Showing papers on "Hydrogen atom abstraction published in 2002"
TL;DR: It is found that the rate-limiting step for this decomposition pathway is the abstraction of hydroxyl hydrogen from methanol, and stable intermediates and transition states are found to obey gas-phase coordination and bond order rules on the Pt(111) surface.
Abstract: A periodic, self-consistent, Density Functional Theory study of methanol decomposition on Pt(111) is presented. The thermochemistry and activation energy barriers for all the elementary steps, starting with O[bond]H scission and proceeding via sequential hydrogen abstraction from the resulting methoxy intermediate, are presented here. The minimum energy path is represented by a one-dimensional potential energy surface connecting methanol with its final decomposition products, CO and hydrogen gas. It is found that the rate-limiting step for this decomposition pathway is the abstraction of hydroxyl hydrogen from methanol. CO is clearly identified as a strong thermodynamic sink in the reaction pathway while the methoxy, formaldehyde, and formyl intermediates are found to have low barriers to decomposition, leading to very short lifetimes for these intermediates. Stable intermediates and transition states are found to obey gas-phase coordination and bond order rules on the Pt(111) surface.
325 citations
TL;DR: The preference for phenxyl/phenol to occur by PCET while methoxyl/methanol exchange occurs by HAT is traced to the greater pi donating ability of phenyl over methyl, which results in greater electron density on the oxygens in the PCET transition structure for phenoxyl /phenol.
Abstract: Degenerate hydrogen atom exchange reactions have been studied using calculations, based on density functional theory (DFT), for (i) benzyl radical plus toluene, (ii) phenoxyl radical plus phenol, and (iii) methoxyl radical plus methanol. The first and third reactions occur via hydrogen atom transfer (HAT) mechanisms. The transition structure (TS) for benzyl/toluene hydrogen exchange has C(2)(h)() symmetry and corresponds to the approach of the 2p-pi orbital on the benzylic carbon of the radical to a benzylic hydrogen of toluene. In this TS, and in the similar C(2) TS for methoxyl/methanol hydrogen exchange, the SOMO has significant density in atomic orbitals that lie along the C-H vectors in the former reaction and nearly along the O-H vectors in the latter. In contrast, the SOMO at the phenoxyl/phenol TS is a pi symmetry orbital within each of the C(6)H(5)O units, involving 2p atomic orbitals on the oxygen atoms that are essentially orthogonal to the O.H.O vector. The transferring hydrogen in this reaction is a proton that is part of a typical hydrogen bond, involving a sigma lone pair on the oxygen of the phenoxyl radical and the O-H bond of phenol. Because the proton is transferred between oxygen sigma orbitals, and the electron is transferred between oxygen pi orbitals, this reaction should be described as a proton-coupled electron transfer (PCET). The PCET mechanism requires the formation of a hydrogen bond, and so is not available for benzyl/toluene exchange. The preference for phenoxyl/phenol to occur by PCET while methoxyl/methanol exchange occurs by HAT is traced to the greater pi donating ability of phenyl over methyl. This results in greater electron density on the oxygens in the PCET transition structure for phenoxyl/phenol, as compared to the PCET hilltop for methoxyl/methanol, and the greater electron density on the oxygens selectively stabilizes the phenoxyl/phenol TS by providing a larger binding energy of the transferring proton.
312 citations
TL;DR: In this article, the role of base is to generate a highly active dihydride catalyst RuH 2 (PPh 3 ) 3, and the mechanism of Ru-, Rh-, and Ir-catalyzed hydrogen transfer was probed by using α-deuterated alcohol as hydrogen donor and measuring the amount of deuterium transferred to the keto carbon of the hydrogen acceptor.
297 citations
TL;DR: In this article, the grafting of a number of vinyl monomers to polycarbon substrates is described in terms of the proposed mechanisms, and the recently developed application of living free radical polymerization to the preparation of block copolymers is discussed.
197 citations
TL;DR: Surface peroxide sites are found to be slightly less favorable for H-abstraction from methane than the O(s)(-) species, and a possible mechanism for oxidative coupling of methane over La(2)O(3)(001) involving surface peroxides as the active oxygen source is suggested.
