Kinetics and mechanisms of bromine chloride reactions with bromite and chlorite ions

Chloride ion catalyzes the reactions of HOBr with bromite and chlorite ions in phosphate buffer (p[H(+)] 5 to 7). Bromine chloride is generated in situ in small equilibrium concentrations by the addition of excess Cl(-) to HOBr. In the BrCl/ClO(2)(-) reaction, where ClO(2)(-) is in excess, a first-order rate of formation of ClO(2) is observed that depends on the HOBr concentration. The rate dependencies on ClO(2)(-), Cl(-), H(+), and buffer concentrations are determined. In the BrCl/BrO(2)(-) reaction where BrCl is in pre-equilibrium with the excess species, HOBr, the loss of absorbance due to BrO(2)(-) is followed. The dependencies on Cl(-), HOBr, H(+), and HPO(4)(2)(-) concentrations are determined for the BrCl/BrO(2)(-) reaction. In the proposed mechanisms, the BrCl/ClO(2)(-) and BrCl/BrO(2)(-) reactions proceed by Br(+) transfer to form steady-state levels of BrOClO and BrOBrO, respectively. The rate constant for the BrCl/ClO(2)(-) reaction [k(Cl)(2)]is 5.2 x 10(6) M(-1) s(-1) and for the BrCl/BrO(2)(-) reaction [k(Br)(2)]is 1.9 x 10(5) M(-1) s(-1). In the BrCl/ClO(2)(-) case, BrOClO reacts with ClO(2)(-) to form two ClO(2) radicals and Br(-). However, the hydrolysis of BrOBrO in the BrCl/BrO(2)(-) reaction leads to the formation of BrO(3)(-) and Br(-).     

Gaseous rust: thermochemistry of neutral and ionic iron oxides and hydroxides in the gas phase

The experimental and theoretical thermochemistry of the gaseous neutral and ionic iron oxides and hydroxides FeO, FeOH, FeO(2), OFeOH, and Fe(OH)(2) and of the related cationic water complexes Fe(H(2)O)(+), (H(2)O)FeOH(+), and Fe(H(2)O)(2)(+) is analyzed comprehensively. A combination of data for the neutral species with those of the gaseous ions in conjunction with some additional measurements provides a refined and internally consistent compilation of thermochemical data for the neutral and ionic species. In terms of heats of formation at 0 K, the best estimates for the gaseous, mononuclear FeO(m)H(n)(-/0/+/2+) species with m = 1, 2 and n = 0-4 are Delta(f)H(FeO(-)) = (108 +/- 6) kJ/mol, Delta(f)H(FeO) = (252 +/- 6) kJ/mol, Delta(f)H(FeO(+)) = (1088 +/- 6) kJ/mol, Delta(f)H(FeOH) = (129 +/- 15) kJ/mol, Delta(f)H(FeOH(+)) = (870 +/- 15) kJ/mol, Delta(f)H(FeO(2)(-)) = (-161 +/- 13) kJ/mol, Delta(f)H(FeO(2)) = (67 +/- 12) kJ/mol, Delta(f)H(FeO(2)(+)) = (1062 +/- 25) kJ/mol, Delta(f)H(OFeOH) = (-84 +/- 17) kJ/mol, Delta(f)H(OFeOH(+)) = (852 +/- 23) kJ/mol, Delta(f)H(Fe(OH)(2)(-)) = -431 kJ/mol, Delta(f)H(Fe(OH)(2)) = (-322 +/- 2) kJ/mol, and Delta(f)H(Fe(OH)(2)(+)) = (561 +/- 10) kJ/mol for the iron oxides and hydroxides as well as Delta(f)H(Fe(H(2)O)(+)) = (809 +/- 5) kJ/mol, Delta(f)H((H(2)O)FeOH(+)) = 405 kJ/mol, and Delta(f)H(Fe(H(2)O)(2)(+)) = (406 +/- 6) kJ/mol for the cationic water complexes. In addition, charge-stripping data for several of several-iron-containing cations are re-evaluated due to changes in the calibration scheme which lead to Delta(f)H(FeO(2+)) = (2795 +/- 28) kJ/mol, Delta(f)H(FeOH(2+)) = (2447 +/- 30) kJ/mol, Delta(f)H(Fe(H(2)O)(2+)) = (2129 +/- 29) kJ/mol, Delta(f)H((H(2)O)FeOH(2+)) = 1864 kJ/mol, and Delta(f)H(Fe(H(2)O)(2)(2+)) = (1570 +/- 29) kJ/mol, respectively. The present compilation thus provides an almost complete picture of the redox chemistry of mononuclear iron oxides and hydroxides in the gas phase, which serves as a foundation for further experimental studies and may be used as a benchmark database for theoretical studies.     

