Hydrogen Atom Abstraction Reactions Research Articles

Hydrogen atom abstraction reaction by triplet pyrazine at low temperature

Hydrogen atom abstraction reaction by triplet pyrazine at low temperature

A Molecular Orbital Study on the Partial Reactivity of Hydrogen of Various Amino Acids in the Abstraction Reaction by Hydroxyl Radical

Abstract The reactivities of hydrogen atoms of aliphatic amino acids toward hydroxyl radicals in the hydrogen atom abstraction reaction have been studied in terms of the energy required for stretching the C–H bond which is attacked. The energy required for stretching the C–H bond by 0.5 Å from the equilibrium bond length has been calculated by the INDO method and compared with the partial rate constants experimentally assigned to the hydrogen atoms of aliphatic amino acids. The energy required has been correlated only with the partial rate constants of the hydrogen atoms on the β-carbon atom. For the hydrogen atoms on the α-carbon atom, although data was insufficient a similar situation has been observed. The reactivities of the hydrogen atoms on the γ and δ-carbon atom failed to correlate with the stretching energy. The reasons are discussed in terms of the effect raised by the configuration of amino acid molecules in solution. The stretching energy has also been found to be consistent with the electrophilicity observed in the hydrogen abstraction reaction by hydroxyl radical.

Selective Hydrogen Atom Abstraction by H Atoms in Radiolysis and Photolysis at 77 K: Decane-d22, Tetramethylsilane, and Xenon Matrices

Abstract The selective formation of HD and solute radicals in the radiolysis of n-C10D22 doped with n-C10H22 at 77 K is explained quantitatively by the mechanism of a selective hydrogen-atom-abstraction reaction by D atoms. The selective hydrogen-atom abstraction by H atoms is found also in tetramethylsilane and xenon matrices as well as in the alkane matrix. The rate constants of the abstraction reaction by H atoms are compared with the bond energies of the solvent and solute molecules. The bond energy of the solvent molecule is always higher than that of the solute molecule. Even if the difference in the bond energy between the solvent and the solute is very small, the k(solute)/k(solvent) ratio is quite large. The k(solute)/k(neo-C5H12) ratios for different solutes in a neopentane matrix are not influenced by the differences in the bond energy among the solutes. Since an isotope effect on the selective hydrogen-atom abstraction reaction is small, the effect may diminish the possibility of the quantum mechanical tunneling abstraction by thermal H atoms. The energy loss of hot H atoms in a Xe matrix is discussed.

Hydrogen atom abstraction reaction by H atoms in γ-radiolysis and U.V.-photolysis of alkane mixtures at 4K

Hydrogen atom abstraction reaction by H atoms in the irradiated alkane mixtures has been studied at 4 K by ESR spectroscopy. When H atoms are produced by the radiolysis of neopentane, 2,2,3,3-tetramethylbutane, or isobutane at 4 K, they react promptly with the neighboring solvent molecules, though they react selectively with the solute alkane at 77 K. When H atoms are produced by the photolysis or by the dissociative electron attachment of HI in neo-C 5H 12 matrix, they react selectively with the solute alkane at 4 K as well as at 77 K. This suggests that the H atoms produced by the photolysis of HI at 4 K migrate a longer distance than those produced by the radiolysis of the solvent alkane at 4 K. Mechanisms of H atom abstraction reaction are discussed.

Selective Hydrogen Atom Abstraction by Hydrogen Atoms in Photolysis and Radiolysis of Alkane Mixtures at 77 K

Abstract A selective hydrogen atom abstraction reaction by H atoms, which are produced at 77K by radiolysis of alkane or photolysis of hydrogen halides, has been found in isobutane, 2,2,3,3-tetramethylbutane (TMB), and cyclopropane matrices as well as in the neopentane matrix. The selective hydrogen atom abstraction reaction is caused by H atoms which have initial kinetic energies in the range from 15 to 67 kcal/mol. The reaction is caused also by D atoms. The competitive reaction between c-C6H12 and HI for H atoms has been studied in the radiolysis and photolysis of neo-C5H12–c-C6H12–HI mixture at 77K. The rate constants of these reactions in the neopentane matrix are quite different from those of a thermal H atom reaction, but similar to those of a hot H atom reaction. The importance of the selective hydrogen atom abstraction reaction by H atoms is pointed out in the radical formation in the radiolysis of pure TMB at 77K.

Minimum energy path equation for A+BC→AB+C type hydrogen atom abstraction reactions

The equation L1XAB+ln(L1XAB)=−L2XBC−ln(L2XBC) is suggested to predict the minimum energy path for A+BC→AB+C type reactions. X=R−Ro, where Ro denotes the equilibrium bond length. Parameters L1 and L2 are assumed to depend on the properties of the bonds formed and cleaved, respectively.