Abstract: Results of gradient-corrected periodic density functional theory calculations are reported for hydrogen abstraction from methane at , , , point defect, and Sr2+-doped surface sites on La2O3(001). The results show that the anionic species is the most active surface oxygen site. The overall reaction energy to activate methane at an site to form a surface hydroxyl group and gas-phase •CH3 radical is 8.2 kcal/mol, with an activation barrier of 10.1 kcal/mol. The binding energy of hydrogen at an site is −102 kcal/mol. An oxygen site with similar activity can be generated by doping strontium into the oxide by a direct Sr2+/La3+ exchange at the surface. The O--like nature of the surface site is reflected in a calculated hydrogen binding energy of −109.7 kcal/mol. Calculations indicate that surface peroxide ( ) sites can be generated by adsorption of O2 at surface oxygen vacancies, as well as by dissociative adsorption of O2 across the closed-shell oxide surface of La2O3(001). The overall reaction energy and appa...
197 citations
TL;DR: In this article, self-consistent periodic DFT-GGA calculations are used to investigate the methanol decomposition pathway on both equilibrium and stretched Cu(111) surfaces.
187 citations
TL;DR: In this article, the surface chain reaction of styrene with hydrogen-terminated Si(111), H−Si(111) initiated at isolated surface dangling bonds is reported, which suggests that the key radical chain propagation step of hydrogen atom abstraction from the silicon surface for the proposed mechanism of reaction of liquid phase terminally unsaturated organic molecules with hydrogenterminated silicon under free-radical conditions readily occurs.
Abstract: We report the surface chain reaction of styrene with hydrogen-terminated Si(111), H−Si(111) initiated at isolated surface dangling bonds. Electrons from a scanning tunneling microscope in ultrahigh vacuum are used to create surface isolated dangling bonds on H−Si(111) prepared by aqueous ammonium fluoride etch. Exposure of dangling bonds on an otherwise hydrogen terminated surface to as little as 5 langmuirs of styrene leads to the formation of compact islands containing multiple styrene adsorbates bonded to the surface through individual C−Si bonds. From this observation we conclude that these adsorbate islands are the result of a surface chain reaction of styrene with H−Si(111). This result suggests that the key radical-chain propagation step of hydrogen atom abstraction from the silicon surface for the proposed mechanism of reaction of liquid phase terminally unsaturated organic molecules with hydrogen-terminated silicon under free-radical conditions readily occurs.
156 citations
TL;DR: Results point to the reversible generation of a 5'-deoxyadenosyl radical intermediate, which reacts directly with the DNA lesion to initiate a radical-mediated beta-scission in a novel radical-based mechanism for the repair of UV-induced DNA damage.
Abstract: Spore photoproduct (SP) lyase, which catalyzes the direct reversal of SP (5-thyminyl-5,6-dihydrothymine) to thymine monomers, is the only identified nonphotoactivatable pyrimidine dimer lyase. Unlike DNA photolyase, SP lyase does not contain a flavin cofactor and does not require light for activation. Instead, preliminary studies point to the presence of an iron−sulfur cluster in SP lyase and the requirement for S-adenosylmethionine (AdoMet) for catalytic activity, suggesting that SP lyase belongs to the growing group of iron−sulfur cluster and AdoMet-dependent radical enzymes. Here we provide evidence for the role of AdoMet as a reversible deoxyadenosyl radical generator, which initiates repair by hydrogen atom abstraction from C-6 of SP. Reaction of 6-3H-SP, but not methyl-3H-SP, with SP lyase and AdoMet results in transfer of 3H to AdoMet, while no tritiated 5‘-deoxyadenosine is observed. When 5‘-tritiated AdoMet is used in the reaction with unlabeled SP, transfer of 3H into the repaired thymine monome...
125 citations
TL;DR: The resulting model extends the predictive capacity of the previous aliphatic hydroxylation model to include the second most important group of oxidations, aromatic hydroxymation, and has an error of about 0.7 kcal/mol.