Total asymmetric syntheses of 3- and 4-deoxy-hexoses and derivatives

(1S,4S)-7-Oxabicyclo[2.2.1]hept-5-en-2-one ((−)-5, a “naked sugar”) has been converted to (−)-(1R,4S,6S)-6-endo-benzyloxy-2-bromo-7-oxabicyclo[2.2.1]hept-2-ene ((−)-12) in a highly stereoselective fashion. Double hydroxylation of the C=C double bond of (−)-12, followed by acetylation and Baeyer-Villiger oxidation of the resulting -acetoxyketone (−)-14 afforded (−)-5-O-acetyl-2-O-benzyl-3-deoxy-β-D-arabino-hexofuranurono-6,1-lactone ((−)-15). This compound was converted readily into (+)-methyl 3-deoxy--D-arabino-hexofuranoside ((+)-6 and (+)-methyl 3-deoxy-β-L-xylo-hexofuranoside ((+)-7) and partially protected derivatives. (−)-15 was also converted into 4-deoxy-D-lyxo-hexopyranose (34) and several partially protected derivatives such as (+)-methyl 4-deoxy-2,3-O-isopropylidene--D-lyxo-hexopyranoside ((+)-8).     

Chemoenzymatic synthesis of (+)-totarol, (+)-podototarin, (+)-sempervirol, and (+)-jolkinolides E and D

The enzymatic resolution products [(1R,4aR,8aR)-1,2,3,4,4a,5,6,7,8,8a-decahydro-5,5,8a-trimethyl-2-oxo-trans-naphthalene-1-methanol-2-ethylene acetal (8aR)-7 (98% ee) and {acetate of (1S,4aS,8aS)-1,2,3,4,4a,5,6,7,8,8a-decahydro-5,5,8a-trimethyl-2-oxo-trans-naphthalene-1-methanol-2-ethylene acetal} (8aS)-9 (>99% ee)] obtained by the lipase-catalyzed enantioselective acetylation of (±)-7 in the presence of vinyl acetate as an acyl donor were converted to the ,β-unsaturated ketones (8aR)-6 and (8aS)-6, respectively. Concise syntheses of (+)-totarol 1, (+)-podototarin 2 and (+)-sempervirol 3 were achieved based on Michael reactions between (8aS)-6 and the appropriate β-keto ester followed by aldol condensation. The first chiral syntheses of (+)-jolkinolides E 4 and D 5 were achieved from (5R,10R,12R)-12-hydroxypodocarpa-8(14)-en-13-one 15 derived from (8aR)-6.     

Temperature dependence and kinetic isotope effects for the OH + HBr reaction and H/D isotopic variants at low temperatures (53-135 K) measured using a pulsed supersonic Laval nozzle flow reactor

The reactions of OH + HBr and all isotopic variants have been measured in a pulsed supersonic Laval nozzle flow reactor between 53 and 135 K, using a pulsed DC discharge to create the radical species and laser induced fluorescence on the A 2sigma <-- X 2pi (v' = 1 <-- v' = 0) transition. All reactions are found to possess an inverse temperature dependence, in accord with previous work, and are fit to the form k = A(T/298)(-n), with k1 (OH + HBr) = (10.84 +/- 0.31) x 10(-12) (T/298)(-0.67+/-0.02) cm3/s, k2 (OD + HBr) = (6.43 +/- 2.60) x 10(-12) (T/298)(-1.19+/-0.26) cm3/s, k3 (OH + DBr) = (5.89 +/- 1.93) x 10(-12) (T/298)(-0.76+/-0.22) cm3/s, and k4 (OD + DBr) = (4.71 +/- 1.56) x 10(-12) (T/298)(-1.09+/-0.21) cm3/s. A global fit of k vs T over the temperature range 23-360 K, including the new OH + HBr data, yields kT = (1.06 +/- 0.02) x 10(-11) (T/298)(-0.90+/-0.11) cm3/s, and (0.96 +/- 0.02) x 10(-11) (T/298)(-0.90+/-0.03) exp((-2.88+/-1.82 K)/T) cm3/s, in accord with previous fits. In addition, the primary and secondary kinetic isotope effects are found to be independent of temperature within experimental error over the range investigated and take on the value of (kH/kD)(AVG) = 1.64 for the primary effect and (kH/kD)(AVG) = 0.87 for the secondary effect. These results are discussed within the context of current experimental and theoretical work.     

