Sulphur(IV) in rain water and antarctic ice by ion chromatography

Sulphur(IV) is quantified as the α-hydroxysulphonic acid, hydroxymethanesulphonate, which is stabilized in samples and standards by a stoichiometric excess of formaldehyde. The addition of 0.2% (v/v) formaldehyde to the very dilute eluent protects against oxidation of the adduct, which is eluted as a single symmetrical peak, completely resolved from ali other anions including methanesulphonate. The detection limit is 0.1 μg l^?1 S(IV). The method is applied in the analysis of Tasmanian rain water and Antarctic ice.     

BEBO Calculations for hydrogen abstraction by 0(3P)

Rate constants for hydrogen transfer reactions from C—H bonds to O(³P) in the gas phase have been calculated by the bond-energy bond-order method. The use of a single or double triplet repulsion term is discussed. Computational results indicate that with carefully selected input parameters, it is unnecessary to use a double-repulsion term. Tunneling corrections were carried out by two different procedures. Results indicate that in this case there is not much difference between the simple Wigner's treatment and more complicated methods.     

Evidence for hydrogen abstraction from C1 of taurine by the high-spin Fe(IV) intermediate detected during oxygen activation by taurine:alpha-ketoglutarate dioxygenase (TauD)

The Fe(II)- and alpha-ketoglutarate-dependent dioxygenases catalyze hydroxylation reactions of considerable biomedical and environmental significance. Recently, the first oxidized iron intermediate in the reaction of a member of this family, taurine:alpha-ketoglutarate dioxygenase (TauD), was detected and shown to be a high-spin, formally Fe(IV) complex. The demonstration in this study that decay of the Fe(IV) complex is approximately 30-fold slower when it is formed in the presence of 1-[2H]2-taurine provides evidence that the intermediate abstracts hydrogen from C1, the site of hydroxylation, and suggests that quantum-mechanical tunneling may contribute to C1-H cleavage.     

Visible light induced oxygenation of cyclohexene with activation of water sensitized by dihydroxy coordinated tetraphenyloprphyrinatotin(IV)

Visible light irradiation of a reaction mixture containing dihydroxy coordinated tetraphenylporphyrinatotin(IV), cyclohexene and potassium hexachloroplatinate induced oxygenation of the cyclohexene under degassed conditions. In the reaction system, a water molecule served as the oxygen donor. Cyclohex-2-enol, 1,2-dichlorocyclohexane and 2-chlorocyclohexanol were the major oxidation products and the quantum yield was around 0.1. An experiment using H₂ ¹⁸O revealed that an ¹⁸O atom was quantitatively incorporated into the oxygenated products. The reaction was initially induced by an electron transfer from an excited triplet porphyrin to potassium hexachloroplatinate producing a cation radical of the porphyrin. Metal-oxo type complexes formed through deprotonation of the hydroxy group of the porphyrin cation radical were key reactive intermediates reacting with cyclohexene. Two kinds of the metal-oxo type complex reactive intermediate were kinetically demonstrated to be involved in the reaction system, producing different oxidation products from cyclohexene.     

Bimolecular hydrogen abstraction from phenols by aromatic ketone triplets

Absolute rate constants for hydrogen abstraction from 4-methylphenol (para-cresol) by the lowest triplet states of 24 aromatic ketones have been determined in acetonitrile solution at 23 degrees C, and the results combined with previously reported data for roughly a dozen other compounds under identical conditions. The ketones studied include various ring-substituted benzophenones and acetophenones, alpha,alpha,alpha-trifluoroacetophenone and its 4-methoxy analog, 2-benzoylthiophene, 2-acetonaphthone, and various other polycyclic aromatic ketones such as fluorenone, xanthone and thioxanthone, and encompass n,pi*, pi,pi*(CT) and arenoid pi,pi* lowest triplets with (triplet) reduction potentials (E(red)*) varying from about -10 to -38 kcal mol(-1). The 4-methylphenoxyl radical is observed as the product of triplet quenching in almost every case, along with the corresponding hemipinacol radical in most instances. Hammett plots for the acetophenones and benzophenones are quite different, but plots of log k(Q) vs E(red)* reveal a common behavior for most of the compounds studied. The results are consistent with reaction via two mechanisms: a simple electron-transfer mechanism, which applies to the n,pi* triplet ketones and those pi,pi* triplets that possess particularly low reduction potentials, and a coupled electron-/proton-transfer mechanism involving the intermediacy of a hydrogen-bonded exciplex, which applies to the pi,pi* ketone triplets. Ketones with lowest charge-transfer pi,pi* states exhibit rate constants that vary only slightly with triplet reduction potential over the full range investigated; this is due to the compensating effect of substituents on triplet state basicity and reduction potential, which both play a role in quenching by the hydrogen-bonded exciplex mechanism. Ketones with arenoid pi,pi* states exhibit the fall-off in rate constant that is typical of photoinduced electron transfer reactions, but it occurs at a much higher potential than would be normally expected due to the effects of hydrogen-bonding on the rate of electron-transfer within the exciplex.     