Abstract: Experimental observations suggest that electronic characteristics play a role in the rates of substrate oxidation for cytochrome P450 enzymes. For example, the tendency for oxidation of a certain functional group generally follows the relative stability of the radicals that are formed (e.g., N-dealkylation > O-dealkylation > 2 degrees carbon oxidation > 1 degree carbon oxidation). In addition, results show that useful correlations between the rates of product formation can be developed using electronic models. In this article, we attempt to determine whether a combined computational model for aromatic and aliphatic hydroxylation can be developed. Toward this goal, we used a combination of experimental data and semiempirical molecular orbital calculations to predicted activation energies for aromatic and aliphatic hydroxylation. The resulting model extends the predictive capacity of our previous aliphatic hydroxylation model to include the second most important group of oxidations, aromatic hydroxylation. The combined model can account for about 83% of the variance in the data for the 20 compounds in the training set and has an error of about 0.7 kcal/mol.
119 citations
TL;DR: In this paper, the degradation of polystyrene was modeled at the mechanistic level by developing differential equations describing the evolution of the moments of structurally distinct polymer species, and the conversion among the species was described using typical free radical reaction types.
Abstract: The degradation of polystyrene was modeled at the mechanistic level by developing differential equations describing the evolution of the moments of structurally distinct polymer species. This work extends our previous modeling work1 by incorporating chain-length-dependent rate parameters, tracking branched species more explicitly, using rate parameters primarily from the literature, and comparing the model results to extensive experimental data on the degradation of polymers of different molecular weights and at different temperatures. Unique polymer groups were devised that allowed the necessary polymeric features for capturing the degradation chemistry to be tracked, while maintaining a manageable model size. The conversion among the species was described using typical free radical reaction types, including hydrogen abstraction, midchain β-scission, end-chain β-scission, 1,5-hydrogen transfer, 1,3-hydrogen transfer, radical addition, bond fission, radical recombination, and disproportionation. The model...
116 citations
TL;DR: In this article, a complex mechanism involving the formation of a stable pre-reactive complex is proposed and the temperature dependence of the rate coefficients is studied over the temperature range of 290-500 K, using conventional transition state theory (CTST).
Abstract: CCSD(T)//BHandHLYP/6-311G(d,p) calculations have been performed to study the OH hydrogen abstraction reaction from C1–C4 aliphatic alcohols. A complex mechanism involving the formation of a stable pre-reactive complex is proposed and the temperature dependence of the rate coefficients is studied over the temperature range of 290–500 K, using conventional transition state theory (CTST). Excellent agreement between calculated and experimental k at 298 K has been obtained. Arrhenius expressions are proposed for 1-propanol and 1-butanol, k1-Prop = 3.06 × 10−12exp(140/T) and k1-But = 2.14 × 10−12exp(440/T) cm3 molecule−1·s−1, respectively. The rate coefficient for the formation of the alpha radical is found significantly larger than that of the competing channels for C1–C3 alcohols. The finding that at room temperature the rate constant of 1-butanolγ is the largest one supports some of the previous experimental results.
TL;DR: A comparison of hydride, hydrogen atom, and proton-coupled electron transfer reactions is presented and theoretical descriptions and simulation methodology for all three types of reactions are presented, as well as experimentally relevant applications to hydrides in enzymes and PCET in solution.
Abstract: A comparison of hydride, hydrogen atom, and proton-coupled electron transfer reactions is presented. Herein, hydride and hydrogen atom transfer refer to reactions in which the electrons and protons transfer between the same donor and acceptor, while proton-coupled electron transfer (PCET) refers to reactions in which the electrons and protons transfer between different centers. Within these definitions, hydride and hydrogen atom transfer reactions are typically electronically adiabatic, hence evolving on a single electronic surface. In contrast, PCET reactions are often electronically nonadiabatic since the electron transfers a longer distance through a proton transfer interface. For all three types of reactions, solute reorganization is important, particularly the hydrogen donor ± acceptor mode. Solvent reorganization is critical for hydride transfer and PCET, which involve significant solute charge redistribution, but not for hydrogen atom transfer. Theoretical descriptions and simulation methodology for all three types of reactions are presented, as well as experimentally relevant applications to hydride transfer in enzymes and PCET in solution.
TL;DR: The results are explained through hydrogen abstraction by the phthalimide-N-oxyl (PINO) radical, whose reactivity with benzyl derivatives is governed by polar effects, so that benzylamides are much more reactive than the corresponding aldehydes.