Structures,fragmentation, and protonation of trideoxynucleotide CCC mono- and dianions

Both quantum chemical calculations and ESI mass spectrometry are used here to explore the gas-phase structures, energies, and stabilities against collision-induced dissociation of a relatively small model DNA molecule--a trideoxynucleotide with the sequence CCC, in its singly and doubly deprotonated forms, (CCC-H)(-) and (CCC-2H)(2-), respectively. Also, the gas-phase reactivity of these two anions was measured with HBr, a potential proton donor, using an ESI/SIFT/QqQ instrument. The computational results provide insight into the gas-phase structures of the electrosprayed (CCC-2H)(2-) and (CCC-H)(-) anions and the neutral CCC, as well as the proton affinities of the di- and monoanions. The dianion (CCC-2H)(2-) was found to dissociate upon CID by charge separation via two competing channels: separation into deprotonated cytosine (C-H)(-) and (CCC-(C-H)-2H)(-), and by w(1)(-)/a(2)(-) cleavage of the backbone. The monoanion (CCC-H)(-) loses a neutral cytosine upon CID, and an H/D-exchangeable proton, presumably residing on one of the phosphate groups, is transferred to the partially liberated (C-H)(-) before dissociation. This was confirmed by MS/MS experiments with the deuterated analog. The reaction of (CCC-2H)(2-) with HBr was observed to be rapid, k=(1.4+/-0.4) x 10(-9) cm(3) molecule(-1) s(-1), and to proceed both by addition (78%) and by proton transfer (22%) while (CCC-H)(-) reacts only by HBr addition, k=(7.1+/-2.1) x 10(-10) cm(3) molecule(-1) s(-1). This is in accord with the computed proton affinities of (CCC-2H)(2-) and (CCC-H)(-) anions that bracket the known proton affinity of Br(-).     

Hydride affinities of cumulated, isolated, and conjugated dienes in acetonitrile

The hydride affinities (defined as the enthalpy changes in this work) of 15 polarized dienes [five phenyl sulfone substituted allenes (1a), the corresponding five isolated dienes (1b), and the corresponding five conjugated dienes (1c)] in acetonitrile solution were determined by titration calorimetry for the first time. The results display that the hydride affinity scales of the 15 dienes in acetonitrile range from -71.6 to -73.9 kcal/mol for 1a, from -46.2 to -49.7 kcal/mol for 1b, and from -45.0 to -46.5 kcal/mol for 1c, which indicates that the hydride-obtaining abilities of the cumulated dienes (1a) are not only much larger than those of the corresponding conjugated dienes (1c) but also much larger than those of the corresponding isolated dienes (1b). The hydrogen affinities of the 15 dienes as well as the hydrogen affinities and the proton affinities of the radical anions of the dienes (1(-*)) in acetonitrile were also evaluated by using relative thermodynamic cycles according to Hess's law. The results show that (i) the hydrogen affinities of the neutral dienes 1 cover a range from -44.5 to -45.6 kcal/mol for 1a, from -20.4 to -21.4 kcal/mol for 1b, and from -17.3 to -18.5 kcal/mol for 1c; (ii) the hydrogen affinities of the radical anions of the dienes (1(-*)) in acetonitrile cover a range from -40.6 to -47.2 kcal/mol for 1a(-*), from -21.6 to -29.6 kcal/mol for 1b(-*), and from -10.0 to -15.4 kcal/mol for 1c(-*); (iii) the proton affinities of the 15 1a(-*) in acetonitrile cover a range from -97.0 to -100.6 kcal/mol for 1a(-*), from -77.8 to -83.4 kcal/mol for 1b(-*), and from -66.2 to -68.9 kcal/mol for 1c(-*). The main reasons for the great difference between the cumulated dienes and the corresponding isolated and conjugated dienes in the hydride affinity, hydrogen affinity, and proton affinity have been examined. It is evident that these experimental results should be quite valuable to facilitate the elucidation of the origins of the especially high chemical potencies of the allenes, the choice of suitable hydride reducing agents to reduce the dienes, and the analyses on the reduction mechanisms.     

