Crossed-beams studies of the dynamics of the H-atom abstraction reaction, O(3P) + CH4 → OH + CH3, at hyperthermal collision energies

The H-atom abstraction reaction, O((3)P) + CH(4) → OH + CH(3), has been studied at a hyperthermal collision energy of 64 kcal mol(-1) by two crossed-molecular-beams techniques. The OH products were detected with a rotatable mass spectrometer employing electron-impact ionization, and the CH(3) products were detected with the combination of resonance-enhanced multiphoton ionization (REMPI) and time-sliced ion velocity-map imaging. The OH products are mainly formed through a stripping mechanism, in which the reagent O atom approaches the CH(4) molecule at large impact parameters and the OH product is scattered in the forward direction: roughly the same direction as the reagent O atoms. Most of the available energy is partitioned into product translation. The dominance of the stripping mechanism is a unique feature of such H-atom abstraction reactions at hyperthermal collision energies. In the hyperthermal reaction of O((3)P) with CH(4), the H-atom abstraction reaction pathway accounts for 70% of the reactive collisions, while the H-atom elimination pathway to produce OCH(3) + H accounts for the other 30%.     

PHOTOCHEMISTRY OF trans-[Rh(BIS(DIPHENYLPHOSPHINO)ETHANE)2X2][PF6]: TRANSIENT ABSORBANCE KINETIC STUDIES OF METAL-HALOGEN BOND HOMOLYSIS

Abstract— Laser flash photolysis of trans -[Rh(dppe)₂X₂][PF₆] (X=Br and I; dppe=bis(diphenylphosphino)ethane) in CH₂Cl₂ or CH₃CN produces the d⁷ Rh(II) radicals, [Rh(dppe)₂X]⁺, and halogen atoms. The kinetics of the disappearance of [Rh(dppe)₂X]⁺ radicals in CH₂Cl₂ or CH₃CN were mixed order: H-atom abstraction from solvent to produce the rhodium hydrides, [RhH(dppe)₂X][PF₆], and Rh/X recombination. In the poor H-atom donor solvent, benzonitrile, Rh/Br recombination was observed to be uncomplicated by competing H-atom abstraction. The hydride complexes [RhH(dppe)²X][PF₆], formed by H-atom abstraction were completely characterized by ³¹P{¹H}-NMR, ¹H-NMR, and mass specrometry. Cyclohexene was used as an effective trap for photogenerated Br atoms and yielded bromocyclohexane and 3-bromocyclohexene in a relative yield, 1:9. The photochemical mechanism is discussed in light of the transient absorbance and trapping studies.     

Kinetics of self-decomposition and hydrogen atom transfer reactions of substituted phthalimide N-oxyl radicals in acetic acid

Kinetic data have been obtained for three distinct types of reactions of phthalimide N-oxyl radicals (PINO(.)) and N-hydroxyphthalimide (NHPI) derivatives. The first is the self-decomposition of PINO(.) which was found to follow second-order kinetics. In the self-decomposition of 4-methyl-N-hydroxyphthalimide (4-Me-NHPI), H-atom abstraction competes with self-decomposition in the presence of excess 4-Me-NHPI. The second set of reactions studied is hydrogen atom transfer from NHPI to PINO(.), e.g., PINO(.) + 4-Me-NHPI <=> NHPI + 4-Me-PINO(.). The substantial KIE, k(H)/k(D) = 11 for both forward and reverse reactions, supports the assignment of H-atom transfer rather than stepwise electron-proton transfer. These data were correlated with the Marcus cross relation for hydrogen-atom transfer, and good agreement between the experimental and the calculated rate constants was obtained. The third reaction studied is hydrogen abstraction by PINO(.) from p-xylene and toluene. The reaction becomes regularly slower as the ring substituent on PINO(.) is more electron donating. Analysis by the Hammett equation gave rho = 1.1 and 1.8 for the reactions of PINO(.) with p-xylene and toluene, respectively.     

