Elimination Of Hydrogen Atom Research Articles

Molecular orbital study of the initial processes in the electrochemical polymerizations of pyrrole, thiophene and furan

The initial processes in the electrochemical polymerizations of pyrrole, thiophene and furan were studied by the molecular orbital method. The oxidation potentials of the three molecules in acetonitrile were calculated to be 0.79, 1.53 and 1.27 V (vs. SHE), respectively, with the use of the Born-Haber process. The overpotentials of the three molecules were estimated to be 0.25, 0.31 and 0.82 V, respectively. If it is assumed that the electrode reactions take place in the normal region of the Marcus electron-transfer theory, the three reorganization energies involved in the electrode reactions may be in the order: pyrrole < thiophene < furan. The Bader-Pearson theory predicts that the coupling reaction of each cation is symmetry-allowed under C i or C 2 symmetry of the cation dimer. The three radical coupling reactions under C i symmetry were followed by calculating the total energy of each cation dimer as a function of the distance between the α- and α'-carbon atoms. The variation in bond length between the α-carbon atom and hydrogen atom and that in the two-centre energy between the two atoms suggest the elimination of hydrogen atoms or protons. The calculation also indicated that while the rate-determining step is a coupling process for the pyrrole cation and furan cation, as shown previously for the former cation, it is an elimination process with a relatively large activation energy for the thiophene cation. This could explain at least partially the difficult formation of a uniform and thick polythiophene film, in comparison with the polypyrrole film.

A model for the surface chemical kinetics of GaAs deposition by chemical-beam epitaxy

Recently we have reported the measurement of reflection high-energy electron diffraction intensity oscillations during chemical-beam epitaxy of GaAs using triethylgallium (TEG) and As2 derived from an arsine cracker [Appl. Phys. Lett. 50, 19 (1987)]. In this study we observed a significant variation of the GaAs growth rate with substrate temperature at constant TEG flux. In addition, the variation of growth rate with incident flux at constant temperature was found to be nonlinear below approximately 500 °C and linear above 500 °C for incident fluxes yielding maximum growth rates between 0.2 and 1.8 monolayers/s. We have developed a model of the surface pyrolysis of triethylgallium which explains the qualitative behavior of the above data. The model assumes the existence of adsorbed triethyl, diethyl, and monoethyl gallium species as well as adsorbed ethyl radicals. As a starting point, the rate limiting step to epitaxial incorporation of atomic gallium is assumed to be cleavage of the second ethyl–gallium bond. Elimination of atomic hydrogen from adsorbed ethyl groups to form ethylene is also included in the model, as is recombination of one ethyl group and a diethylgallium to form adsorbed triethylgallium. Assuming Arrhenius behavior for the rate constants of the elementary steps of the reaction mechanism and steady-state behavior an expression for the temperature dependence of the growth rate is derived. Using realistic values for the preexponential factors and activation energies of the elementary steps a reasonable fit to the experimental data was obtained. The model reproduces the nonlinear dependence of growth rate on incident flux below 500 °C and the fall-off growth rate at higher temperature. The nonlinearity is due to the second-order recombination of an adsorbed diethyl gallium radical with an adsorbed ethyl radical followed by desorption of triethylgallium. The fall-off in growth rate above 500 °C is due to desorption of diethyl gallium radicals.

Additions and Corrections - Biosynthesis of Polyprenols in Higher Plants. The Elimination of the pro-4S Hydrogen Atom of Mevalonic Acid during the Formation of Their (Z)-Isoprene Chain

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTAdditions and Corrections - Biosynthesis of Polyprenols in Higher Plants. The Elimination of the pro-4S Hydrogen Atom of Mevalonic Acid during the Formation of Their (Z)-Isoprene ChainTakayuki Suga, Toshifumi Hirata, Tadashi Aoki, and Tsuyoshi ShishiboriCite this: J. Am. Chem. Soc. 1984, 106, 24, 7654Publication Date (Print):November 1, 1984Publication History Published online9 April 2004Published inissue 1 November 1984https://doi.org/10.1021/ja00336a602RIGHTS & PERMISSIONSArticle Views14Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (144 KB) Get e-Alerts Get e-Alerts

STEREOCHEMISTRY OF THE C-4 PROCHIRAL HYDROGEN ATOM ELIMINATION OF MEVALONATE IN THE BIOSYNTHESIS OF POLYPRENOLS IN HIGHER PLANTS

Abstract In the biosynthesis of the polyprenols in the five higher plants examined hitherto, it was found that stereochemistry of the C-4 prochiral hydrogen atom elimination of mevalonate in the formation of their E-isoprene chain follows Cornforth’s basic priciple for mevalonoid biosynthesis, but the formation of their Z-isoprene chain involves, contrary to this principle, the elimination of the pro-4s hydrogen atom of mevalonate