指示稳定性的反相超高效液相色谱评价原料药和制剂配方中奈必洛尔杂质的方法建立与验证(英文)

A sensitive,stability-indicating gradient reverse phase ultra performance liquid chromatographic method has been developed for the quantitative estimation of nebivolol impurities in active pharmaceutical ingredient(API)and pharmaceutical formulation.Efficient chromatographic separation was achieved on an Acquity BEH C18 column(100 mm×2.1 mm,1.7μm)with mobile phase of a gradient mixture.The flow rate of the mobile phase was 0.18 mL/min with column temperature of 30℃and detection wavelength of 281 nm.The relative response factor values of(R*)-2-(benzylamino)-1-((S*)-6-fluorochroman-2-yl)ethanol((R*S*)NBV-1),(R)-1-((R)-6-fluorochroman-2-yl)-2-((S)-2-((S)-6-fluoro-chroman-2-yl)-2-hydroxyethylamino)ethanol((RRSS)NBV-3),1-(chroman-2-yl)-2-(2-(6-fluorochroman-2-yl)-2-hydroxy ethyl amino)ethanol(monodesfluoro impurity),(S)-1-((R)-6-fluorochroman-2-yl)-2-((R)-2((S)-6-fluoro-chroman-2-yl)-2-hydroxyethylamino)ethanol hydrochloride((RSRS)NBV-3)and(R*)-1-((S*)-6-fluorochroman-2-yl)-2-((S*)-2-((S*)-6-fluoro-chroman-2-yl)-2-hydroxyethylamino)ethanol((R*S*S*S*)NBV-2)were 0.65,0.91,0.68,0.92 and 0.91 respectively.Nebivolol formulation sample was subjected to the stress conditions of acid,base,oxidative,hydrolytic,thermal,humidity and photolytic degradation.Nebivolol was found to degrade significantly under peroxide stress condition.The degradation products were well resolved from nebivolol and its impurities.The peak purity test results confirmed that the nebivolol peak was homogenous and pure in all stress samples and the mass balance was found to be more than 98%,thus proving the stability-indicating power of the method.The developed method was validated according to International Conference on Hormonization(ICH)guidelines with respect to specificity,linearity,limits of detection and quantification,accuracy,precision and robustness.     

Determination of absolute configuration of a metabolite (−)-hydroxyhexamide from acetohexamide, syntheses of (−)- and (+)-hydroxyhexamides and (−)- and (+)-acetoxyhexamides

Enantioselective acetylation of (±)-4-(1-hydroxyethyl)benzenesulfonamide 6 with ‘Acylase I’ (No. A 2156) from Aspergillus melleus in the presence of vinyl acetate gave (R)-4-(1-acetoxyethyl)benzenesulfonamide 7 (98% ee) and (S)-6 (98% ee). Both (S)-6 and (R)-7 were individually converted to the (S)-hydroxyhexamide 2 (>99% ee) and (R)-hydroxyhexamide 2 (>99% ee), respectively. The absolute configuration of a metabolite (−)-hydroxyhexamide 2 from acetohexamide 1 was found to be S based on unequivocal chemical methods including X-ray analysis.     