Abstract: The oxidation of N-alkylamides by O2, catalyzed by N-hydroxyphthalimide (NHPI) and Co(II) salt, leads under mild conditions to carbonyl derivatives (aldehydes, ketones, carboxylic acids, imides) whose distribution depends on the nature of the alkyl group and on the reaction conditions. Primary N-benzylamides lead to imides and aromatic aldehydes at room temperature without any appreciable amount of carboxylic acids, while under the same conditions nonbenzylic derivatives give carboxylic acids and imides with no trace of aldehydes, even at very low conversion. These results are explained through hydrogen abstraction by the phthalimide-N-oxyl (PINO) radical, whose reactivity with benzyl derivatives is governed by polar effects, so that benzylamides are much more reactive than the corresponding aldehydes. The enthalpic effect is, however, dominant with nonbenzylic amides, making the corresponding aldehydes much more reactive than the starting amides. The importance of the bond dissociation energy (BDE) of th...
TL;DR: In this paper, it was shown that the α-alkyl-hydroperoxy radical formed on free radical abstraction of the tertiary hydrogen in α-position to the hydroperoxide group is very labile.
TL;DR: Observations support the conclusion that the NH(2) group in amido complex 1 is exceptionally basic and as a result prefers to abstract a proton rather than a hydrogen atom from a reactive C-H bond.
Abstract: trans-(DMPE)(2)Ru(H)(NH(2)) (1) dehydrogenates cyclohexadiene and 9,10-dihydroanthracene to yield benzene (or anthracene), (DMPE)(2)Ru(H)(2), and ammonia. Addition of fluorene to 1 results in the formation of the ion pair [trans-(DMPE)(2)Ru(H)(NH(3))(+)][A(-)] (A(-) = fluorenide, 4a). Complex 1 also reacts with weak acids A-H (A-H = phenylacetylene, 1,2-propadiene, phenylacetonitrile, 4-(alpha,alpha,alpha-trifluoromethyl)phenylacetonitrile, cyclobutanone, phenol, p-cresol, aniline) to form ammonia and trans-(DMPE)(2)Ru(H)(A) (7, 8, 9a, 9b, 10, 11b, 11c, 12, respectively). In the cases where A-H = phenylacetylene, cyclobutanone, aniline, phenol, and p-cresol, the reaction was observed to proceed via ion pairs analogous to 4a. Compound 1 is reactive toward even weaker acids such as toluene, propylene, ammonia, cycloheptatriene, and dihydrogen, but in these cases deuterium labeling studies revealed that only H/D exchange between A-H and the ND(2) group is observed, rather than detectable formation of ion pairs or displacement products. Addition of triphenylmethane to 1 results in the formation of an equilibrium mixture of 1, triphenylmethane, and the ruthenium/triphenylmethide ion pair 4h. If the energetics of ion-pair association are ignored, this result indicates that the basicity of 1 is similar to that of triphenylmethide. All these observations support the conclusion that the NH(2) group in amido complex 1 is exceptionally basic and as a result prefers to abstract a proton rather than a hydrogen atom from a reactive C-H bond. The energetics and mechanism of these proton-transfer and -exchange reactions are analyzed with the help of DFT calculations.
TL;DR: In this paper, the authors determined the yield of hydroxyl radicals by illumination of TiO 2 layers immersed in air saturated aqueous methanol solutions and showed that the resulting yield is equal to half the measured formaldehyde yield in the pH range 7-13.
Abstract: The yield of hydroxyl radicals has been determined by illumination of TiO 2 layers immersed in air saturated aqueous methanol solutions. This yield is equal to half the measured formaldehyde yield in the pH range 7-13. A detailed mechanism is proposed, accounting for the lack of accumulation of hydrogen peroxide. The effect of changing methanol concentration, pH and light intensity (the latter by three orders of magnitude) is in agreement with a very simple mechanism. In contrast to hydroxyl radicals, which react via hydrogen abstraction, leading to formation of HCHO, there is no sign for reaction of methanol with mobile holes. Thus, the limiting quantum yield observed at high methanol concentration is related to the maximum yield of ○OH ads under air saturated conditions at the given pH and light intensity. The effect of light intensity shows the expected inverted square root dependency. The yield of ○OH ads is nearly constant in the range 7 < pH < 12. This system may be useful for comparative tests of different TiO 2 preparations.
