Migration Of Hydrogen Atom Research Articles

Hydrogen rearrangement in molecular ions of alkyl benzenes: Appearance potentials and substituent effects on the formation of [C7H8]+ ions

AbstractThe mechanism of the formation of [C7H8]+ ions by hydrogen rearrangement in the molecular ions of 1‐phenylpropane and 1,3‐diphenylpropane has been investigated by looking at the effects of CH3O and CF3 substituents in the meta and para positions on the relative abundances of the corresponding ions and on the appearance energies. The formation of [C7H8]+ ions from 1,3‐diphenylpropane is much enhanced at the expense of the formation of [C7H7]+ ions by benzylic cleavage, due to the localized activation of the migrating hydrogen atom by the γ phenyl group. A methoxy substituent in the 1,3‐diphenylpropane, exerts a site‐specific influence on the hydrogen rearrangement, which is much more distinct than in 1‐phenylpropane and related 1‐phenylalkanes, the rearrangement reaction being favoured by a meta methoxy group. The mass spectrum of 1‐(3‐methoxyphenyl)‐3‐(4‐trideuteromethoxyphenyl)‐propane shows that this effect is even stronger than the effect of para methoxy groups on the benzylic cleavage. From measurements of appearance potentials it is concluded that the substituent effect is not due to a stabilization of the [C7H7X]+ product ions. Whereas the [C7H7]+ ions are formed directly from molecular ions of 1‐phenylpropane and 1,3‐diphenylpropane, the [C7H8]+ ions arise by a two‐step mechanism in which the s̀ complex type ion intermediate can either return to the molecular ion or fragment to [C7H8]+ by allylic bond cleavage. Obviously the formation of this s̀ complex type ion, is influenced by electron donating substituents in specific positions at the phenyl group. This is borne out by a calculation of the ΔHf values of the various species by thermochemical data. Thus, the relative abundances of the fragment ions are determined by an isomerization equilibrium of the molecular ions, preceding the fragmentation reaction.

The mechanism and the kinetics of polyethylene branching studied on a model system

The γ-ray initiated radical telomerization of ethylene by bromodecane has been used as the model reaction in the study of the mechanism and kinetics of non-linear structure formation in low density (high pressure) polyethylene (HPPE). The identification of the reaction products showed a 1,5-rearrangement to take place of the intermediate alkyl radicals which is accompanied by a hydrogen atom migration. This type of isomerization explains the presence of the short butyl and ethyl branches in the PE molecules. The competing kinetics method has been used to find the isometrization constants in the 30–150°C range for which the parameters of the Arrhenius equation have been determined. The findings permitted the calculation of the branching intensity for various process technology parameters (temperature and pressure).

Mass spectral studies of some dithioethers and diamine-dithioethers

The mass spectra of bis(phenylthio)methane, 1,2-bis(phenylthio)ethane, 1,3-bis(phenylthio)propane, 1,6-bis(phenylthio)hexane, 1,8-bit(phenylthio)octane, cis-1,2-bis(phenylthio)ethylene, o-phenylenebis(phenylthioether), some methyl-substituted analogues, and some tetradentate diamine-dithioethers are reported. The fragmentation patterns are illustrated and discussed with respect to differences between the ligands and the fragments derived from different ligands. The completely aliphatic ligands exhibit a complex fragmentation of α and β-cleavages with and without hydrogen atom migration. A number of novel sulphur- and nitrogen-containing heterocycles have been detected.

Cyclic unsaturated compounds. 65. Cycloheptadienyl anions

1. The thermal isomerization of 1,3- and 1,4-cycloheptadiene is not observed up to 500°C. 2. In the presence of a strong base, 1,4-cycloheptadienes isomerize almost irreversibly to the corresponding 1,3-dienes; bases do not catalyze double-bond migration in the 1,3-cycloheptadiene series. 3. For the reaction of 1,3- and 1,4-cycloheptadienes with a strong base the formation of anions that can be alkylated occurs; these anions are not identical and, therefore, do not have the structure of a homocyclopentadienyl anion. 4. A correlation between the acidity of the hydrocarbons in the series eyclopentadiene-cyclohexadiene-cycloheptadiene and the enthalpy of activation for double-bond migration in the same series does not exist. Consequently, in this isomerization process the migrating hydrogen atom does not have a positive charge.

Recovery of Pure Iron Quenched-in Liquid Hydrogen

A new method of hydrogenation and quenching of metals in liquid hydrogen, i.e. the hydrogen quenching method, is developed and applied to investigate the recovery of pure iron after the hydrogenation and quenching by measuring the electrical resistivity change during annealing at higher temperatures. A new stage corresponding to the migration and disappearance of solute hydrogen atoms in pure iron appears in the resistivity decay curve, and by an approximate analysis based upon the rate process calculation the migration energy of single interstitial hydrogen atoms is determined to be 2.2±0.2 kcal/mol. In the diffusion process, most of the solute hydrogen atoms are discharged at the free surface of the specimen, but, in order to fully explain the resistivity decay curves, the influence of trapping sites or sinks for hydrogen in the interior must be also considered.