Hydrogen isotope effects and mechanism of aqueous ozone and peroxone decompositions

Hydrogen peroxide exalts the reactivity of aqueous ozone by reasons that remain obscure. Should H2O2 enhance free radical production, as it is generally believed, a chain mechanism propagated by (.OH/.O2-) species would account for O3 decomposition rates in neat H2O, HR-O3, and in peroxone (O3 + H2O2) solutions, HPR-O3. We found, however, that: (1) the radical mechanism correctly predicts HR-O3 but vastly overestimates HPR-O3, (2) solvent deuteration experiments preclude radical products from the (O3 + HO2-) reaction. The modest kinetic isotope effect (KIE) we measure in H2O/D2O: HR-O3/DR-O3 = 1.5 +/- 0.3, is compatible with a chain process driven by electron- and/or O-atom transfer processes. But the large KIE found in peroxone: HPR-O3/DPR-O3 = 19.6 +/- 4.0, is due to an elementary (O3 + HO2-) reaction involving H-O2- bond cleavage. Since the KIE for the hypothetical H-atom transfer: O3 + HO2- HO3. +.O2-, would emerge as a KIE1/2 factor in the rates of the ensuing radical chain, the magnitude of the observed KIE must be associated with the hydride transfer reaction that yields a diamagnetic species: O3 + HO2- HO3- + O2. HO3-/H2O3 may be the bactericidal trioxide recently identified in the antibody-catalyzed addition of O2(1Deltag) to H2O.     

Potentiometric flow-injection determination of iodide and iodine

A potentiometric flow-injection system is described, in which iodide in the concentration range 10(-6)-10(-1)M may be determined at rates of up to 360 samples/hr at a flow-rate of 17.8 ml/min. Iodine, after on-line reduction to iodide with 0.1M sodium metabisulphite, can also be determined, with a throughput of 60 samples/hr for 10(-5)-10(-3)M iodine. Analyses of two pharmaceutical preparations for iodide and iodine are reported, and the results are in reasonable agreement with titrimetric values.     

Association interaction and voltammetric determination of 1-aminopyrene and 1-hydroxypyrene at cyclodextrin and DNA based electrochemical sensors

The aim of this work was voltammetric determination of 1-aminopyrene and 1-hydroxypyrene using carbon paste electrodes modified with cyclodextrin derivatives and double stranded deoxyribonucleic acid (dsDNA). The detection schemes based on a preconcentration and differential pulse voltammetric (DPV) determination at beta-cyclodextrin and gamma-cyclodextrin modified carbon paste electrode (beta-CD/CPE, gamma-CD/CPE), neutral beta-cyclodextrin polymer and carboxymethyl-beta-cyclodextrin polymer modified screen-printed electrode (beta-CDP/SPE, beta-CDPA/SPE) and dsDNA modified screen-printed electrode (DNA/SPE) are proposed for the trace determination of studied analytes within the concentration range from 2 x 10(-8) to 4 x 10(-7) mol dm(-3) and from 2 x 10(-7) to 4 x 10(-6) mol dm(-3) with the limits of quantification down to 10(-8) mol dm(-3). Depending on pH, 1-aminopyrene interacts with both surface attached CD and DNA by electrostatic bonds and supramolecular complexation while 1-hydroxypyrene associates with the CD hosts via complexation. The 1-aminopyrene interaction with dsDNA was confirmed by fluorimetric measurements in the solution phase using a competing DNA-TO-PRO-3 dye complex. In addition, the effect of temperature on this association was investigated using an electrically heated DNA-modified carbon paste electrode (DNA/CPE).     

Nucleofugality of phenyl and methyl carbonates

A series of X,Y-substituted benzhydryl phenyl carbonates 1 and X,Y-substituted benzhydryl methyl carbonates 2 were subjected to solvolysis in different methanol/water, ethanol/water, and acetone/water mixtures at 25 degrees C. The LFER equation, log k = sf(Ef + Nf), was used to derive the nucleofuge-specific parameters (Nf and sf) for phenyl carbonate (1LG) and methyl carbonate (2LG) leaving groups in a given solvent in SN1 type reaction. Kinetic measurements showed that phenyl carbonates solvolyze one order of magnitude faster than methyl carbonates. Optimized geometries of 1LG and 2LG at B3LYP/6-311G(d,p), B3LYP/6-311++G(d,p), and MP2(full)/6-311++G(d,p) levels revealed that negative charge delocalization in carbonate anions to all three oxygen atoms occurs due to negative hyperconjugation. Phenyl carbonate (1LG) is a better leaving group (Nf = -0.84 +/- 0.07 in 80% v/v aq EtOH) than methyl carbonate 2LG (Nf = -1.84 +/- 0.07 in 80% v/v aq EtOH) because of more pronounced negative hyperconjugation, which is characterized with a more elongated RO-C bond and more increased RO-C-CO angle in 1LG than in 2LG. Calculated affinities of benzhydryl cation toward methyl and phenyl carbonate anions (DeltaDeltaEaff = 11.7 kcal/mol at the B3LYP/6-311++G(d,p) level and DeltaDeltaEaff = 2.7 kcal/mol at the PCM-B3LYP/6-311++G(d,p) level in methanol, respectively) showed that 1LG is more stabilized than 2LG, which is in accordance with greater solvolytic reactivity of 1 than 2.     