TL;DR: Kinetic effects on H-atom abstractions from the diols in HB acceptor (HBA) solvents can be quantitatively accounted for over at least 50% of the available range of solvent HBA activities as measured by their beta2H values.
Abstract: Catechols and 1,8-naphthalene diols contain one “free” hydroxyl and one intramolecularly H-bonded hydroxyl group. The “free” hydroxyls are strong hydrogen-bond donors (HBDs) with α2H values (Abraham et al. J. Chem. Soc., Perkin Trans. 2 1989, 699) ranging from 0.685 to 0.775, indicating that these compounds have similar HBD properties to those of strongly acidic phenols such as 4-chlorophenol (α2H = 0.670) and 3, 5-dichlorophenol (α2H = 0.774). Kinetic effects on H-atom abstractions from the diols in HB acceptor (HBA) solvents can be quantitatively accounted for over at least 50% of the available range of solvent HBA activities (as measured by their β2H values; see Abraham et al. J. Chem. Soc. Perkin Trans. 2 1990, 521) on the basis of a single reactive OH group, the “free” OH. This free OH group is an outstanding H-atom donor in poor HBA solvents; e.g., in hexane rate constants for reaction with the DPPH• radical are 2.1 × 104 M-1 s-1 for 3,5-di-tert-butyl catechol and 2 × 106 M-1 s-1 for 4-methoxy-1,8-n...
TL;DR: In this article, the formation of polymer-like hydrocarbon films from beams of methyl radicals and atomic hydrogen is considered as a model system for a-C:H growth in low-temperature plasmas from a hydrocarbon precursor gas.
TL;DR: In this paper, a review of recent cross-beam and ab-initio studies of ground-state carbon atoms C(3P j ) with unsaturated hydrocarbons and their radicals is presented.
Abstract: This article reviews recent crossed-beams and ab-initio studies of reactions of ground-state carbon atoms C(3P j ) with unsaturated hydrocarbons and their radicals. All reactions have no entrance barrier and are initiated via an addition of the carbon atom to the ~ system. With the exception of the carbon atom reaction with acetylene, which also shows a significant fraction of molecular hydrogen loss, these bimolecular collisions are dominated by an atomic carbon versus hydrogen atom exchange mechanism. In some systems, homolytic cleavages of carbon-carbon bonds present additional decomposition routes of typically a few per cent at the most. The impact-parameter-dependent chemical dynamics are interpreted in terms of statistical and non-statistical decomposition of reaction intermediates involved. The polyatomic reaction products are highly hydrogend-eficient resonance-stabilized free radicals. The latter are strongly suggested as suitable precursors to form the first (substituted) aromatic ring molecule,...
TL;DR: In this paper, a modified and recalibrated potential energy surface for the gas-phase CH4+H→CH3+H2 reaction and its deuterated analogs is reported and tested, which is completely symmetric with respect to the permutation of the four methane hydrogen atoms.
Abstract: A modified and recalibrated potential energy surface for the gas-phase CH4+H→CH3+H2 reaction and its deuterated analogs is reported and tested, which is completely symmetric with respect to the permutation of the four methane hydrogen atoms, and is calibrated with respect to updated experimental and theoretical stationary point (reactants, products, and saddle point) properties, and experimental forward thermal rate constants. The forward and reverse rate constants are calculated using variational transition-state theory with multidimensional tunneling effect over a wide temperature range, 300–2000 K. The theoretical results reproduce the available experimental data, with a small curvature of the Arrhenius plot which indicates the role of the tunneling in this reaction. Five sets of kinetic isotope effects are also calculated. In general, they agree with experimental values within the experimental errors. This surface is then used to analyze dynamical features, such as reaction-path curvature, the couplin...
TL;DR: The reaction of specifically protected anhydroalditols with (diacetoxyiodo)benzene or iodosylbenzenes and iodine is a mild and selective procedure for the synthesis of chiral 6,8-dioxabicyclo[3.2.1]heptane ring systems under neutral conditions.