Predictive studies of H-atom abstraction reactions by an iron(IV)-oxo corrole cation radical oxidant

Density functional theory calculations compare the reactivity of iron(IV)-oxo porphyrin and corrole cation radical species in H-atom abstraction reactions.     

H-atom abstraction reaction for organic substrates via mononuclear copper(II)-superoxo species as a model for DbetaM and PHM

Hydrogen atom abstraction reactions have been implicated in oxygenation reactions catalyzed by copper monooxygenases such as peptidylglycine alpha-hydroxylating monooxygenase (PHM) and dopamine beta-monooxygenase (DbetaM). We have investigated mononuclear copper(I) and copper(II) complexes with bis[(6-neopentylamino-2-pyridyl)methyl][(2-pyridyl)methyl]amine (BNPA) as functional models for these enzymes. The reaction of [Cu(II)(bnpa)]2+ with H2O2, affords a quasi-stable mononuclear copper(II)-hydroperoxo complex, [Cu(II)(bnpa)(OOH)]+ (4) which is stabilized by hydrophobic interactions and hydrogen bonds in the vicinity of the copper(II) ion. On the other hand, the reaction of [Cu(I)(bnpa)]+ (1) with O2 generates a trans-mu-1,2-peroxo dicopper(II) complex [Cu(II)2(bnpa)2(O2(2-]2+ (2). Interestingly, the same reactions carried out in the presence of exogenous substrates such as TEMPO-H, produce a mononuclear copper(II)-hydroperoxo complex 4. Under these conditions, the H-atom abstraction reaction proceeds via the mononuclear copper(II)-superoxo intermediate [Cu(II)(bnpa)(O2-)]+ (3), as confirmed from indirect observations using a spin trap reagent. Reactions with several substrates having different bond dissociation energies (BDE) indicate that, under our experimental conditions the H-atom abstraction reaction proceeds for substrates with a weak X-H bond (BDE < 72.6 kcal mol(-1)). These investigations indicate that the copper(II)-hydroperoxo complex is a useful tool for elucidation of H-atom abstraction reaction mechanisms for exogenous substrates. The useful functionality of the complex has been achieved via careful control of experimental conditions and the choice of appropriate ligands for the complex.     

PHOTOCHEMICAL REACTIONS OF AROMATIC KETONES WITH NUCLEIC ACIDS AND THEIR COMPONENTS I—. PURINE AND PYRIMIDINE BASES AND NUCLEOSIDES*.

Abstract— The photochemical reactions of benzophenone and acetophenone with purine and pyrimidine derivatives in aqueous solutions have been investigated by flash photolysis and steady-state experiments. Upon excitation of these two ketones in aqueous solutions, two transient species are observed: molecules in their triplet state and ketyl radicals. The triplet state lifetimes are 65 μsec for benzophenone and 125 μsec for acetophenone. The ketyl radicals disappear by a second order reaction, controlled by diffusion. In the presence of pyrimidine derivatives, the triplet state is quenched and the ketyl radical concentration is decreased without any change in its kinetics of disappearance. Ketone molecules in their triplet state react with purine derivatives leading to an increase in the yield of ketyl radicals due to H-atom abstraction from the purines. Steady-state experiments show that benzophenone and acetophenone irradiated in aqueous solution at wavelengths longer than 290 nm undergo photochemical reactions. The rate of these photochemical reactions is increased in the presence of pyrimidine derivatives and even more in the presence of purine derivatives. Following energy transfer from the triplet state of benzophenone to diketopyrimidines, cyclobutane dimers are formed. The energy transfer rate decreases in the order orotic acid > thymine > uracil. Benzophenone molecules in their triplet state can also react chemically with pyrimidine derivatives to give addition photoproducts. All these results are discussed with respect to photosensitized reactions in nucleic acids involving ketones as sensitizers.     