Simple and automatic determination of aldehydes and acetone in water by headspace solid-phase microextraction and gas chromatography-mass spectrometry

We describe a simple and automatic method to determine nine aldehydes and acetone simultaneously in water. This method is based on derivatization with 2,2,2-trifluoroethylhydrazine (TFEH) and consecutive headspace-solid-phase microextraction and gas chromatography-mass spectrometry. Acetone-d(6) was used as the internal standard. Aldehydes and acetone in water reacted for 30 min at 40°C with TFEH in a headspace vial and the formed TFEH derivatives were simultaneously vaporized and adsorbed on polydimethylsiloxane-divinylbenzene. Under the established condition, the method detection limit was 0.1-0.5 μg/L in 4 mL water and the relative standard deviation was less than 13% at concentrations of 0.25 and 0.05 mg/L. This method was applied to determine aldehydes and acetone in 5 mineral water and 114 surface water samples. All mineral water samples had detectable levels of methanal (24.0-61.8 μg/L), ethanal (57.7-110.9 μg/L), propanal (11.5-11.7 μg/L), butanal, pentanal (3.3-3.4 μg/L) and nonanal (0.3-0.4 μg/L). Methanal and ethanal were also detected in concentration range of 2.7-117.2 and 1.2-11.9 μg/L, respectively, in surface water of 114 monitoring sites in Korea.     

Enantiodivergent synthesis of 3-amino-4,4-dimethyl-1-phenylpyrrolidin-2-one and derivatives: amino analogues of pantolactone

An enantiodivergent preparation of (+)-(R)- and (−)-(S)-3-amino-4,4-dimethyl-1-phenylpyrrolidin-2-one, (R)- and (S)-9, and several derivatives, from 4,4-dimethyl-1-phenylpyrrolidin-2,3-dione, 4, and (R)- or (S)-1-phenylethylamine, (R)- or (S)-5, as the chirality transfer agents, is described. Amine (S)-9 has also been used as a chiral auxiliary in a diastereoselective Michael reaction.     

Antioxidant capacity and larvicidal and antifungal activities of essential oils and extracts from Piper krukoffii

The leaves and twigs of Piper krukoffii, collected in the Carajás National Forest, north Brazil, yielded essential oils (2.0% and 0.8%), the main constituents of which were myristicin (40.3% and 26.7%), apiole (25.4% and 34.1%) and elemicin (2.8% and 3.0%). The antioxidant activities of the oils, methanol extract and its sub-fractions were evaluated. The DPPH EC50 values varied from the ethyl acetate sub-fraction (73.4 +/- 3.7 microg/mL) to the methanol extract (24.9 +/- 0.8 microg/mL), and the ABTS TEAC values ranged in the same order from 265.7 to 349.2 microMol TE/g. These results indicated a significant antioxidant activity for the plant. The lignans (-)-kusunokin, yatein, (-)-hinokin and cubebin were identified in the methanol extract. The hydro-methanolic sub-fraction showed a high value for total phenol content (106.5 +/- 0.7 mg GAE/g), as well as 1H NMR signals for sugar moieties. Crude extracts and sub-fractions were also able to inhibit beta-carotene bleaching, varying from 22.4 to 47.1%. The oils from the leaves and twigs showed strong larvicidal (21.4 and 3.6 microg/mL) and fungicide (0.5 and 0.1 microg/mL) activities.     