Biosynthesis of polyprenols in higher plants. The elimination of the pro-4S hydrogen atom of mevalonic acid during the formation of their (Z)-isoprene chain

Biosynthesis of polyprenols in higher plants. The elimination of the pro-4S hydrogen atom of mevalonic acid during the formation of their (Z)-isoprene chain

Doubly charged ion mass spectra: VI—Amines

AbstractDoubly charged ion mass spectra of 22 amines (2–10 carbon atoms) were determined using an Hitachi RMU‐7L double focusing mass spectrometer. Molecular ions were not observed in the spectra of aliphatic amines. The most intense product ion peaks in the spectra of lower molecular weight amines resulted from hydrogen elimination from the molecular ion; however, as amine molecular weight increased the largest peaks resulted from both hydrogen and heavy atom elimination from the molecular ion. Dominant ions in the doubly charged ion spectra of lower molecular weight aliphatic amines were from reactions of [CnH3N]2+ (n:=2, 3, 4) type ions. The spectra of higher molecular weight aliphatic amines spanned a wide mass range. Appearance energies for some of the more prominent ions were measured in the range from 25 to 49 eV. A geometry optimized quantum mechanical self‐consistent field molecular orbital treatment was used to compute the energies and structural parameters of prominent ions in the doubly charged ion mass spectra.

Metastable transitions of the 1,3-butadiene ion and of its perdeutero analog: Implications for an RRKM calculation

The ratios of metastable abundances for two competing reactions of methyl-radical and hydrogen-atom elimination have been measured in the mass spectra of 1.3-butadiene and of its perdeutero analog as obtained by means of electron-impact ionization. Systematic studies of the transition state in the reaction path were made by changing the transition-state structures so as to reproduce the isotope effects on the relative intensities on the basis of RRKM theory. The most plausible transition state for the methyl-elimination reaction is a 2-methylcyclopropenium ion, by comparison of the calculated rate constant with PIPECO results.

The Biosynthesis of Protoanemonin in Ranunculus glaber. The Pivotal Biosynthetic Intermediate and the Stereospecific Hydrogen Elimination from the Intermediate

Abstract The pivotal biosynthetic intermediate directly leading to protoanemonin was established to be 2-oxoglutarate on the basis of the labeling patterns in protoanemonin biosynthesized from sodium acetate-1-14C, sodium malonate-2-14C, sodium succinate-2,3-14C2, sodium glutamate-5-14C, and sodium 2-oxoglutarate-5-14C by Ranunculus glaber. Also the 3H/14C ratio in protoanemonin biosynthesized from (3R)-2-oxoglutaric-3-3H1-3,4-14C2 and 2-oxoglutaric-3-3H2-3,4-14C2 acids demonstrated that the formation of the double bond at the 3-position of protoanemonin involves the stereospecific elimination of the pro-3S hydrogen atom from 2-oxoglutaric acid.

Anion mass spectrometry of barbiturates.

Recent trends towards increasing abuse of the barbiturates has led to a proposal to legally restrict some of them. The implementation of the resulting legislation might require specific identification of the barbiturates. Such identification is not readily available from electron impact mass spectra and, even when these are supplemented with chemical ionization data, barbiturates differing only in isomeric sidechains are not completely characterized. In this study the anion mass spectra of 30 barbiturates, including all of those commonly available, are presented. The spectra are simple; ions arising from hydrogen atom and sidechain elimination from the initially formed [M]- ion are diagnostic of the barbiturate. For all but two of the barbiturates (butalbital and idobutal) relative peak intensities will discriminate between barbiturates differing only in isomeric sidechains.

A crossed molecular beam study of the O(1D2)+CH4 reaction

A cross molecular beam experiment was performed to study the O(1D2)+CH4 reaction. The results show that hydrogen atom elimination reaction greatly exceeds molecular hydrogen elimination. (AIP)

Photochemical conversion from methylamine to hydrogencyanide with an arf laser at 193 nm

Fluorescence from small free radicals of NH, CN, CH and NH 2 has been observed in the 193 nm laser photolysis of methylamine. Mass analysis shows that it is converted to HCN (70%) and acetonitrile (6%) after 3600 shots. It is suggested that the two-photon excitation to a superexcited state participates in the elimination of atomic and/or molecular hydrogen.