Liquid-phase hydrogen abstraction from chlorinated ethanes by chlorine atoms

The hydrogen abstraction from the chlorinated ethanes by chlorine atoms has been investigated in the liquid phase. Rate constants relative to that for hydrogen abstraction from chloroform have been measured between 267° and 333°K using a competition technique. The results are compared with gas-phase data.     

Activation-controlled hydrogen abstraction by benzophenone triplet

Through nanosecond flash photolysis measurements, activation energies for hydrogen abstraction by benzophenone triplet have been shown to correlate with solvent molecular structure. We demonstrate that two successive processes result in photoproduct generation. One, triplet quenching, is independent of macroscopic viscosity, and is not simply reflected in the observed quantum yield. The second process involves separation of two caged radicals, and does not occur in rigid media.     

Gas-phase hydrogen abstraction from asymmetrically halogenated ethanes by chlorine atoms

The hydrogen abstraction from asymmetrically fluorinated and chlorofluorinated ethanes by chlorine atoms has been investigated in the gas phase between 264 and 333°K using the competition method. Arrhenius parameters for the reaction on both sites of the molecules are discussed.     

Barriers of hydrogen abstraction from primary, secondary, and tertiary alkane sites by O(3P)

We present an extensive study of the barriers of hydrogen abstraction from primary, secondary, and tertiary sites of acyclic alkanes by ground-state oxygen atoms. Our studies include the characterization of the lowest-energy transition states of the O(3P) reactions with methane, ethane, propane, isobutane, and isopentane using high-level ab initio methods. The order of the calculated barriers heights is primary > secondary > tertiary, in agreement with the trends gleaned from kinetic measurements. Analysis of the transition-state geometry reveals a shift toward more reagents-like structures in the primary --> secondary --> tertiary sequence, which concurs with the expectation from Hammond's postulate. Using the ab initio data, we calculate thermal rate constants via transition-state theory. Our highest-level calculations indicate that the room-temperature relative reactivities of primary, secondary, and tertiary alkane sites in hydrogen-abstraction reactions by ground-state oxygen atoms are 1, 29, and 422, respectively. These results are used to interpret recent experiments on the reactions of O(3P) with liquid alkanes.     

Disproportionation of O-methylhydroxylamine catalyzed by aquapentacyanoferrate(II)

The aquapentacyanoferrate(II) ion, [Fe(II)(CN)(5)H(2)O](3-), catalyzes the disproportionation reaction of O-methylhydroxylamine, NH(2)OCH(3), with stoichiometry 3NH(2)OCH(3) → NH(3) + N(2) + 3CH(3)OH. Kinetic and spectroscopic evidence support an initial N coordination of NH(2)OCH(3) to [Fe(II)(CN)(5)H(2)O](3-) followed by a homolytic scission leading to radicals [Fe(II)(CN)(5)(?)NH(2)](3-) (a precursor of Fe(III) centers and bound NH(3)) and free methoxyl, CH(3)O(?), thus establishing a radical path leading to N-methoxyamino ((?)NHOCH(3)) and 1,2-dimethoxyhydrazine, (NHOCH(3))(2). The latter species is moderately stable and proposed to be the precursor of N(2) and most of the generated CH(3)OH. Intermediate [Fe(III)(CN)(5)L](2-) complexes (L = NH(3), H(2)O) form dinuclear cyano-bridged mixed-valent species, affording a catalytic substitution of the L ligands promoted by [Fe(II)(CN)(5)L](3-). Free or bound NH(2)OCH(3) may act as reductants of [Fe(III)(CN)(5)L](2-), thus regenerating active sites. At increasing concentrations of NH(2)OCH(3) a coordinated diazene species emerges, [Fe(II)(CN)(5)N(2)H(2)](3-), which is consumed by the oxidizing CH(3)O(?), giving N(2) and CH(3)OH. Another side reaction forms [Fe(II)(CN)(5)N(O)CH(3)](3-), an intermediate containing the nitrosomethane ligand, which is further oxidized to the nitroprusside ion, [Fe(II)(CN)(5)NO](2-). The latter is a final oxidation product with a significant conversion of the initial [Fe(II)(CN)(5)H(2)O](3-) complex. The side reaction partially blocks the Fe(II)-aqua active site, though complete inhibition is not achieved because the radical path evolves faster than the formation rates of the Fe(II)-NO(+) bonds.     