Reactivity of Diarylnitrenium Ions

Hydride abstraction from diarylamines with the trityl ion is explored in an attempt to generate a stable diarylnitrenium ion, Ar₂N⁺. Sequential H-atom abstraction reactions ensue. The first H-atom abstraction leads to intensely colored aminium radical cations, Ar₂NH^.+, some of which are quite stable. However, most undergo a second H-atom abstraction leading to ammonium ions, Ar₂NH₂⁺. In the absence of a ready source of H-atoms, a unique self-abstraction reaction occurs when Ar=Me₅C₆, leading to a novel iminium radical cation, Ar=N^.+Ar, which decays via a second self H-atom abstraction reaction to give a stable iminium ion, Ar=N⁺HAr. These products differ substantially from those derived via photochemically produced diarylnitrenium ions.     

Direct hydrogen-atom abstraction by activated bleomycin: an experimental and computational study

Bleomycin (BLM), a glycopeptide antibiotic chemotherapy agent, is capable of single- and double-strand DNA damage. Activated bleomycin (ABLM), a low-spin Fe(III)-OOH complex, is the last intermediate detected prior to DNA cleavage following hydrogen-atom abstraction from the C-4' of a deoxyribose sugar moiety. The mechanism of this C-H bond cleavage reaction and the nature of the active oxidizing species are still open issues. We have used kinetic measurements in combination with density functional calculations to study the reactivity of ABLM and the mechanism of the initial attack on DNA. Circular dichroism spectroscopy was used to directly monitor the kinetics of the ABLM reaction. These experiments yield a deuterium isotope effect, kH/kD approximately 3 for ABLM decay, indicating the involvement of a hydrogen atom in the rate-determining step. H-atom donors with relatively weak X-H bonds accelerate the reaction rate, establishing that ABLM is capable of hydrogen-atom abstraction. Density functional calculations were used to evaluate the two-dimensional potential energy surface for the direct hydrogen-atom abstraction reaction of the deoxyribose 4'-H by ABLM. The calculations confirm that ABLM is thermodynamically and kinetically competent for H-atom abstraction. The activation and reaction energies for this pathway are favored over both homolytic and heterolytic O-O bond cleavage. Direct H-atom abstraction by ABLM would generate a reactive Fe(IV)=O species, which would be capable of a second DNA strand cleavage, as observed in vivo. This study provides experimental and theoretical evidence for direct H-atom abstraction by ABLM and proposes an attractive mechanism for the role of ABLM in double-strand cleavage.     

Photoinduced Electron and H-atom Transfer Reactions of Xanthone by Laser Flash Photolysis

The property of the lowest excited triplet states of xanthone in acetonitrile was investigated using time-resolved laser °ash photolysis at 355 nm. The transient absorption spectra and the quenching rate constants(kq) of the excited xanthone with several amines were determined. Good correlation between lgkq and the driving force of the reactions suggests the electron transfer mechanism, except aniline and 3-nitroaniline (3-NO2-A) which showed energy transfer mechanism. With the appearance of ketyl radical, hydrogen atom transfer also happened between xanthone and dimethyl-p-toluidine, 3,5,N,N-tetramethylaniline, N,N-dimethylaniline, and triethylamine. Therefore, both electron transfer and H-atom transfer occured in these systems. Great discrepancies of kq values were discovered in H-atom abstraction reactions for alcohols and phenols, which can be explained by di?erent abstraction mechanisms. The quenching rate constants between xanthone and alcohols correlate well with the ?-C?H bonding energy of alcohols.     

Dynamics of H and D abstraction in the reaction of Cl atom with butane-1,1,1,4,4,4-d6

We report the primary (D-atom) and secondary (H-atom) abstraction dynamics of chlorine atom reaction with butane-1,1,1,4,4,4-d(6). The H- and D-atom abstraction channels were studied over a range of collision energies: 10.4 kcal mol(-1) and 12.9 kcal mol(-1); 5.2 kcal mol(-1) to 12.8 kcal mol(-1), respectively, using crossed molecular beam dc slice ion imaging techniques. Single photon ionization at 157 nm was used to probe the butyl radical products resulting from the H- and D-atom abstraction reactions. These two channels manifest distinct dynamics principally in the translational energy distributions, while the angular distributions are remarkably similar. The reduced translational energy distribution for the primary abstraction showed marked variation with collision energy in the backward direction, while the secondary abstraction showed this variation in the forward direction.     