ICR studies of some hydrogen atom abstraction reactions: X+ + H2 → XH+ + H

Ion cyclotron resonance methods are used to identify and to measure the rate constants for the abstraction of a hydrogen atom from H2 by CH+, CH2+, CH4+, N+, NH+, NH2+, NH3+, O+, OH+, H2O+, CO+, N2+, C2+, and C2H+ ions. Although in most cases hydrogen atom abstraction is the only available exothermic pathway for these reactions at thermal energies, the rate constants measured show that except for O+, CO+, and N2+, a large fraction of collisions between these ions and H2 are not reactive. The rate constants measured range from a low of (3±1) × 10−13 cm3/sec for the NH3+−H2 reaction to (1.73±0.04) × 10−9 cm3/sec for the N2+−H2 reaction. These values compare to the Langevin value of about 1.5 × 10−9 cm3/sec for collisions between these ions and H2. An examination was also made for possible thermoneutral hydrogen atom exchange reactions for those ions which do not react with H2 (CH5+, CH3+, NH4+, H3O+, H2S+, H3S+). The only exchange reaction observed was for collisions between CD3+ ions and H2, for which a rate constant of (5.1±0.5) × 10−10 cm3/sec was measured.

INTERMEDIATES IN THE PHOTOCHEMICAL REACTIONS OF CAMPHORQUINONE WITH ALIPHATIC ALDEHYDES

Abstract The dynamic aspects of photochemical reactions of camphorquinone with aliphatic aldehydes were investigated by means of CIDNP. Besides that the two types of stable products (1)–(3) could be produced through ionic rearrangement of unstable intermediates (6), (7) arose from radical pair (5), the reverse hydrogen atom abstraction reaction was found to occur between the two radicals resulted from the reaction of photo-excited camphorquinone and aldehyde.

Effect of Phase on Hot Hydrogen Atom Abstraction Reaction with Solid Isobutane as Studied by Electron Spin Resonance Spectroscopy

Abstract The effect of phase on the hot-hydrogen-atom abstraction reaction with isobutane at 77°K has been studied by electron spin resonance spectroscopy in the photolysis and the radiolysis of solid isobutane containing hydrogen iodide. While only the isobutyl radical is formed by the UV-illumination of isobutane–hydrogen iodide systems in the polycrystalline state, the tertiary butyl radical, accompanied with a small amount of isobutyl radicals, is produced from isobutane-methylcyclohexane-hydrogen iodide systems in the glassy state. The hot hydrogen atom produced by dissociative electron attachment to hydrogen iodide abstracts the tertiary hydrogen atom from isobutane at 77°K, independent of whether the matrix is polycrystalline or glassy. It appears that the H-abstraction reaction from isobutane by hot hydrogen atoms depends upon the energy of the hot hydrogen atoms.

Some hydrogen atom abstraction reactions of CF2H and CFH2 radicals, and the CH bond dissociation energy in CF2H2

AbstractBy photolyzing (CF2H)2CO and (CFH2)2CO the hydrogen atom abstraction reactions of CF2H radicals with (CF2H)2CO, H2, D2, CH4, C2H6, nC4H10 and isoC4H10, and the reactions of CFH2 radicals with (CFH2)2CO and nC4H10, have been studied. Arrhenius parameters for these reactions are compared with related systems. From a knowledge of the activation energies for the forward and reverse reactions a value of the bond dissociation energy, D(CF2HH) = 97.4 ± 1.3 kcal mole−1 at a mean temperature of 543°K is equation image obtained. This value is subject to much uncertainty due to possible compensation effects in the Arrhenius parameters. These effects are discussed for this and the other reactions, and the data suggest that D(CF2HH) is approximately 100 kcal mole−1, and that D(CFH2H) is very similar. Other literature data tend to confirm these approximate values.

Hydrogen Formation in the Radiolysis of Liquid Olefins. The Role of Hot Hydrogen Atoms and Superexcited States