Photolyse du trans-butène-2 vers 7.1 eV

The photolysis of trans-2-butene (tB-2) was studied at 7.10–7.11 eV (174.5–174.3 nm) in a static system using a nitrogen lamp. The main primary photochemical processes are the C—C split leading to the formation of CH3 and CH3CHCH radicals, the primary quantum yield, Ф = 0.33 and the molecular or/and atomic hydrogen elimination, Ф = 0.27 with concurrent formation of 1,3-butadiene. The excited CH3CHCH radicals isomerize, followed by aliene formation, or are stabilized by collisions. The stabilization of the photoexcited tB-2 molecules is an inefficient process in the pressure region studied (10–20 000 N m−2). Only small isomerization of tB-2 was observed below 133 N m−2[Formula: see text]

Pyrolysis of &lt;i&gt;n&lt;/i&gt;-Propylbenzene in the Presence of Hydrogen

Kinetics and machanism were studied on the pyrolysis of n-propylbenzene in the presence of hydrogen with a flow reactor made of quartz. The reaction conditions were in the ranges of temperature from 500 to 650°C, of molar ratio of hydrogen to n-propylbenzene from 3 to 6, and of residence time from 2.5 to 8.5 sec at the atmospheric pressure. The main reaction products were styrene, methane, ethane, ethylene, toluene, ethylbenzene and benzene. Small amounts of the other several compounds were also identified by GC and GC/MS.It was suggested that the reaction proceeds mainly by the free radical chain mechanism as follows, initiation: the dissociation of n-propylbenzene to benzyl and ethyl radicals, propagation: the formation of 1-phenylpropyl radical by hydrogen atom abstraction of n-propylbenzene, and its consecutive decompostion into styrene and methyl radical, termination: the recombination of methyl, ethyl and benzyl radicals. Methane, ethane and toluene are formed by the atom transfer of methyl, ethyl and benzyl radicals respectively with hydrogen, and ethylene by the hydrogen atom elimination of ethyl radical, etc. Benzene is believed to be formed by hydrogenolysis of n-propylbenzene in the same mechanism as that of toluene reported previously.

Metastable transitions in n‐butane and n‐butane‐d7

AbstractMetastable ions have been investigated for n‐butane d7 molecular ions using a tandem mass spectrometer which samples unimolecular decay processes occuring during the time interval of c.2 μs to 4 ms after ion formation. Some 37% of ions formed by 70 eV electron impact decay on this time scale. The competing unimolecular processes observed, in order of relative importance, are methance, methyl radical and hydrogen atom elimination. The slow metastables sample the threshold energy regime of unimolecular reactions responsible for forming the ordinary mass spectrum of butance and very large isotope effect are noted for the deuterated molecule.

Mode of formulation of cholesta-5,7-dien-3beta-ol from Cholest-5-en-3beta-ol by Larvae of Calliphora erythrocephala.

1. The conversion of cholest-5-en-3beta-ol (cholesterol) into cholesta-5,7-dien-3beta-ol by axenic Calliphora erythrocephala larvae was demonstrated. 2. The transformation is probably direct (Delta(5)-->Delta(5,7)) and does not involve a Delta(0) intermediate (Delta(5)-->Delta(0)-->Delta(7)--> Delta(5,7)). 3. Delta(7)-bond formation involves the stereospecific elimination of the 7beta hydrogen atom. 4. The relative amounts of free and esterified sterols were determined in larvae grown on cholesterol as sole sterol source and on 5alpha-cholestan-3beta-ol supplemented with minimal amounts of cholesterol. 5. The significance of the results is assessed in relation to the probable role of cholesta-5,7-dien-3beta-ol as an intermediate in the biosynthesis of ecdysones.

Pyrolysis of gem-dichlorides: 2,2-dichlorobutane and 2,2-dichloropropane

2,2-Dichlorobutane decomposes at 308–370° to hydrogen chloride and a non-equilibrium mixture of chlorobutenes. In ‘seasoned’ reaction vessels the reaction is homogeneous and of the first-order, and for initial pressures within the range 27–320 mm. the rate is expressible as k1= 1014·49 exp(– 50,200/RT)sec.–1 The results are consistent with a unimolecular mechanism for the elimination of hydrogen chloride, with negligible subsequent decomposition or isomerisation of the chlorobutenes. In contrast to previous results for other gem-dichlorides the Arrhenius parameters are closer to those reported for the corresponding monochlorides. This was also observed for the decomposition of 2,2-dichloropropane, which was re-examined at 322–385° with initial pressures from 30 to 300 mm., and for which the first-order rate coefficient is expressible as k1= 1014·52 exp(–51,400/RT)sec.–1 Substitution of a β-methyl group in 2,2-dichloropropane increases the rate of elimination of hydrogen atoms both from the carbon centre to which the methyl group is attached and also from the carbon centre on the other side of the molecule.