Intramolecular hydrogen bonding and hydrogen atom abstraction in gas-phase aliphatic amine radical cations

The intramolecular hydrogen atom abstraction by the nitrogen atom in isolated aliphatic amine radical cations is examined experimentally and with composite high-level ab initio methods of the G3 family. The magnitude of the enthalpy barriers toward H-atom transfer varies with the shape and size of the cyclic transition state and with the degree of substitution at the nitrogen and carbon atoms involved. The lower barriers are found for 1,5- and 1,6-abstraction, for chairlike transition states, for abstraction reactions in ionized primary amines, and for abstraction of H from tertiary carbon atoms. In most cases, the internal energy required for 1,4-, 1,5-, and 1,6-hydrogen atom abstraction to occur is less than that required for gas-phase fragmentation by simple cleavage of C-C bonds, which explains why H-atom transfer can be reversible and result in extensive H/D exchange prior to the fragmentation of many low-energy deuterium labeled ionized amines. The H-atom transfer to nitrogen is exothermic for primary amine radical cations and endothermic for tertiary amines. It gives rise to a variety of distonic radical cations, and these may undergo further isomerization. The heat of formation of the gauche conformers of the gamma-, delta-, and epsilon-distonic isomers is up to 25 kJ mol(-1) lower than that of the corresponding trans forms, which is taken to reflect C-H-N hydrogen bonding between the protonated amino group and the alkyl radical site.     

A theoretical study of reactivity and regioselectivity in the hydroxylation of adamantane by ferrate(VI)

The conversion of adamantane to adamantanols mediated by ferrate (FeO(4)(2)(-)), monoprotonated ferrate (HFeO(4)(-)), and diprotonated ferrate (H(2)FeO(4)) is discussed with the hybrid B3LYP density functional theory (DFT) method. Diprotonated ferrate is the best mediator for the activation of the C-H bonds of adamantane via two reaction pathways, in which 1-adamantanol is formed by the abstraction of a tertiary hydrogen atom (3 degrees ) and 2-adamantanol by the abstraction of a secondary hydrogen atom (2 degrees ). Each reaction pathway is initiated by a C-H bond cleavage via an H-atom abstraction that leads to a radical intermediate, followed by a C-O bond formation via an oxygen rebound step to lead to an adamantanol complex. The activation energies for the C-H cleavage step are 6.9 kcal/mol in the 1-adamantanol pathway and 8.4 kcal/mol in the 2-adamantanol pathway, respectively, at the B3LYP/6-311++G level of theory, whereas those of the second reaction step corresponding to the rebound step are relatively small. Thus, the rate-determining step in the two pathways is the C-H bond dissociation step, which is relevant to the regioselectivity for adamantane hydroxylation. The relative rate constant (3 degrees )/(2 degrees ) for the competing H-atom abstraction reactions is calculated to be 9.30 at 75 degrees C, which is fully consistent with an experimental value of 10.1.     

Reactions of an aromatic σ,σ-biradical with amino acids and dipeptides in the gas phase