Abstract The γ-radiolysis of liquid olefins was undertaken to obtain reliable evidence for a possible role of hot hydrogen atoms and superexcited states in radiation chemistry. The contribution of thermal hydrogen atoms may reasonably be excluded in this system. The effect of various electron or positive-ion scavengers on hydrogen formation in the radiolyses of liquid propylene and ethylene has been examined. The hydrogen formation is not appreciably affected by these additives, which indicates ionic species may not play an important role in the hydrogen formation. The liquidphase radiolyses of C3H6-C3D6 and C2H4-C2D4 mixtures have also been investigated and compared with those previously reported of C4H8-l - C4D8-l and trans-C4H8 - 2-trans-C4D8-2 mixtures. The data indicate that the hydrogen from the olefins except ethylene may be formed for the most part by bimolecular processes. A large part of the hydrogen from liquid ethylene may be interpreted in terms of the molecular detachment of a hydrogen molecule, while from the other olefins it may be interpreted in terms of the hydrogen-atom abstraction reaction of hot hydrogen atoms formed by direct excitation which must at least partially involve superexcitation. To attempt quantitative substantiation of the above viewpoint a simple calculation has been made based on the so-called optical approximation. Calculated yields of the hydrogen formation are apparently in good agreement with observed ones.

Thermal-energy ion-neutral reaction rates. 7. Some hydrogen atom abstraction reactions: F C Fehsenfeld et al,J chem phys, 46 (7), 1 st April 1967, 2802–2808

Thermal-energy ion-neutral reaction rates. 7. Some hydrogen atom abstraction reactions: F C Fehsenfeld et al,J chem phys, 46 (7), 1 st April 1967, 2802–2808

Ion—Molecule Reactions by a Photoionization Mass Spectrometer. I. Propylene and 1,3-Butadiene

Ion—molecule reactions in propylene and 1,3-butadiene have been studied using a mass spectrometer which utilizes photoionization as an ion source instead of electron impact. The light source used was a low-pressure hydrogen-discharge lamp with a LiF window (substantially Lyman alpha, 1216 Å). It has been shown that this method has two major advantages in the study of ion—molecule reactions compared with the electron-impact method; one is the lack of unimolecular fragmentation of the parent ion and the other the absence of hot filament, which enables one to get high pressure without suffering pyrolysis. Five reactions of C3H6+ with C3H6 and eight reactions of C4H6+ with C4H6 have been identified and the cross sections determined for these reactions. In propylene, the reaction leading to the formation of C3H7+ was found to be of the largest cross section and, although cross section was small, the reaction forming C3H3+ was found as a new reaction. It was shown that C3H6+ could not be the precursor of secondary C4H6+ in propylene, criticizing the Fuchs assignment. In 1,3-butadiene, three new reactions have been found; the reaction leading to the formation of C3H3+, the hydrogen-atom abstraction reaction just as in propylene, and the parent-minus-one reaction. The first one was of the largest cross section in all reactions in 1,3-butadiene. This reaction has been discussed in connection with the structure and the heat of formation of C3H3+.

On the Rate of the Hydrogen Atom Abstraction Reaction in Liquid Hexane1

On the Rate of the Hydrogen Atom Abstraction Reaction in Liquid Hexane¹

Hydrogen and deuterium atom abstraction from trideuteromethyl mercaptan by methyl radicals. Reactivity of methylthio radicals

The reaction of methyl radicals with trideuteromethyl mercaptan has been studied in the temperature range 130–200°C in order to determine the rate at which hydrogen and deuterium atom abstraction occurs. Abstraction of the hydrogen atom is facile, the S—H bond being broken about 120 times faster than is the C—D bond. An unusual feature of the reaction is the abstraction of the mercapto group from the substrate by methyl radicals. Arrhenius parameters (based on a value of 1013.34 mole–1 cm3 sec–1 for the rate of recombination of methyl radicals) have been measured for the reactions:[graphic omitted] Using the relationships 1n (A1/A–1)=ΔS/R and ΔH=E1–E–1, Arrhenius parameters have been calculated for the reaction of methylthio radicals with methane. It appears that methylthio radicals do not readily abstract hydrogen atoms from methane and it is suggested that radical-radical reactions are likely to be a more important feature of their chemistry than hydrogen atom abstraction reactions.

Hydrogen Atom Abstraction Reactions by Cyanide Ion-Radicals

Ion-molecule reactions of low energy (0–0.15 ev) CHN+ and CH3N+ are shown conclusively to follow a hydrogen atom abstraction mechanism of the type, XH++YH→XH2++Y. These reactions are presumably bimolecular being first order with respect to each reactant over the pressure range 10—7—10—3 mm Hg. The rates of these reactions are markedly influenced by the molecular structure of both the cyanide ion and the hydrogen donor (YH). Ion-molecule reactions of this type are similar to the hydrogen atom abstraction processes of free radicals, but differ in having no appreciable temperature coefficients (in the range of 100–215°C), and in having much larger rate constants. On the basis of this study, chemical species having unpaired electrons whether charged (ion-radicals) or uncharged (free radicals) are shown to undergo similar types of reactions. Possible implications of these observations to the field of radiation chemistry are discussed.