Showing papers on "Hydrogen atom abstraction published in 1987"

A structure-activity relationship for the estimation of rate constants for the gas-phase reactions of OH radicals with organic compounds

Abstract: A previous technique for the calculation of rate constants for the gas-phase reactions of the OH radical with organic compounds has been updated and extended to include sulfur- and nitrogen-containing compounds. The overall OH radical reaction rate constants are separated into individual processes involving (a) H-atom abstraction from CH and OH bonds in saturated organics, (b) OH radical addition to >CC NH, >N, SH, and S groups. During its development, this estimation technique has been tested against the available database, and only for 18 out of a total of ca. 300 organic compounds do the calculated and experimental room temperature rate constants disagree by more than a factor of 2. This suggests that this technique has utility in estimating OH radical reaction rate constants at room temperature and atmospheric pressure of air, and hence atmospheric lifetimes due to OH radical reaction, for organic compounds for which experimental data are not available. In addition, OH radical reaction rate constants can be estimated over the temperature range ca. 250–1000 K for those organic compounds which react via H-atom abstraction from CH and OH bonds, and over the temperature range ca. 250–500 K for compounds containing >CC< bond systems.

Organic solid state chemistry

Abstract: Organic Solid State Reactions: Topochemistry and Mechanism. Geometric Requirements for Intramolecular Photochemical Hydrogen Atom Abstraction: Studies Based on a Combination of Solid State Chemistry and X-Ray Crystallography (J.R. Scheffer). Topotactic and Topochemical Photodimerisation of Benzylidenecyclopentanones (C.R. Theocharis, W. Jones). The Prediction of Chemical Reactivity Within Organic Crystals Using Geometric Criteria (S.K. Kearsley). Phonon Spectroscopy of Organic Solid State Reactions (P.N. Prasad). Four-Center Type Photopolymerization of Diolefin Crystals (M. Hasegawa). Some Stereochemical Questions: Pure and Applied Chemistry. Organic Molecules in Constrained Environments (J.M. Thomas, K.D.M. Harris). Clathrates (G. Tsoucaris). Solid State Chemistry of Phenols and Possible Industrial Applications (R. Perrin et al.). Gas-Solid Reactions and Polar Crystals (I.C. Paul, D.Y. Curtin). Intermolecular Interactions: From Crystallography to Chemical Physics. Phase Transitions in Organic Solids (C.N.R. Rao). Molecular Motions in Organic Crystals: The Structural Point of View (A. Gavezzotti, M. Simonetta). Interatomic Potentials and Computer Simulations of Organic Molecules in the Solid State (S. Ramdas, N.W. Thomas,). Conformational Polymorphism (J. Bernstein). Crystal Engineering A 4 A - Short Axis Structure for Planar Chloro Aromatics (G.R. Desiraju).

Reactivity of Phthalimide- N -oxyl : A Kinetic Study

Abstract: The reactivity of phthalimide-N-oxyl radical (2), which is produced by an electrochemical one-electron oxidation of N-hydroxyphthalimide (1), was investigated. The self-decomposition of 2 obeyed second-order kinetics with respect to itself and the rate was little affected by changing the temperature or the concentration of dissolved oxygen, water or added base. In the reaction between 2 and benzhydrol or its α-deuterated derivative, the decay of the radical was first-order with respect to each component. The observed large deuterium isotope effect (kH/kD= 10.6 at 25 °C) is attributed to the rate-limiting hydrogen abstraction. Rate measurements were performed on the reaction with 24 other substrates which have one or more hydrogen atoms on a benzylic or allylic position or on a carbon adjacent to a hetero atom.

Kinetics of the reaction of a secondary alkyl radical with tri-n-butylgermanium hydride and calibration of a secondary alkyl radical clock reaction

Abstract: Les parametres d'Arrhenius par la reaction du titre sont mesures en utilisant la cyclisation du radical hexene-5yle comme reaction d'etalonnage

Structure of the boron-hydrogen complex in crystalline silicon.