Gas-phase reactivity of a positively charged aromatic σ,σ-biradical (N-methyl-6,8-didehydroquinolinium) was examined toward six aliphatic amino acids and 15 dipeptides by using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR) and laser-induced acoustic desorption (LIAD). While previous studies have revealed that H-atom and NH₂ abstractions dominate the reactions of related monoradicals with aliphatic amino acids and small peptides, several additional, unprecedented reaction pathways were observed for the reactions of the biradical. For amino acids, these are 2H-atom abstraction, H₂O abstraction, addition — CO₂, addition — HCOOH, and formation of a stable adduct. The biradical reacts with aliphatic dipeptides similarly as with aliphatic amino acids, but undergoes also one additional reaction pathway, addition/C-terminal amino acid elimination (addition — CO — NHCHR_C). These reactions are initiated by H-atom abstraction by the biradical from the amino acid or peptide, or nucleophilic addition of an NH₂ or a HO group of the amino acid or peptide at the radical site at C-6 in the biradical. Reactions of the unquenched C-8 radical site then yield the products not observed for related monoradicals. The biradical reacts with aromatic dipeptides with an aromatic ring in N-terminus (i.e., Tyr-Leu, Phe-Val, and Phe-Pro) similarly as with aliphatic dipeptides. However, for those aromatic dipeptides that contain an aromatic ring in the C-terminus (i.e., Leu-Tyr and Ala-Phe), one additional pathway, addition/N-terminal amino acid elimination (addition — CO — NHCHR_N), was observed. This reaction is likely initiated by radical addition of the biradical at the aromatic ring in the C-terminus. Related monoradicals add to aromatic amino acids and small peptides, which is followed by Cα-Cβ bond cleavage, resulting in side-chain abstraction by the radical. For biradicals, with one unquenched radical site after the initial addition, the reaction ultimately results in the loss of the N-terminal amino acid. Similar to monoradicals, the C-S bond in amino acids and dipeptides was found to be especially susceptible to biradical attack.     

Ortho effect in the Bergman cyclization: interception of p-benzyne intermediate by intramolecular hydrogen abstraction

Intramolecular hydrogen atom (H-atom) abstraction from the o-OCH3 group effectively intercepts the p-benzyne intermediate in the Bergman cycloaromatization of 2,3-diethynyl-1-methoxybenzene (1) before this intermediate undergoes either retro-Bergman ring opening or external H-atom abstraction. This process leads to the formation of a new diradical and renders the cyclization step essentially irreversible. Chemical and kinetic consequences of this phenomenon were investigated through the combination of computational and experimental studies.     

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

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.     

Oxidation of C-H bonds by [(bpy)2(py)RuIVO]2+ occurs by hydrogen atom abstraction

Anaerobic oxidations of 9,10-dihydroanthracene (DHA), xanthene, and fluorene by [(bpy)(2)(py)Ru(IV)O](2+) in acetonitrile solution give mixtures of products including oxygenated and non-oxygenated compounds. The products include those formed by organic radical dimerization, such as 9,9'-bixanthene, as well as by oxygen-atom transfer (e.g., xanthone). The kinetics of these reactions have been measured. The kinetic isotope effect for oxidation of DHA vs DHA-d(4) gives k(H)/k(D) > or = 35 +/- 1. The data indicate a mechanism of initial hydrogen-atom abstraction forming radicals that dimerize, disproportionate and are trapped by the oxidant. This mechanism also appears to apply to the oxidations of toluene, ethylbenzene, cumene, indene, and cyclohexene. The rate constants for H-atom abstraction from these substrates correlate well with the strength of the C-H bond that is cleaved. Rate constants for abstraction from DHA and toluene also correlate with those for oxygen radicals and other oxidants. The rate constant for H-atom transfer from toluene to [(bpy)(2)(py)Ru(IV)O](2+) appears to be close to that predicted by the Marcus cross relation, using a tentative rate constant for hydrogen atom self-exchange between [(bpy)(2)(py)Ru(III)OH](2+) and [(bpy)(2)(py)Ru(IV)O](2+).     

Reactions of methyl radicals with poly(vinylacetophenone) during photolysis

The photochemistry of isotopically labelled poly(vinylacetophenone) (PVAP) has been investigated, with a view to determining the point of H-atom abstraction from the polymer. Poly(α-d-vinylacetophenone) (DPVAP) and poly β-β dideuterovinyl(acetophenone) (D₂PVAP) were irradiated under high vacuum with long wave u.v. radiation (⩾ 300 nm). From the nature of the gaseous products, it was confirmed that both underwent initial photo-decomposition by a Norrish Type I reaction. Rates of abstraction reactions by CH₃ and CH₃CO radicals from DPVAP were subject to an isotope effect of 8.3; the corresponding rates for D₂PVAP were about 85% of those observed for PVAP. In addition, no significant yields of deuterated products were obtained from D₂PVAP. It is concluded that abstractions occur preferentially at the α-C-atom, in line with predictions based on the energetics of abstraction reactions. Molecular weight increases (due to cross-linking which involves interactions of macro-radicals generated by abstraction reactions) confirm the involvement of the α-C-atom.     