Abstract: The reaction of specifically protected anhydroalditols with (diacetoxyiodo)benzene or iodosylbenzene and iodine is a mild and selective procedure for the synthesis of chiral 6,8-dioxabicyclo[3.2.1]octane and 2,7-dioxabicyclo[2.2.1]heptane ring systems under neutral conditions. This reaction can be considered to be an intramolecular glycosidation that goes through an intramolecular hydrogen abstraction promoted by an alkoxy radical followed by oxidation of the transient C-radical intermediate to an oxycarbenium ion. This methodology is useful not only for the preparation of chiral synthons but also for the selective oxidation of specific carbons of the carbohydrate skeleton, constituting a good procedure for the synthesis of protected uloses.
TL;DR: In this article, a complex mechanism involving the formation of a stable prereactive complex is proposed, and the rate coefficients are calculated over the temperature range 296-445 K, using classical transition state theory.
Abstract: Unrestricted second and fourth order Moller-Plesset perturbation theory (MP2 and MP4), density functional theory (B3LYP and BHandHLYP), coupled cluster (CCSD(T)), and quadratic configuration interaction (QCI) calculationshave been performed using both the 6-11 1++G(d,p) and 6-311++G(2d,2p) basis sets, to study the OH hydrogen abstraction reaction from formic acid. A complex mechanism involving the formation of a very stable prereactive complex is proposed, and the rate coefficients are calculated over the temperature range 296-445 K, using classical transition state theory. The following expressions, in L mol- 1 s - 1 , are obtained for the acidic, for the formyl, and for the overall temperature-dependent rate constants: k 1 = (1.37 ′ 0.40) x 10 7 exp[(786 ′ 87)/T], k 1 1 = (5.93 ′ 1.39) × 10 8 exp[(-1036 ′ 72)/T], and k = (5.28 ′ 2.35) × 10 7 exp[(404 ′ 125)/T], respectively. An extremely large tunneling factor results for the acidic path, as a consequence of the presence of a high and narrow effective activation barrier. The contribution of the formyl path to the overall rate coefficient, as well as the magnitude of the tunneling effect, explain the observed non-Arrhenius behavior.
TL;DR: Results have an important bearing on the generalization of the Baldwin-Beckwith rules, which classified the 5-endo-trig radical cyclization as a "disfavored" process.
Abstract: Relative kinetic data were determined for the 5-endo-trig cyclization of radical 12 compared to hydrogen abstraction from (TMS)(3)SiH in the temperature range of 344-430 K, which allows for the estimation of a rate constant of 2 x 10(4) s(-)(1) at 298 K with an activation energy of ca. 9 kcal/mol for the cyclization process. The 5-endo-trig cyclization of a variety of radicals that afford five-membered nitrogen-containing heterocycles was addressed computationally at the UB3LYP/6-31G level. The 5-endo vs 4-exo mode of cyclication and the effect of delocalization of the unpaired electron in the transition state were investigated. Because the ring formed during cyclization contains five sp(2) centers, electrocyclization via a pentadienyl-like resonance form was also considered. For comparison, similar calculations were performed for 4-penten-1-yl and related radicals. The factors that affect the activation energies of homolytic 5-endo-trig cyclization were determined. In the absence of steric or conformational effects, the endo cyclization to form the five-membered ring was strongly favored over exo cyclization to form the four-membered ring not only on thermodynamic grounds but also kinetically. When a substituent on the double bond was able to delocalize the unpaired electron in the transition state of the 4-exo path, the two modes of cyclization became kinetically comparable. These results have an important bearing on the generalization of the Baldwin-Beckwith rules, which classified the 5-endo-trig radical cyclization as a "disfavored" process.
TL;DR: In this article, the first examples of catalytic catalysis using a manganese-oxo complex with a Mn 4 O 4 cubane core were reported. But, the results were limited to the case where the cubane was filled with Mn ions in a mixed valence oxidation state, formally Mn 4 (2III, 2IV).