Mass-spectrometric study of hydrocarbons of bicyclo[3.2.1]octane series

1. 1. Mass-spectra of 33 methyl-, dimethyl- and ethyl-substituted bicyclo[3.2.1]octane geomers were studied. 2. 2. Main trends of the breakdown of M + ions of bicyclo[3.2.1]octanes are due to bond ruptures at tertiary and quaternary carbon atoms. These ruptures are often accompanied by the migration of hydrogen atoms. 3. 3. Some agreement between the relative thermodynamic stability of stereo-isomers of a given structure and their resistance to electron impact was established. 4. 4. “Unfavourable” interactions of substituents with each other and with the system of bicyclo [3.2.1] octane determine the main directions of breakdown of molecular ions, while methyl substituents are retain d in neutral or charged fragments.

Stereochemistry of hydrogen migration from C-24 to C-25 during isofucosterol biosynthesis in Pinus pinea

During the biosynthesis of isofucosterol from lanosterol in Pinus pinea the stereochemistry of the hydrogen atom migration from C-24 to C-25 has been established.

Behavior of the13C lines in the ESR spectra of 3,6-di-tert-butyl-2-oxyphenoxyl with intramolecular hydrogen atom migration

1. Study has been made of the effect of unpaired electron hyperfine interaction with the13C nucleus on the ESR spectrum of 3,6-di-tert-butyl-2-oxyphenoxyl. 2. The spin density distribution has been calculated for the radical under both rapid and slow exchange, and13C hfi constants obtained for various positions.

Chain length effects in the competition between intramolecular hydrogen migrations in the molecular ions of hydrogen migration in the molecular ions of α‐benzylamino‐ω‐benzyloxyalkanes as studied by deuterium labelling

AbstractThe unimolecular decompositions upon electron impact of the bifunctional compounds ØCH2‐NH(CH2)n‐OCH2Ø(n=2 to 7)have been studied by deuterium labelling. The molecular ions lose a benzyl radical and/or a benzaldehyde molecule, depending on the length of the aliphatic chain. Both reactions appear to be the result of the transfer of a hydrogen atom to the charge localized amino function in a cyclic transition state. The ring size of this transition state turns out to play a far more important role than the difference in reactivity of the migrating hydrogen atoms.

The adsorption of hydrogen atoms on support oxides active in promoting hydrogen atom spillover

The adsorption of hydrogen atoms generated in the gas phase to a variety of oxide supports, reported to be active in promoting hydrogen atom spillover, has been studied at 77 and 273 °K. Hydrogen uptake was insufficient to account for catalytic enhancement due to hydrogen atom migration followed by adsorption. Supports prepared under alkali conditions in general adsorbed more active hydrogen. Hydrogen uptake by tungsten trioxide was about two orders of magnitude greater than on the other support oxides investigated.

Mass-spectrometric study of sulfenamide derivatives of 2-substituted benzothiazoles

It is shown that the dissociative ionization of 2-mercaptobenzothiazole derivatives with a sulfenamide bond is accompanied by cleavage of the labile N-S bond and simultaneous migration of the hydrogen atom from the thiol substituent at the site of cleavage to the sulfur atom. Ion peaks with mass 167, 166, R, and (R-H), where R is an N-substituted cycloalkylamine, are maximum peaks and are used for the identification of the compounds.

Coordination complexes containing multidentate ligands. VII. Mass spectral studies on some open-chain tetradentate thioether chelates

The mass spectra of six open-chain tetradentate thioether ligands are reported; two containing all aliphatic linkages, 1,2-bis(2-methylthioethylthio)ethane and 1,3-bis(2-methylthioethylthio)propane, and four containing both aliphatic and aromatic linkages, 1,2-bis (o-methylthiophenylthio)ethane. 1,3-bis(o-methyl-thiophenylthio)propane, 1,4-bis(o-methylthiophenyl-thio)butane, and cis-l,2-bis(o-methylthiophenylthio)-ethylene. The fragmentation patterns are illustrated and discussed with respect to differences between the ligands and the fragments derived from different ligands. The completely aliphatic ligands exhibit a complex fragmentation of α-cleavages with and without hydrogen atom migration; the formation of heterocyclic intermediates in those containing aromatic groups is dependent on the number of methylene groups in the aliphatic chain. An unusual feature of the latter type of ligand is the formation of ion m/e = 91 (C 7H 7) directly from m/e = 155 (C 7H 7S 2).

Theoretical study of hydrogen atom migration models in cyclic conformation of 1-pentene

Migration of hydrogen atoms in the cyclic conformation of 1-pentene is assumed. Four migration models are proposed and their relative stability studied by semiempirical methods. According to the results obtained by the MINDO/2 method all model structures assumed are more stable than the original cyclic conformation, whereas by CNDO/2 only one structure is relatively stable. From the results achieved the cyclic conformation of 1-pentene appears as a barrier structure behind which there are other relatively stable structures.