Theoretical investigation on the kinetics and thermochemisty of H-atom abstraction reactions of 2-chloroethyl methyl ether (CH<Subscript>3</Subscript>OCH<Subscript>2</Subscript>CH<Subscript>2</Subscript>Cl) with OH radical at 298 K

Kinetics and thermochemistry of the H-atom abstraction reaction of CH₃OCH₂CH₂Cl with OH radical have been carried out using dual level of methods. Initially, geometry optimization and frequency calculations are performed at M06-2X/6-31+G(d, p) level of theory, and energetic calculations are further refined using CCSD(T)/6-311++G(d, p) level of theory in order to characterized all stationary points on potential energy surface (PES). The result shows that H-atom abstraction from –OCH₂ site of CH₃OCH₂CH₂Cl is dominant path. The rate constants are calculated using canonical transition state theory at 298 K, which are found to be in good agreement with the experimental data. We have presented the standard enthalpies of formation for CH₃OCH₂CH₂Cl and the radicals generated during the H-atom abstraction using group-balanced isodesmic reactions scheme. The atmospheric lifetime of title molecule is also calculated.     

Oxygen activation by the noncoupled binuclear copper site in peptidylglycine alpha-hydroxylating monooxygenase. Reaction mechanism and role of the noncoupled nature of the active site

Reaction thermodynamics and potential energy surfaces are calculated using density functional methods to investigate possible reactive Cu/O(2) species for H-atom abstraction in peptidylglycine alpha-hydroxylating monooxygenase (PHM), which has a noncoupled binuclear Cu active site. Two possible mononuclear Cu/O(2) species have been evaluated, the 2-electron reduced Cu(II)(M)-OOH intermediate and the 1-electron reduced side-on Cu(II)(M)-superoxo intermediate, which could form with comparable thermodynamics at the catalytic Cu(M) site. The substrate H-atom abstraction reaction by the Cu(II)(M)-OOH intermediate is found to be thermodynamically accessible due to the contribution of the methionine ligand, but with a high activation barrier ( approximately 37 kcal/mol, at a 3.0-A active site/substrate distance), arguing against the Cu(II)(M)-OOH species as the reactive Cu/O(2) intermediate in PHM. In contrast, H-atom abstraction from substrate by the side-on Cu(II)(M)-superoxo intermediate is a nearly isoenergetic process with a low reaction barrier at a comparable active site/substrate distance ( approximately 14 kcal/mol), suggesting that side-on Cu(II)(M)-superoxo is the reactive species in PHM. The differential reactivities of the Cu(II)(M)-OOH and Cu(II)(M)-superoxo species correlate to their different frontier molecular orbitals involved in the H-atom abstraction reaction. After the H-atom abstraction, a reasonable pathway for substrate hydroxylation involves a "water-assisted" direct OH transfer to the substrate radical, which generates a high-energy Cu(II)(M)-oxyl species. This provides the necessary driving force for intramolecular electron transfer from the Cu(H) site to complete the reaction in PHM. The differential reactivity pattern between the Cu(II)(M)-OOH and Cu(II)(M)-superoxo intermediates provides insight into the role of the noncoupled nature of PHM and dopamine beta-monooxygenase active sites, as compared to the coupled binuclear Cu active sites in hemocyanin, tyrosinase, and catechol oxidase, in O(2) activation.     

Addition of radicals to quinones—II. ESR spectra of radicals derived from fluoranil

In systems consisting of thermally decomposing benzoyl peroxide in alkylbenzenes, fluoranil behaves as a selective spin trap. As a result of charge transfer (CT) complex formation between alkylbenzenes and fluoranil, the latter traps secondary radicals formed via H-atom abstraction by phenyl radicals from the alkyl group of the aromatic solvents. Weaker CT interaction leads to the trapping of phenyl radicals. The kinetic isotope effect in H-atom abstraction leads to the same result.