Abstract: The oxidation of a variety of substrates (thioethers, hydrocarbons, alkenes, benyzl alcohol and benzaldehyde) by t BuOOH catalyzed by Mn 4 O 4 (O 2 PPh 2 ) 6 ( 1 ) and Mn 4 O 4 (O 2 P( p -MePh) 2 ) 6 ( 2 ) is reported. These reactions illustrate the first examples of oxidative catalysis using a manganese-oxo complex with a Mn 4 O 4 cubane core. These uncharged complexes contain Mn ions in a mixed valence oxidation state, formally Mn 4 (2III, 2IV), and are bridged by bulky diphenylphosphinate chelates across each of the six faces of the cube. Using this system, methyl phenyl sulfide is selectively mono-oxygenated to methyl phenyl sulfoxide with high catalytic efficiency, and no evidence for further oxidation to the thermodynamically preferred sulfone. Toluene is oxidized to a mixture of benzyl alcohol, benzaldehyde, and benzoic acid with high catalytic efficiencies. Lower catalytic efficiencies are observed in the oxidation of styrene to a mixture of styrene oxide and benzaldehyde, of cyclohexene to a mixture of cyclohexene oxide, 2-cyclohexen-1-ol, and 2-cyclohexen-1-one, and of cyclohexane to a mixture of cyclohexanol and cyclohexanone. The observed product distribution from the oxidation of hydrocarbons has the characteristics of a free radical-based oxidation mechanism. However, the sulfoxidation and epoxidation activity of the 1 / t BuOOH system, as well as the observed steric preferences for less congested substrates, suggest that a metal-oxo centered oxidation mechanism is active in the reactions studied here. An intermediate species, characterized by a UV–VIS band centered at 610 nm is observed in all reaction mixtures, and forms upon reaction of 1 or 2 with t BuOOH . Preliminary evidence suggests this reactive intermediate may correspond to a Mn(V)O species. Kinetic studies suggest two pathways for oxidation: one involving an oxygen atom transfer (two-electron branch), and the other involving a hydrogen atom abstraction (one-electron branch).
TL;DR: The ene reactions of nitroso compounds were studied with B3LYP geometry optimizations and energy calculations, along with single point energy evaluations using CASPT2/6-31G** and UCCSD(T)/6-311+G* methods.
Abstract: The ene reactions of nitroso compounds were studied with B3LYP/6-31G* geometry optimizations and energy calculations, along with single point energy evaluations using CASPT2/6-31G** and UCCSD(T)/6-311+G* methods. Reactions of HNO with propene and of MeNO and p-NO2C6H4NO with propene or substituted alkenes were also studied. The reaction mechanism is stepwise and involves a polarized diradical intermediate. The electronic structure of this intermediate is between that of a closed shell polar species and that of a pure diradical, and it is stabilized by polar solvents. A weak C−N bonding interaction combined with a CH−O hydrogen bond leads to heightened barriers to rotation about formally single bonds compared to conventional diradicals. Consequently, rotation is slower than hydrogen abstraction and cyclization to form an aziridine N-oxide. This aziridine N-oxide does not lead to ene products without subsequent ring opening but provides a mechanism for the RNO moiety to translate from one end of the alkene ...
TL;DR: The B3LYP/6-311++G(d,p) (ZPE) calculations revealed that HOOO radicals are considerably stabilized by forming intermolecularly hydrogen-bonded complexes with acetone and dimethyl ether and this type of interaction appears to be crucial for the relatively fast reactions (and the formation of the polyoxides in relatively high yields) in these solvents.
