Showing papers on "Hydrogen atom abstraction published in 1983"

Absolute rate constants for reactions of cumyloxy in solution

Abstract: Determination de la constante de vitesse de la reaction du radical cumyloxyle avec 10 substrats parmi lesquels le cumene et le peroxyde de cumyle, et de la β-scission du radical cumyloxyle en solution en utilisant les techniques de photolyse eclair laser

Intermediates in the reduction of 5-halouracils by eaq-

Abstract: The radical anions of 5-bromo- and 5-iodouracil produced by the reaction of e/sub aq//sup -/ with the halouracils have been observed in nanosecond pulse radiolysis experiments by their optical absorption at 330 nm. They decay with half-periods of 7.0 +- 0.5 and 1.7 +- 0.3 ns. These short lifetimes contrast with the much longer periods observed for the electron adducts to 5-chloro- and 5-fluorouracil (4.9 and > 15 ..mu..s). The uracilyl radical produced by halide elimination from the radical anions has an extinction coefficient not greater than 150 M/sup -1/cm/sup -1/ at 330 nm and no appreciable absorption at wavelengths longer than 310 nm but reacts with 5-bromouracil to form an addition product which has a broad absorption at 365 nm. The rate constant for this latter reaction, (2.66 +- 0.09) x 10/sup 8/ M/sup -1/s/sup -1/, provides a reference against which to compare the rates of competing reactions. The absolute rate constant for abstraction of hydrogen from tert-butyl alcohol by uracilyl radical has been determined to be (2.3 +- 0.2) x 10/sup 7/M/sup -1/s/sup -1/. It is clear that uracilyl radical reacts very rapidly by both addition and abstraction processes, reflecting the sigma character of this radical. 6 figures.

Effects of diffusion on the self‐termination kinetics of isopropylol radicals in solution

Abstract: Rate constants for the bimolecular self-reaction of isopropylol radicals [(CH3)2ĊOH] in various solvents are determined as functions of temperature by kinetic electron spin resonance. For hydrocarbon solvents they are well described by theoretical equations for reactions controlled by translational diffusion if diffusion coefficients of 2-propanol, a constant reaction distance, and a spin statistical factor of 1/4 are applied. Deviations from 2kt ∼ D at high diffusion constants agree with trends expected from recent theoretical models. For hydrogen-bonding solvents large negative deviations are observed. They are attributed to steric constraints and slower rotational diffusion of radical–solvent aggregates. The disproportionation-to-combination ratio of isopropylol increases with solvent viscosity. As previously for tert-butyl, this is explained by anisotropic reorientation during encounters. Further, rate data are given for the decarbonylation of the 2-hydroxy-2-methylpropanoyl radical and for several hydrogen abstraction reactions of isopropylol.

Electron Gain and Electron Loss Radicals Stabilized on the Purine and Pyrimidine of a Cocrystal Exhibiting Base-Base Interstacking: ESR-ENDOR of X-Irradiated Adenosine:5-Bromouracil

Abstract: KAR L., AND BERNHARD, W. A. Electron Gain and Electron Loss Radicals Stabilized on the Purine and Pyrimidine of a Cocrystal Exhibiting Base-Base Interstacking: ESR-ENDOR of XIrradiated Adenosine:5-Bromouracil. Radiat. Res. 93, 232-253 (1982). The predominant free radicals trapped in cocrystals of adenosine:5-bromouracil X-irradiated at 12?K were identified by ESR-ENDOR spectroscopy and the radical reactions were followed upon annealing to 480?K. The dominant electron abstraction and electron addition products stabilized on the bases at 12?K are observed to be the bromouracil 7r-cation and the adenine rcation and 7r-anion. The formation of an anion on bromouracil is inferred from the presence of a radical formed by deuterium addition to C6 of bromouracil at higher temperatures. Above 40?K the bromouracil 7r-cation appears to decay by recombination and is reduced to undetectable levels at - 170?K. Both adenine ir-ions are also observed to decay within the same temperature range. Above 200?K hydrogen adducts are stabilized on the bases. Experiments using partially deuterated cocrystals indicate that the H-adducts are formed via both hydrogen addition and protonation of the respective anions. Two hydrogen abstraction radicals stabilized on the sugar residue are detectable at temperatures above 200?K, but these may be present at much lower temperatures. The results presented here question the generally accepted hypothesis that, in the presence of purine:pyrimidine stacking interactions, holes are predominantly transferred to the purines while electrons are predominantly transferred to the pyrimidines.

The Uranyl Ion, Fluorescent and Fluorine‐Like: A Review

Abstract: The uranyl ion has rather unique stereochemistry, and its ultraweak electron transfer bands are compared with other transition‐group spectra. The 6 most recent MO calculations are shown to be inconclusive about the distribution of the many excited levels. The first excited state has very different chemical properties compared with the ground state, reacts with polarizable cations and anions, and shows a potent photochemistry of electron and of hydrogen atom abstraction. Its fluorescence (with much higher quantum yield in glasses than in aqueous solution) can be used for flat‐plate concentrators of solar energy.