Abstract: The lattice site occupied by the boron and hydrogen atoms in hydrogenated crystalline silicon is investigated by use of ion channeling and nuclear-reaction analysis. After hydrogenation, boron atoms are found to be displaced from substitutional sites. Analysis of the data indicates that the shift is 0.22\ifmmode\pm\else\textpm\fi{}0.04 \AA{}. The hydrogen atoms are found to preferentially occupy the bond-center site. These findings are consistent with a structure for the boron-hydrogen complex in which the hydrogen atom occupies a site between a silicon and a boron atom, while the boron atom relaxes back towards a threefold-coordinated position. The data are compared with recent predictions of the structure of the B-H complex in silicon.

Photoreduction of benzophenone by acetonitrile: correlation of rates of hydrogen abstraction from RH with the ionization potentials of the radicals R.

Abstract: Triplet benzophenone (/sup 4/K) abstracts a hydrogen from acetonitrile with a rate constant of 130 +/- 30 M/sup -1/ s/sup -1/. Despite this low rate constant acetonitrile is not really an inert solvent and at low light fluxes, where T-T annihilation is not a major fate of the triplet, a major decay path for /sup 4/K is hydrogen abstraction with resulting pinacol (K/sub 2/H/sub 2/) formation (phi/sub K/sub 2/H/sub 2/ approx. = 0.1 at t = 0). Both K/sub 2/H/sub 2/ formation and /sup 3/K lifetime rapidly decrease with irradiation due to the light absorbing transients (LAT's) which are formed along with K/sub 7/H/sub 2/ from ketyl radicals (KH). The rate constants per hydrogen (k/sub H/) for abstraction from R-H by the electrophilic /sup 3/K correlate well with the ionization potentials (IP) of the corresponding radicals (R).

Investigation of hydroxyl radical reactions with o-xylene and m-xylene in a continuous stirred tank reactor.

Abstract: The gas-phase reactions of hydroxyl radicals with o-xylene and m-xylene were studied in a continuous stirred tank reactor. Gas and aerosol products accounted for 65-85% of the reacted carbon. Approximately 19 and 13% of the original o-xylene and m-xylene oxidation were estimated to have occurred through methyl hydrogen abstraction by OH, primarily leading to methylbenzyl nitrates and tolualdehydes. The remaining mass reacted through the OH addition pathway forming dimethylphenols, nitrodimethylphenols, nitroxylenes, and stable products resulting from reaction of metastable O/sub 2/-OH adducts. For o-xylene, the ratio of the rate constants for formation of nitroxylenes vs. dimethylphenols was estimated to be 5.9 x 10/sup 4/, while the same value for m-xylene was only about 1.0 x 10/sup 4/. The ratios of the dimethylphenol formation rates to the oxygen addition rates were found to be greater than or equal to 0.15 for o-xylene and 0.27 for m-xylene. 44 references, 3 figures, 4 tables.

Mechanisms of the hydrometalation (insertion) and stoichiometric hydrogenation reactions of conjugated dienes effected by manganese pentacarbonyl hydride: processes involving the radical pair mechanism

Abstract: Manganese pentacarbonyl hydride (I) reacts with 1,3-butadiene, isoprene, and 2,3-dimethyl-1,3-butadiene to form predominantly the hydrometalated products of 1,4-addition of H-Mn(CO)/sub 5/ to the dienes, i.e. I + CH/sub 2/=CRCR'=CH/sub 2/ ..-->.. MeCR=CR'CH/sub 2/Mn(CO)/sub 5/ R,R' = H, Me. Monoolefins, the products of stoichiometric 1,2- and 1,4-addition of two hydrogen atoms to the dienes, are also formed as minor products, i.e. 2I + CH/sub 2/=CRCR'=CH/sub 2/ ..-->.. MeCHRCR'=CH/sub 2/ + CH/sub 2/=CRCHR'Me + MeCR=CR'Me + Mn/sub 2/(CO)/sub 10/. Hydrogenation is the predominant process with 1,3-cyclohexadiene. The hydrometalation reactions are first order in both I and diene, and the reaction rates are unaffected by added carbon monoxide (1 atm), while careful NMR monitoring of the early stages of the reactions under appropriate conditions reveals striking CIDNP polarization is for the /sup 1/H resonances of both the products of hydrometalation and of hydrogenation. The experimental evidence thus suggests that the reactions do not proceed via prior coordination of the olefin to the metal followed by conventional migratory insertion and reductive elimination processes but rather via hydrogen atom abstraction from I by the diene to give the corresponding (ally) + Mn(CO)/sub 5/) radical pair.

