Product branching between reactive and nonreactive pathways in the collisional quenching of OH A 2Sigma+ radicals by H2

The collisional quenching of OH radicals in their excited A 2Sigma+ electronic state by molecular hydrogen is examined to determine the partitioning between reactive and nonreactive pathways. This is achieved using a pump-probe laser technique to compare the population prepared in the excited OH A 2Sigma+ state with that produced in the OH X 2Pi ground state from nonreactive quenching. Only a small fraction of the products, less than 15%, arise from nonreactive quenching; reactive quenching is the dominant product channel. The branching between the product channels provides a new dynamical signature of the conical intersection region(s) that couple the excited state potential for OH A 2Sigma++H2 with OH X 2Pi+H2 and H2O+H products.     

Collision dynamics and reactive uptake of OH radicals at liquid surfaces of atmospheric interest

The inelastic scattering of OH radicals from the surfaces of a sequence of potentially reactive organic liquids: squalane (C(30)H(62), 2,6,10,15,19,23-hexamethyltetracosane); squalene (C(30)H(50), trans-2,6,10,15,19,23-hexamethyltetracosa-2,6,10,14,18,22-hexaene); and oleic acid (C(18)H(34)O(2), cis-9-octadecanoic acid) was studied experimentally. A liquid long-chain perfluorinated polyether (PFPE, Krytox? 1506) was compared as a chemically inert reference. Gas-phase OH with an average laboratory-frame kinetic energy of 54 kJ mol(-1) was generated by 355-nm photolysis of a low-pressure of HONO a short distance (9 mm) above the liquid surface. Scattered OH was detected at the same distance by laser-induced fluorescence (LIF). Appearance profiles as a function of photolysis-probe delay were recorded for selected OH v' = 0, N' rotational levels. The efficiency of momentum transfer to the surface is least for PFPE and highest for squalane, with squalene and oleic acid intermediate, but in all cases the speed distributions are markedly too hot to be consistent with a thermal accommodation mechanism. The rotational distribution is found to be a function of scattered OH speed. The generally high rotational temperatures implied by the relative fluxes for N' = 1 and 5 were confirmed by LIF excitation spectra at the peak of the profile for each liquid. The trends in translational-to-rotational energy transfer were broadly consistent with the sequence in surface stiffness inferred from the translational inelasticity. The non-statistical distribution of OH fine-structure and Λ-doublet states produced by HONO photolysis appears to be effectively completely scrambled in collisions with the liquid surfaces. With due account taken of the product rotational distributions, and assuming that 100% of the OH scatters from PFPE, the integrated OH survival probabilities were: squalane (0.70 ± 0.08), squalene (0.61 ± 0.07) and oleic acid (0.76 ± 0.10). The 'missing' OH is presumed to have reacted at the liquid surface. Detailed comparison of the appearance profiles suggests that the main difference between squalane and squalene is loss of slower-moving OH, consistent with an additional capture mechanism at the vinyl sites.     

Supersonic expansion cooling of electronically excited OH radicals

Supersonic nozzle cooling of excited-state melecular radicals produced in a corona discharge is demonstrated and investigated using hydroxyl radicals. Rotational temperatures of 11 ± 2 K and vibrational temperatures of 3400 ± 300 K are typically observed in the OH A ²Σ⁺ state.     

Oxidation of naphthalene by radiolytically produced OH radicals

Radiolytically produced OH radicals react with naphthalene in aqueous solution in the presence of ferricyanide mainly to yield 1- and 2-naphthol in a ratio of 2.1:1. Reaction takes place via addition of OH to the naphthalene with a rate constant of 1.2 x 10¹⁰M^-1S^-1. Because of the low solubility of naphthalene (∼ 2 x 10^-4M) γ-radiolysis studies were carried out at doses of ∼ 10³ rad, where the naphthols were produced at micromolar concentrations. Analysis was by high-performance liquid chromatography. The initial yields of naphthols from solutions saturated with N₂O and naphthalene total 4.7 molec/100 eV, indicating near to quantitative conversion of the OH adducts to the naphthols by the ferricyanide. The observed product ratio is taken to reflect a two-fold preference for attack of OH at the 1-position as compared with that at the 2-position of naphthalene with only little (<10%) reaction at the 9-position.     