Second hydrogen atom abstraction by molecular ions

We report the observation of a new physical phenomenon of the addition of 2 hydrogen atoms to molecular ions thus forming [M + 2H]⁺ ions. We demonstrate such second hydrogen atom abstraction onto the molecular ions of pentaerythritol and trinitrotoluene (TNT). We used both gas chromatography mass spectrometry (GC‐MS) with supersonic molecular beam (SMB) with methanol added into its make‐up gas and electron ionization (EI) liquid chromatography mass spectrometry (LC‐MS) with SMB with methanol as the LC solvent. We found that the formation of methanol clusters resulted upon EI in the formation of dominant protonated pentaerythritol ion at m/z = 137 plus about 70% relative abundance of pentaerythritol molecular ion with 2 additional hydrogen atoms at m/z = 138 which is well above the 5.7% natural C¹³ isotope abundance of protonated pentaerythritol. Similarly, we found an abundant protonated TNT ion at m/z = 228 and a similar abundance of TNT molecular ion with 2 additional hydrogen atoms at m/z = 229. Upon the use of deuterated methanol (CD₃OD) as the solvent, we observed an abundant m/z = 231 (M + 2D)⁺ of TNT with 2 deuterium atoms. We found such abundant second hydrogen atom abstraction with butylglycolate and at low abundances in dioctylphthalate, Vitamin K3, phenazine, and RDX. At this time, we are unable to report the magnitude and frequency of occurrence of this phenomenon in standard electrospray LC‐MS. This observation could have important implications on the provision of elemental formula from mass spectra that are involved with protonated molecules. Accordingly, while accurate mass measurements can serve for the generation of elemental formula, their further support and improvement via isotope abundance analysis are questionable. Consequently, if a given compound can be analyzed by both GC‐MS and LC‐MS, its GC‐MS analysis can be superior for the provision of accurate elemental formulae if its EI mass spectrum exhibits abundant molecular ions such as with GC‐MS with SMB (also known as cold EI).     

PMO theory for hydrogen abstraction from saturated hydrocarbons

Saturated hydrocarbon radicals in the hybrid orbital all-valence-electron basis possess a nonbonding molecular orbital whose eigencoefficients can be written by inspection. Perturbational MO calculations using these coefficients lead to good correlations of hydrogen abstraction reactivity data.     

Rate constants for abstraction of hydrogen from ethylene by methyl and ethyl radicals relative to abstraction from propane and isobutane

A method is described for the measurement of relative rate constants for abstraction of hydrogen from ethylene at temperatures in the region of 750 K. The method is based on the effect of the addition of small quantities of propane and isobutane on the rates of formation of products in the thermal chain reactions of ethylene. On the assumption that methane and ethane are formed by the following reactions, (1) measurements of the ratio of the rates of formation of methane and ethane in the presence and absence of the additive gave the following results: Values for k₂ and k₃ obtained from these ratios are compared with previous measurements.     

Fluoride ion detection by 8-hydroxyquinoline-Zr(IV)-EDTA complex

A simple fluorescent detection based on the ligand exchange mechanism is proposed for the fluoride ion in aqueous media. This procedure is based on the exchange of 8-hydroxyquinoline (oxine) coordinated to Zr(IV) by fluoride ion without interference from other common anions. The ternary complex of oxine with [Zr(H(2)O)(2)EDTA].2H(2)O formed by replacing two water molecules in aqueous solution provides a sensitive signalling system for fluoride ion in the concentration range from 6 x 10(-7)M to 8 x 10(-4)M. The green fluorescence (lambda(max)=532 nm) exhibited by the complex upon excitation at 247 nm decreases in intensity with fluoride addition with a detection limit of 12 ppb. The complexation reaction between oxine and Zr(IV)-EDTA and the ligand exchange reaction with fluoride ion has been investigated by UV-vis and fluorescence spectroscopies combined with the PM3 semi-empirical quantum chemical calculations. Job's method of continuous variation and the molar ratio method ascertain a 1:1 stoichiometry composition of the chelate in aqueous media.     

On the possible catalysis by single water molecules of gas-phase hydrogen abstraction reactions by OH radicals

The gas phase hydrogen abstraction reaction between OH and CY(2)XH, where X = H, F, OH, or NH(2) and Y = H, CH(3) or F, in the absence and presence of a single water molecule is investigated using both density function theory, B3LYP, and explicitly correlated coupled cluster theory, CCSD(T)-F12. We find that a single water molecule could have a catalytic effect at low temperatures possible in laboratory experiments, but does not seem to catalyze these reactions at 298 K, and will not play a role under relevant atmospheric conditions.