Intramolecular functionalization of N-cyanamide radicals: synthesis of 1,4-and 1,5-N-cyanoepimino compounds

Abstract: Photolysis of 3β-methoxymethoxy-5α-cholestan-6β-ylcyanamide (6), 29-methoxyfriedelan-3β-yl-cyanamide (15), (22R,25R)-5α-furostan-26-ylcyanamide (23), 8α,12-(12R/12S)-epoxylabdan-15-ylcyanamides, (32) and (33), in the presence of iodine and lead tetra-acetate leads to neutral cyanimyl radicals which undergo intramolecular hydrogen abstraction to produce N-cyanoepimino compounds. Better results are obtained with the system iodine and diacetoxyiodobenzene. The starting cyanamides have been prepared by reduction of the oximes (3) and (13) and of the amides (20), (28), and (29), respectively, with lithium aluminium hydride followed by cyanation with cyanogen bromide or with sodium cyanate and subsequent dehydration of the urea derivative with methanesulphonyl chloride.

Polarity reversal catalysis of hydrogen atom abstraction reactions

Abstract: Overall transfer of electron deficient hydrogen to the electrophilic t-butoxyl radical is catalysed by Me3N → BH2Hext; hydrogen atom abstraction is actually brought about by the nucleophilic Me3N → ḂHHext.

Kinetic and spectroscopic study of a ketyl-phenoxy biradical produced by a remote hydrogen abstraction

Abstract: Etude de la photolyse eclair laser d'(hydroxy-4-phenethyloxy)-4 aceto- et valerophenones en solution

Binuclear platinum(II) photochemistry. Rates of hydrogen atom transfer from organometallic hydrides to electronically excited Pt2(P2O5H2)44

Abstract: Previous investigations have established that the triplet dsigma*psigma excited states of d/sup 8/-d/sup 8/ binuclear complexes (/sup 3/M/sub 2/*, M = Rh, Ir, Pt) are readily oxidized by one-electron acceptors. A related but largely unexplored reaction for these /sup 3/M/sub 2/* states is hydrogen atom abstraction, RH + /sup 3/M/sub 2/* ..-->.. R/sup ./ + /sup ./M/sub 2/H, a step that was proposed by Roundhill to be a primary process in the Pt/sub 2/-photocatalyzed conversion of isopropyl alcohol to acetone and dihydrogen (Pt/sub 2/ = Pt/sub 2/(P/sub 2/O/sub 5/H/sub 2/)/sub 4//sup 4 -/). Support for Roundhill's proposal has come from work in their laboratory that has shown that /sup 3/Pt/sub 2/* will abstract hydrogen atoms from a large number of different substrates. Because of the potential importance of H atom transfer in hydrocarbon and related small-molecule activation processes, they are engaged in studies involving systematic variations in the substrate donor to /sup 3/Pt/sub 2/*: their aim is to elucidate the factors that control the reaction rates. Here they report the results of kinetic studies of H atom transfers from organometallic hydrides to /sup 3/Pt/sub 2/*.

Dominant reaction channels and the mechanism of silane decomposition in a H2-Si(s)-SiH4 glow discharge

Abstract: Chemical relaxation mass spectrometry has been used to study the kinetics and mechanism in the silane-hydrogen-solid silicon system under conditions of glow discharge. The emphasis was on the main processes related to the deposition of amorphous and nanocrystalline silicon thin films. It is shown that under conditions of the deposition of a-Si and nc-Si the dominant reaction channel is the electron impact induced fragmentation of silane into molecular hydrogen and SiH2 radical. The role of other processes, such as hydrogen abstraction, is discussed.