ChemInform Abstract: CNDO/2 AND INDO CALCULATIONS OF A REACTION PATHWAY FOR THE SIGMATROPIC (1,5)H‐SHIFT IN CYCLOPENTADIENE

CNDO/2 and INDO calculations have been carried out in order to construct a suitable model for the activated complex during the reaction. In this reaction model the migrating hydrogen atom moves along an edge of the cyclopentadiene ring. An analysis of this situation suggests a partial electron transfer from the migrating hydrogen to the nascent cyclopentadienyl system. This charge transfer is discussed in terms of aromaticity. The calculated activation enthalpies are 10 kcal/mole (CNDO/2) and 17 kcal/mole (INDO), whereas the experimental value is ca. 24 kcal/mole [1].

CNDO/2 and INDO calculations of a reaction pathway for the sigmatropic [1, 5] H-shift in cyclopentadiene

CNDO/2 and INDO calculations have been carried out in order to construct a suitable model for the activated complex during the reaction. In this reaction model the migrating hydrogen atom moves along an edge of the cyclopentadiene ring. An analysis of this situation suggests a partial electron transfer from the migrating hydrogen to the nascent cyclopentadienyl system. This charge transfer is discussed in terms of aromaticity. The calculated activation enthalpies are 10 kcal/mole (CNDO/2) and 17 kcal/mole (INDO), whereas the experimental value is ca. 24 kcal/mole [1].

The mass spectra of the trimethylsilyl derivatives of glycerophosphoric acids. Inter- and intra-molecular rearrangements of siliconium ions

The trimethylsilyl derivatives of several glycerophosphoric acids are taken as model compounds for an investigation of the fragmentation mechanisms producing the characteristic ions present in the mass spectra of trimethylsilyl derivatives of organic phosphates of this type. A considerable number of rearrangement ions formed by migration of hydrogen atoms and/or trimethylsilyl groups are observed. The presence of a fully substituted phosphonium system is thought to contribute to the stability of these ions. A close similarity between the migratory properties of hydrogen atoms and trimethylsilyl groups is apparent. The spectra of the trimethylsilyl derivatives of the diphosphates exhibit ions produced by the interaction and rearrangement of intact trimethylsilyl phosphate groups. Ion–molecule reactions involving the combination of a neutral molecular species with an abundant fragment ion containing a siliconium centre are also observed. These ion–molecule reactions provide additional information regarding the structure of some of the fragment ions.

Isomerization of vibrationally excited 3-methyl-1-buten-1-yl radicals via hydrogen atom migration. Quantum statistical weight effect

Isomerization of vibrationally excited 3-methyl-1-buten-1-yl radicals via hydrogen atom migration. Quantum statistical weight effect

Studies in mass spectrometry. Part XIV. The effect of conformation on the electron impact-induced fragmentation of adducts of p-benzoquinone and 1,1′-bicycloalkenyls

Electron impact-induced double hydrogen migration leading to the formation of ions a from adducts of p-benzoquinone and 1,1′-bicycloalken-1-yls has been found to be suppressed when the sizes of rings C and D of the adducts are increased. This effect is explained in terms of different distances between the migrating hydrogen atoms and the carbonyl groups, which are affected by the conformations of the central ring B and of rings C and D.

Studies in mass spectrometry—XV: Retro‐diels‐alder fragmentation and double hydrogen migration in some polycyclic diketones

AbstractIn contrast to adducts I of bi‐1‐cycloalken‐1‐yls and p‐benzoquinone, their reduction products II do not exhibit a double hydrogen migration from δ positions accompanying a ‘retro‐Diels‐Alder’ type fragmentation. An ordinary retro‐Diels‐Alder fragmentation was found to take place, with charge retention in the diene portion of the molecule. A double hydrogen migration has been detected in II leading to m/e 112 ion c2, which differed, however, from that in I in charge retention and in the origin of the migrating hydrogen atoms. Adducts III of di‐1‐cycloalken‐1‐yls and naphthoquinone behave similarly to II. They exhibit relatively low abundance ions a, however, due to a double hydrogen migration from δ positions, similarly to I. The origin of the migrating hydrogen atoms have been determined by deuterium labelling. Mechanistic suggestions are presented to explain the observed facts.

Mass spectral studies of metal chelates. VI. Mass spectra and appearance potentials of β-diketonates of copper(II)

The mass spectra of several copper complexes of β-diketones are discussed. The extent of fragmentation of the molecular ions depends upon the nature of the substituents on the chelate ring, being larger for electron-withdrawing, and smaller for electron-releasing substituents. In a number of fragmentations, migration of an aryl group to the copper atom occurs. Hydrogen atom migrations are also observed in some fragmentations. Attempts are made to correlate the appearance potentials of the molecular ions with acid dissociation constants of the β-diketones, formation constants and polarographic reduction potentials of the complexes, and Hammett σ constants of the substituents.