The reaction of OH radicals with dimethyl sulfide

The kinetics of the gas phase reaction of OH radicals with dimethyl sulfide (CH₃SCH₃) have been studied at various temperatures and total pressures using two relative rate methods and a flash photolysis technique. For the relative rate methods, rate constants were measured at 296 ± 2 K as a function of the O₂ pressure at a total pressure of ca. 740 torr. Data from these three experimental techniques were not in agreement. It is concluded that the relative rate techniques are subject to secondary reactions, possibly involving CH₃S radicals. A rate constant of (2.5) × 10^?12 e^{(130 = 102)/T} cm³ molecule^?1 s^?1 obtained using the flash photolysis-resonance fluorescence data in the absence of O₂, and which is in agreement with the lower range of values previously reported in the literature, is recommended.     

Mechanistic insights on how hydroquinone disarms OH and OOH radicals

Phenol derivatives are distinguished as successful free radical scavengers. We present a detailed analysis of hydroxyl hydrogen abstraction from hydroquinone by hydroxyl and hydroperoxyl radical with emphasis on changes that take place in the vicinity of the transition state. Quantum theory of atoms in molecules is employed to elucidate the sequence of positive and negative charge transfer by studying selected properties of the three key atoms (the transferring hydrogen, the donor atom, and the acceptor atom) along intrinsic reaction path. The presented results imply that in both reactions, which are examples of proton coupled electron transfer, proton, and electron get simultaneously transferred to the radical oxygen atom. The fact that the hydrogen's charge and volume do not monotonously change in the vicinity of the transition state in the product valley results from the adjacency of the proton and the electron to the donor and the acceptor oxygen atoms. Obtaining a detailed understanding of mechanisms by which free radicals are disarmed is of paramount importance given the effects of those highly reactive species on biological systems. A comprehensive analysis of hydroxyl hydrogen abstraction from hydroquinone by hydroxyl and hydroperoxyl radicals, based on changes of selected electronic properties of the three most relevant atoms (hydrogen donor, hydrogen acceptor, and the hydrogen itself), along the reaction coordinate, can be obtained by first‐principles calculations.     

Reactivity of allylic hydrogen in cyclohexadiene towards OH radicals

The yield of benzene in the reaction of 1,4- and 1,3-cyclohexadiene with OH radicals in the presence of oxygen was determined using H₂O₂ and CH₃ONO as OH radical sources. Both in the H₂O₂ and the CH₃ONO systems, the yield of benzene from 1,4-cyclohexadiene was 15.3% and the yield from 1,3-cyclohexadiene was 8.9%. On the basis of the obtained yields, the rate constant for allylic hydrogen abstraction per C? H in cyclohexadiene was determined to be 3.8 × 10^?12 cm³ molecule^?1 s^?1. The branching ratio of the hydrogen abstraction to overall reaction for 1-butene and 1-pentene was estimated to be (25–14)% by applying the obtained rate constants. The result was in good agreement with the branching ratio determined directly by use of the discharge flow photoionization mass spectrometer by Biermann, Harris, and Pitts [4].     

The photoelectron spectra of the OH and OD radicals

He(I) photoelectron spectra of the hydroxyl radical show ionizations corresponding to the production of OH⁺ (OD⁺) in the states X ³Σ^? and ¹Δ at ionization potentials of 13.01 eV and 15.20 eV respectively. The excitation energy of the ¹Δ state is in very good agreement with that predicted by calculation and experiment; vibrational spacings in the X ³Σ^? state agree with emission spectroscopic data.     