The rotational spectrum and molecular properties of the hydrogen cyanide hydrogen bromide complex

Abstract: Rotational spectra have been assigned for four isotopic species of a linear hydrogen‐bonded complex formed between hydrogen cyanide and hydrogen bromide The vibrational ground state spectroscopic constants are The order of the atoms in HCNHBr with a N⋅⋅⋅Br separation of 361A

On the regioselectivity of free radical processes ; reactions of benzoyloxy, phenyl and t-butoxy radicals with some α,β-unsaturated esters

Abstract: The reactions of three electrophilic radicals, benzoyloxy, phenyl, and t-butoxy, have been examined with a series of α, β unsaturated esters; methyl acrylate, methyl methacrylate, and methyl crotonate. For the three monomers the ratio of hydrogen abstraction to double bond addition was found to increase and the ratio of head against tail addition to decrease in the series: benzoyloxy; phenyl; t-butoxy. Relative rates for these reactions have been determined and the factors influencing the mode of reaction are discussed.

An electron spin resonance study of fatty acids and esters. Part 1. Hydrogen abstraction from olefinic and acetylenic long-chain esters

Abstract: The radicals generated from unsaturated fatty acid esters by hydrogen abstraction with photochemically formed t-butoxyl radicals have been investigated by e.s.r. spectroscopy. The substituted allyl and pentadienyl radicals generated from monoenic and dienoic esters were conformationally stable under the conditions of the experiments. The main species observed on hydrogen-abstraction from monoynoic esters were substituted propynyl radicals, and the same radical type was generated from diynoic esters with more than one methylene unit separating the triple bonds. Substituted penta-1,4-diynyl and penta-1,3-diynyl radicals were identified on hydrogen-abstraction from methylene-interrupted and conjugated diynoic esters. Hydrogen abstraction from triacylglycerols containing one or more double bonds was also examined. Most of the unsaturated esters also gave rise to secondary radicals produced by hydrogen-abstraction from the chain methylene groups. From the measured concentrations of the secondary and delocalised radicals the following approximate relative rates of hydrogen-abstraction by t-butoxyl radicals from the different sites were obtained: secondary: propynylic: allylic: bisayllic = 1 : 18 : 36 : 116 at 293 K.

Hydrogen in aromatics. III. Chemisorption of hydrogen in graphite–alkali metal intercalation compounds

Abstract: The hydrogen chemisorption mechanism in the graphite–alkali metal intercalation compounds has been investigated by means of ESR and electrical resistivity under hydrogen atmosphere. In the first stage rubidium compound C8Rb, hydrogen molecules are dissociated into atoms which are stabilized in the interstices of Rb atoms in the gaps of the graphite layers. C8K has a two‐stage hydrogen absorption. In the initial stage, the dissociated hydrogen atoms are transiently yielded and then, in the second stage, the formation of hydride ions H− takes place through the charge transfer from C8K to H. C8Cs shows no change in the ESR signal under hydrogen atmosphere. This finding suggests that its catalytic activity for an H–D exchange reaction is due to the surface activity. For the second stage compounds C24K and C24Rb, there exists a one‐stage hydrogen absorption process, i.e., absorbed hydrogen directly becomes hydride ion H−. C24Cs shows no change in the ESR signal under hydrogen atmosphere. For the chemisorption activity, the order of the strength is K≳Rb≳Cs in both first and second stage compounds.

Solvent effects on the reaction of t-butoxy radicals with methyl methacrylate

Abstract: Examination of the title reaction in a range of solvents shows that the ratio of hydrogen abstraction to t-butoxy radical addition increases with increasing solvent polarity. In several cases there is a competition between solvent and methyl methacrylate for reaction with t-butoxy radicals. The implications of these findings for polymer structure are discussed.

Crystal lattice control of unimolecular photorearrangements. Medium-dependent photochemistry of cyclohexenones

Abstract: Photolyse en solution et a l'etat solide de compose de structure oxy-1 cis-tetrahydro-4a,5,8,8- naphtoquinol-4. En solution, formation de composes cage [2+2]. A l'etat solide, produits resultant de l'enlevement intramoleculaire d'atome H allylique par l'atome Cβ de l'enone suivi d'une rupture diradicalaire. Etude par diffraction des types possibles de conformation a l'etat solide. Mecanismes

Detection of hydroxymethyl (CH2OH) radicals by resonance-enhanced multiphoton ionization spectroscopy

Abstract: The hydroxymethyl radical, CH/sub 2/OH, has been detected by the method of resonance-enhanced multiphoton ionization mass spectroscopy. Radicals were produced via hydrogen abstraction from CH/sub 3/OH by fluorine atoms in a discharge flow reactor. Tunable laser radiation between 427 and 490 nm ionized the CH/sub 2/OH radicals. The identity of the radical was determined from mass spectrometry of deuterium-substituted radicals. The isomerization reaction which transforms methoxy radicals into hydroxymethyl radicals was not observed.

Aryl isomerization during aliphatic carbon-hydrogen bond activation

Abstract: The transition-metal-stablized benzyne or o-phenylene (n/sup 2/-C/sub 6/H/sub 4/) ligand has been shown to be both an interesting and reactive group. The ligand is normally generated in mononuclear systems by ..beta..- (ortho-) hydrogen abstraction from an aryl group, and this synthetic approach has allowed a stable example to be isolated and structurally characterized. Here the conclusive identification of a benzyne intermediate during the isomerization of a tantalum-aryl compound is reported. The reaction is interesting in that the ortho hydrogen is transferred to the carbon atom of a cyclometalated chelate, the reverse (isomerization) step thus involving the activation of an aliphatic CH bond by the intermediate benzyne.