Low temperature NH(X 3sigma-) radical reactions with NO, saturated, and unsaturated hydrocarbons studied in a pulsed supersonic laval nozzle flow reactor between 53 and 188 K

The reactions of ground-state imidogen radicals (NH(X 3sigma-)) with NO and select saturated and unsaturated hydrocarbons have been measured in a pulsed supersonic expansion Laval nozzle flow reactor in the temperature range 53-188 K. The rate coefficients for the NH + NO system display negative temperature dependence in the temperature regime currently investigated and a global temperature-dependent fit is best represented in a modified power law functional form, with k1(NH + NO) = (4.11 +/- 0.31) x 10(-11) x (T/300)(-0.30+/-0.17) x exp(77+/-21/T) cm3/s. The reactions of NH with ethylene, acetylene, propene, and diacetylene were measured over the temperature range 53-135 K. In addition, the reactions of NH with methane and ethane were also measured at 53 K, for reasons discussed later. The temperature dependence of the reactions of NH with the unsaturated hydrocarbons are fit using power law expressions, k(T) = A(T/300)(-n), and are as follows: k4 = (2.3 +/- 1.2) x 10(-12) x (T/300)(-1.09+/-0.33) cm3/s, k5 = (4.5 +/- 0.3) x 10(-12) x (T/300)(-1.07+/-0.04) cm3/s, k6 = (5.6 +/- 1.9) x 10(-12) x (T/300)(-1.23+/-0.21) cm3/s, and k7 = (7.4 +/- 1.8) x 10(-12) x (T/300)(-1.23+/-0.15) cm3/s for ethylene, acetylene, propene, and diacetylene, respectively. The rate for NH + ethane at 53 K is measured to be k3 = (6.8 +/- 1.7) x 10(-12) cm3/s, while that for methane at the same temperature represents an upper bound of k2 < or = (1.1 +/- 4.3) x 10(-12) cm3/s, as this is at the limits of measurement with our current technique. The behavior of these systems throughout the temperature range explored indicates that these reactions occur over a potential energy surface without an appreciable barrier through a complex formation mechanism. Implications for chemistry in low temperature environments where these species are found are briefly discussed.     

Cation and pressure effects on the electrochemistry of 12-tungstocobaltate and 12-tungstophosphate ions in acidic aqueous solution

The effects of supporting electrolytes and of pressure on the electrode reactions of the aqueous CoW(12)O(40)(5-/6-) couple at 25 degrees C are reported, together with limited data on PW(12)O(40)(3-)/4-) and PW(12)O(40)(4-/5-). The half-wave potentials E(1/2) for the CoW(12) couple become moderately more positive with increasing electrolyte concentration and cationic charge, and also in the sequences Li(+) approximately Na(+) < NH(4)(+) < or = H(+) < K(+) < Rb(+) < Cs(+) and Na(+) < Mg(2+) < Ca(2+) < Eu(3+). The mean diffusion coefficients for CoW(12) with the 1:1 electrolytes are independent of electrolyte concentration and rise only slightly from Li(+) to Cs(+), averaging (2.4 +/- 0.3) x 10(-6) cm(2) s(-1). Neither the volumes of activation for diffusion Delta V(diff)(++) (average -0.9 +/- 1.1 cm(3) mol(-1)) nor the electrochemical cell reaction volumes Delta V(Ag/AgCl) (average -22 +/- 2 cm(3) mol(-1)) for the CoW(12) couple show significant dependence on electrolyte identity or concentration. For the PW(12)(3-/4-) and PW(12)(4-/5-) couples, Delta V(Ag/AgCl) = -14 and -26 cm(3) mol(-1), respectively, suggesting a dependence on Delta(z(2)) (z = ionic charge number) as predicted by the Born-Drude-Nernst theory of electrostriction of solvent, but comparison with Delta V(Ag/AgCl) for CoW(12) and other anion-anion couples shows that the Born-Drude-Nernst approach fails in this context. For aqueous electrode reactions of CoW(12), as for other anionic couples such as cyanometalates, the standard rate constants k(el) show specific cation catalysis (Na(+) < K(+) < Rb(+) < Cs(+)), and Delta V(el++) is invariably positive, in the presence of supporting electrolytes. For the heavier group 1 cations, Delta V(el++) is particularly large (10-15 cm(3) mol(-1)), consistent with a partial dehydration of the cation to facilitate catalysis of the electron-transfer process. The positive values of Delta V(el++) for the CoW(12) couple cannot be attributed to rate control by solvent dynamics, which would lead to Delta V(el++) < or = Delta V(diff++), i.e., to negative or zero Delta V(el++) values. These results stand in sharp contrast to those for aqueous cationic couples, for which k(el) shows relatively little influence of the nature of the counterion and Delta V(el++) is always negative.     