Abstract: Low-temperature ozonation of cumene (1a) in acetone, methyl acetate, and tert-butyl methyl ether at -70 degrees C produced the corresponding hydrotrioxide, C(6)H(5)C(CH(3))(2)OOOH (2a), along with hydrogen trioxide, HOOOH. Ozonation of triphenylmethane (1b), however, produced only triphenylmethyl hydrotrioxide, (C(6)H(5))(3)COOOH (2b). These observations, together with the previously reported experimental evidence, seem to support the "radical" mechanism for the first step of the ozonation of the C-H bonds in hydrocarbons, i.e., the formation of the caged radical pair (R(**)OOOH), which allows both (a) collapse of the radical pair to ROOOH and (b) the abstraction of the hydrogen atom from alkyl radical R(*) by HOOO(*) to form HOOOH. The B3LYP/6-311++G(d,p) (ZPE) calculations revealed that HOOO radicals are considerably stabilized by forming intermolecularly hydrogen-bonded complexes with acetone (BE = 8.55 kcal/mol) and dimethyl ether (7.04 kcal/mol). This type of interaction appears to be crucial for the relatively fast reactions (and the formation of the polyoxides in relatively high yields) in these solvents, as compared to the ozonations run in nonbasic solvents. However, HOOO radicals appear to be not stable enough to abstract hydrogen atoms outside the solvent cage, as indicated by the absence of HOOOH among the products in the ozonolysis of triphenylmethane. The decomposition of alkyl hydrotrioxides 2a and 2b involves a homolytic cleavage of the RO-OOH bond with subsequent "in cage" reactions of the corresponding radicals, while the decomposition of HOOOH is most likely predominantly a "pericyclic" process involving one or more molecules of water acting as a bifunctional catalyst to produce water and singlet oxygen (Delta(1)O(2)).
TL;DR: In this article, the hydrogen bonding complexes formed between the H2O and OH radical have been completely investigated for the first time in this study using density functional theory (DFT), and a larger basis set 6-311++G(2d,2p) has been employed in conjunction with a hybrid density functional method.
Abstract: The hydrogen bonding complexes formed between the H2O and OH radical have been completely investigated for the first time in this study using density functional theory (DFT). A larger basis set 6-311++G(2d,2p) has been employed in conjunction with a hybrid density functional method, namely, UB3LYP/6-311++G(2d,2p). The two degenerate components of the OH radical 2Π ground electronic state give rise to independent states upon interaction with the water molecule, with hydrogen bonding occurring between the oxygen atom of H2O and the hydrogen atom of the OH radical. Another hydrogen bond occurs between one of the H atoms of H2O and the O atom of the OH radical. The extensive calculation reveals that there is still more hydrogen bonding form found first in this investigation, in which two or three hydrogen bonds occur at the same time. The optimized geometry parameter and interaction energy for various isomers at the present level of theory was estimated. The infrared (IR) spectrum frequencies, IR intensities, and vibrational frequency shifts are reported. The estimates of the H2O · OH complex's vibrational modes and predicted IR spectra for these structures are also made. It should be noted that a total of 10 stationary points have been confirmed to be genuine minima and transition states on the potential energy hypersurface of the H2O · HO system. Among them, four genuine minima were located. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
TL;DR: The alkoxy radical generated by reaction of 3,7-anhydro-2-deoxyoctitols with (diacetoxyiodo)benzene (DIB) and iodine abstracts regioselectively either the proton at C7 or that at C4 depending on the electronegativity of the substituent at C3.
TL;DR: In this article, the self-directed growth mechanism of molecular nanowires on the Si (100)-2×1 monohydride surface from the molecular precursors styrene (H2C=CH-C6H5) and propylene (H 2C =CH-CH-3) was investigated using density functional theory.
Abstract: We use density functional theory to investigate the self-directed growth mechanism of molecular nanowires on the Si (100)-2×1 monohydride surface from the molecular precursors styrene (H2C=CH–C6H5) and propylene (H2C=CH–CH3). The reaction is initiated using a scanning tunneling microscope tip to create a Si dangling bond on the surface. This dangling bond then attacks the C=C π bond to form a Si–C bond and a C radical. Next, the C radical abstracts a H atom from a neighboring surface site, which results in a new Si dangling bond to propagate the chain reaction. For the case of H2C=CH–C6H5 the predicted hydrogen abstraction barrier of 18.0 kcal/mol from a neighboring dimer along the dimer row for C–H bond formation is smaller than H2C=CH–C6H5 desorption energy of 22.6 kcal/mol. On the other hand, for the case of H2C=CH–CH3 the predicted hydrogen abstraction barrier of 10.8 kcal/mol for C–H bond formation from a neighboring dimer is significantly larger than H2C=CH–CH3 desorption barrier of 2.7 kcal/mol. Consequently, the predicted barriers indicate that the self-directed growth of nanowires on (100) silicon using styrene occurs while a self-directed chain reaction using propylene should not occur, in agreement with experimental observations.