The mechanism and rate parameters for the pyrolysis of n-hexane in the range 723–823 K

Abstract: The pyrolysis of n-hexane has been investigated in the ranges 723–823 K and 10–100 Torr at up to 3% decomposition. The reaction is homogeneous and free from the self-inhibition by olefin products observed for several other alkanes. The products of the reaction are hydrogen, methane, ethane, ethene, propene, but-1-ene, and pent-1-ene, with smaller amounts of propane. It is shown that the results are in quantitative agreement with a conventional Rice-Herzfeld chain mechanism terminated by the combination and disproportionation of ethyl radicals, but with the mechanism extended so as to include the unimolecular isomerizations via a six-membered cyclic transition state between 1-hexyl and 2-hexyl (1-methylpentyl) radicals. The overall rate constant of initiation is estimated to be given by The rate constant for the reaction is given by which when combined with published data gives an Arrhenius plot curved upwards at low values of 1/T as has been observed for several other hydrogen abstraction reactions of methyl and of ethyl. Estimates are made of rate constants and ratios of rate constants for several reactions of the free radicals involved in the reaction. It is suggested that the minor product propane arises mainly from a hydrogen abstraction by 1-propyl from hexane with a contribution from a minor termination process involving ethyl and methyl.

N-nitrosoamine and N-nitroamine formation: factors influencing the aqueous reactions of nitrogen dioxide with morpholine

Abstract: The formation of nitrosamine and nitroamine from low levels of nitrogen dioxide (1-99 ppm) and morpholine in aqueous solution was examined. We have shown for the first time that significant quantities of N-nitromorpholine form at physiological pH; the rate of N-nitration was more than double that of N-nitrosation at 37/sup 0/C, pH 7.4. The effects of temperature, pH, reactant concentration, and various anions on both nitration and nitrosation reactions are presented. In addition to pH effects, a deuterium effect was observed for the nitration reaction, suggesting that hydrogen abstraction is a rate-limiting step in product formation. Possible reaction mechanisms are described, and the implications for carcinogen formation in vivo due to low-level NO/sub 2/ exposure are discussed. 16 references, 7 figures, 4 tables.

Pulse radiolysis study of the reaction of OH radicals with C2H4 over the temperature range 343-1173 K

Abstract: The reaction of OH and OD radicals with ethylene in the presence of 1 atm argon and 6 Torr water vapor was studied in the temperature range 343–1173 K. The results reveal three kinetically separate temperature regions: (1) 343–563 K, where the disappearance of OH radical is dominated by the addition of OH to the double bond of ethylene; (2) 563–748 K, where concurrent reactions of addition, the reverse reaction of addition and H-atom abstraction is dominant; and (3) 748–1173 K, where H-atom abstraction is likely the main reaction. The rate for hydrogen abstraction is 2.4 × 10−11 exp[(−2104 ± 125)/T] cm3/molec-s (for OD 2.1 × 10−11 exp[(−2130 ± 172)/T] cm3/molec-s). There was no obvious pyrolysis of ethylene below 1073 K. The study of OD radical with ethylene shows a small isotope effect.

Product state distributions from the reaction O(3P)+ HBr

Abstract: An experimental investigation has been performed of the hydrogen atom abstraction reaction O(3P)+ HBr → OH(X2Π)+ Br. A photolytic method was used to produce O(3P) in the presence of HBr, and laser-induced fluorescence detection employed to obtain the nascent vibrational, rotational and fine-structure state partitioning in the product OH. Distributions were found to be highly internally excited, with a strong vibrational inversion and substantial rotational excitation, extending to the limit of available energy. Non-statistical distributions of the fine-structure states were observed. The possibility of a subsidiary reactive channel producing spin–orbit excited Br(2P1/2) is proposed as an explanation for an apparent anomaly in the OH(v″= 1) rotational distribution. The dynamics generally appear consistent with the kinematic constraints imposed by the heavy + light–heavy mass combination.

Absolute rate constants for hydrogen abstraction from hydrocarbons by the trichloromethylperoxyl radical

Abstract: Absolute rate constants have been measured for the reactions of trichloromethylperoxyl radicals with cyclohexane, cyclohexene, and hexamethylbenzene. The CCl3O2 radicals were produced by pulse radiolysis of air-saturated CCl4 solutions containing various amounts of the hydrocarbons. The rate constants were determined by competition with the one-electron oxidation of metalloporphyrins, using the rate of formation of the metalloporphyrin radical cation absorption to monitor the reaction by kinetic spectrophotometry. The rate constants for hydrogen abstraction from cyclohexane, cyclohexene, and hexamethylbenzene were found to be 1 × 103, 1.0 × 105, and 7.5 × 104 M−1 s−1, respectively.