Mixed Micellization of Dimeric (Gemini) Surfactants and Conventional Surfactants

The aqueous solutions of mixtures of various conventional surfactants and dimeric anionic and cationic surfactants have been investigated by electrical conductivity, spectrofluorometry, and time-resolved fluorescence quenching to determine the critical micelle concentrations and the micelle aggregation numbers in these mixtures. The following systems have been investigated: 12-2-12/DTAB, 12-2-12/C(12)E(6), 12-2-12/C(12)E(8), 12-3-12/C(12)E(8), Dim3/C(12)E(8), and Dim4/C(12)E(8) (12-2-12 and 12-3-12=dimethylene-1,2- and trimethylene-1,3-bis(dodecyldimethylammonium bromide), respectively; C(12)E(6) and C(12)E(8)=hexa- and octaethyleneglycol monododecylethers, respectively; Dim3 and Dim4=anionic dimeric surfactants of the disodium sulfonate type, Scheme 1; DTAB=dodecyltrimethylammonium bromide). For the sake of comparison the conventional surfactant mixtures DTAB/C(12)E(8) and SDS/C(12)E(8) (SDS=sodium dodecylsulfate) have also been investigated (reference systems). Synergism in micelle formation (presence of a minimum in the cmc vs composition plot) has been observed for the Dim4/C(12)E(8) mixture but not for other dimeric surfactant/nonionic surfactant mixtures investigated. The aggregation numbers of the mixed reference systems DTAB/C(12)E(8) and SDS/C(12)E(8) vary monotonously with composition from the value of the aggregation number of the pure C(12)E(8) to that of the pure ionic component. In contrast, the aggregation number of the dimeric surfactant/C(12)E(8) mixtures goes through a minimum at a low value of the dimeric surfactant mole fraction. This minimum does not appear to be correlated to the existence of synergism in micelle formation. The initial decrease of the aggregation number of the nonionic surfactant upon addition of ionic surfactant, up to a mole fraction of ionic surfactant of about 0.2 (in equivalent per total equivalent), depends little on the nature the surfactant, whether conventional or dimeric. The results also show that the microviscosity of the systems containing dimeric surfactants is larger than that of the reference systems. Copyright 2001 Academic Press.     

Photoelectron anisotropy and channel branching ratios in the detachment of solvated iodide cluster anions

Photoelectron spectra and angular distributions in 267 nm detachment of the I(-)Ar, I(-)H(2)O, I(-)CH(3)I, and I(-)CH(3)CN cluster anions are examined in comparison with bare I(-) using velocity-map photoelectron imaging. In all cases, features are observed that correlate to two channels producing either I((2)P(3/2)) or I((2)P(1/2)). In the photodetachment of I(-) and I(-)Ar, the branching ratios of the (2)P(1/2) and (2)P(3/2) channels are observed to be approximately 0.4, in both cases falling short of the statistical ratio of 0.5. For I(-)H(2)O and I(-)CH(3)I, the (2)P(1/2) to (2)P(3/2) branching ratios are greater by a factor of 1.6 compared to the bare iodide case. The relative enhancement of the (2)P(1/2) channel is attributed to dipole effects on the final-state continuum wave function in the presence of polar solvents. For I(-)CH(3)CN the (2)P(1/2) to (2)P(3/2) ratio falls again, most likely due to the proximity of the detachment threshold in the excited spin-orbit channel. The photoelectron angular distributions in the photodetachment of I(-), I(-)Ar, I(-)H(2)O, and I(-)CH(3)CN are understood within the framework of direct detachment from I(-). Hence, the corresponding anisotropy parameters are modeled using variants of the Cooper-Zare central-potential model for atomic-anion photodetachment. In contrast, I(-)CH(3)I yields nearly isotropic photoelectron angular distributions in both detachment channels. The implications of this anomalous behavior are discussed with reference to alternative mechanisms, affording the solvent molecule an active role in the electron ejection process.