineralization; (iii) with an initial MMTD concentration of 50 mg/l, 4 hours of UV/H2O2 treatment lead to an almost complete mineralization of the MMTD organic sulfur and to a partial mineralization of carbon (59%) and nitrogen (14%). The identification of degradation by-products, performed by HPLC-UV-MS analysis, revealed that the sole UV irradiation gives rise to the MMTD transformation into a single by-product that continuously accumulates in the solution. Conversely, the UV/H2O2 treatment forms seven intermediates that undergo further degradation through the breakdown of the thiadiazole ring. On the basis of the obtained results a degradation pathway has been proposed.     

Kinetics of the disproportionation reaction of HIO2 in acidic aqueous solutions

We review and discuss kinetic studies of the disproportionation reaction of iodous acid (HIO₂) in the presence of excess of Hg²⁺‐ions. The reactions are followed at different temperatures in water solution with strongly defined acidity. The rate constants of disproportionation are determined between 285 and 303 K based on kinetic data obtained under steady‐state conditions. The calculated rate constants increase with increasing temperature and acid concentration. The corresponding values of activation energy as well as enthalpy and entropy of activation for this reaction have been calculated. The enthalpy of activation as well as entropy is higher at higher sulfuric acid concentration. Also, it was considered that the values of Gibbs energy of formation of HgI⁺ are generated during the process. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 687–691, 2010     

A fluorescein-based fluorogenic and chromogenic chemodosimeter for the sensitive detection of sulfide anion in aqueous solution

A highly sensitive chromo- and fluorogenic chemodosimeter for sulfide anion was developed based on its nucleophilicity. 2,4-Dinitrobenzenesulfonyl-fluorescein (I) is a weakly fluorescent compound. Upon mixing with sulfide anion in aqueous acetone solution, the 2,4-dinitrobenzenesulfonyl group of I was efficiently removed and highly fluorescent fluorescein was released, hence leading to the dramatic increases in both fluorescence and absorbance of the reaction solution. The fluorescence increment is linear with sulfide anion concentration in the range 50-1000 nmol L⁻¹ with a detection limit of 4.3 nmol L⁻¹ (3σ). The proposed chemodosimeter showed excellent selectivity toward sulfide anion and was successfully applied to the determination of sulfide anion in synthetic wastewater samples.     

Mechanism of protection of adenosine from sulphate radical anion and repair of adenosine radicals by caffeic acid in aqueous solution

The photooxidation of adenosine in presence of peroxydisulphate (PDS) has been studied by spectrophotometrically measuring the absorbance of adenosine at 260 nm. The rates of oxidation of adenosine by sulphate radical anion have been determined in the presence of different concentrations of caffeic acid. Increase in [caffeic acid] is found to decrease the rate of oxidation of adenosine suggesting that caffeic acid acts as an efficient scavenger of SO ₄ ^• and protects adenosine from it. Sulphate radical anion competes for adenosine as well as for caffeic acid. The quantum yields of photooxidation of adenosine have been calculated from the rates of oxidation of adenosine and the light intensity absorbed by PDS at 254 nm, the wavelength at which PDS is activated to sulphate radical anion. From the results of experimentally determined quantum yields (Φ_expt1) and the quantum yields calculated (Φ_cal) assuming caffeic acid acting only as a scavenger of SO ₄ ^• show that Φ_expt1 values are lower than Φ_expt1 values. The ǵf values, which are experimentally found quantum yield values at each caffeic acid concentration and corrected for ₄ ^• scavenging by caffeic acid, are also found to be greater than Φ_expt1 values. These observations suggest that the transient adenosine radicals are repaired by caffeic acid in addition to scavenging of sulphate radical anions.     

Enthalpy of solution of CO2 in aqueous solutions of 2-amino-2-methyl-1-propanol

The enthalpies of solution of CO₂ in aqueous solution of 2-amino-2-methyl-1-propanol (AMP) 15 wt% and 30 wt% were measured at 322.5 K and pressures range from (0.2 to 5) MPa using a flow calorimetric technique. The gas solubilities were simultaneously determined from the calorimetric data. The solubilities were compared to available literature values obtained by direct measurements. The experimental enthalpies of solution were compared to the values derived from the literature vapor liquid equilibrium data. This work provides calorimetric data that will be used later for the development of a thermodynamic model to predict both solubilities and enthalpies of solution of acid gases in aqueous amine solutions.     

Aminium cation radical of glycylglycine and its deprotonation to aminyl radical in aqueous solution

The photochemical reaction between glycylglycine and triplet 4-carboxybenzophenone has been investigated using time-resolved chemically induced dynamic nuclear polarization (CIDNP). It is shown that the mechanism of the peptide reaction with triplet excited carboxybenzophenone is electron transfer from the amino group of the peptide, leading to the formation of an aminium cation radical that deprotonates to a neutral aminyl radical. Simulation of the CIDNP kinetics leads to an estimation of the paramagnetic relaxation time for the alpha-protons at the N-terminus at 20 to 40 mus with the best-fit value of 25 mus.