Gas-phase reactions of brominated diphenyl ethers with OH radicals

A small volume reaction chamber coupled to a mass spectrometer was used to study the gas-phase kinetics and mechanism of the reaction of OH radicals with diphenyl ether and seven polybrominated diphenyl ethers (PBDEs) with 1-2 bromines. Relative rate constants for these reactions were determined using isopropyl nitrite photolysis in He-air mixtures at approximately 740 Torr between the temperatures of 326-388 K. The Arrhenius expression for each compound was used to extrapolate the following OH rate constants at 298 K (in units of 10(-12) cm3 molecule(-1) s(-1), with 95% confidence intervals): diphenyl ether, 7.45 +/- 0.13; 2-bromodiphenyl ether, 4.7; 3-bromodiphenyl ether, 4.6; 4-bromodiphenyl ether, 5.7; 2,2'-dibromodiphenyl ether, 1.3; 2,4-dibromodiphenyl ether, 3.8; 3,3-dibromodiphenyl ether, 3.2; and 4,4'-bromodiphenyl ether, 5.1. The measured OH rate constants are in reasonable agreement with those predicted by structure activity relationships. Positive temperature dependences of these OH rate constants are observed for all compounds measured except for diphenyl ether and 4,4'-dibromodiphenyl ether. Bromophenols (in yields up to 20% relative to the amount of PBDE consumed) and Br2 were characterized as products of these reactions, suggesting that OH addition to ipso positions of these brominated aryls may be an important reaction pathway.     

High-temperature reactions of OH radicals with benzene and toluene

The rate constants for the reactions of OH radicals with benzene and toluene have been measured directly by a shock tube/pulsed laser-induced fluorescence imaging method at high temperatures. The OH radicals were generated by the thermal decomposition of nitric acid or tert-butyl hydroperoxide. The derived Arrhenius expressions for the rate constants were k(OH + benzene) = 8.0 x 10(-11) exp(-26.6 kJ mol(-1)/RT) [908-1736 K] and k(OH + toluene) = 8.9 x 10(-11) exp(-19.7 kJ mol(-1)/RT) [919-1481 K] in the units of cubic centimeters per molecule per second. Transition-state theory (TST) calculations based on quantum chemically predicted energetics confirmed the dominance of the H-atom abstraction channel for OH + benzene and the methyl-H abstraction channel for OH + toluene in the experimental temperature range. The TST calculation indicated that the anharmonicity of the C-H-O bending vibrations of the transition states is essential to reproduce the observed rate constants. Possible implications to the other analogous H-transfer reactions were discussed.     

Generation and consumption rates of OH radicals in sonochemical reactions

In this study, a KI aqueous solution or Methyl Orange (MO) aqueous solution was irradiated by an ultrasonic wave under the same experimental condition. The rates of oxidation of KI and MO by OH radicals differed by an order of magnitude. When the consumption of OH radicals by chemical reactions with species other than KI or MO is taken into account, numerical analysis of chemical kinetics model yields the same generation rate of OH radicals by the action of an ultrasonic wave for the experiments of KI and MO solutions.     

First rate constants for reactions of OH radicals with amides

Rate constants have been measured for the reaction of OH radicals with four amides, R₁N(CH₃)—C(O)R₂ (R₁ = H or Methyl, R₂ = Methyl or Ethyl), at 300 and 384 K using flash photolysis/resonance fluorescence. Reactants are introduced under slow flow conditions and are controlled by two independent methods, gas saturation and continuous injection. It turns out that the reactivities of the amides are considerably lower than those of the corresponding amines. The pattern of rate constants obtained at 300 K: 14, 21, 5.2, and 7.6 · 10⁻¹² cm³/s for N,N-Dimethylacetamide (dmaa), N,N-Dimethylpropionamide (dmpa), N-Methylacetamide (maa), and N-Methylpropionamide (mpa), respectively, indicates a single, dominating reaction center and strong electronic effects of the substituents at both sides of the amide function. Correspondingly, the observed negative temperature dependence (E/R = − 400 to − 600 K) excludes a direct abstraction mechanism. © 1997 John Wiley & Sons, Inc.     

Binary amorphous solids consisting of 2,4,6-triarylphenoxyl radicals and their dimers

Although molecular amorphous materials represent an important area of research in solid-state chemistry, studies pertaining to these systems have been restricted almost exclusively to amorphous solids based on a single molecule. In this study, we found that, while the 2,4,6-bis(4-tert-butylphenyl)phenoxyl radical (2_M) and its dimer (2_D) did not give single-component amorphous solids, they rapidly formed the corresponding binary amorphous solid IIa following their condensation from benzene, dichloromethane, chloroform, and ethyl acetate solutions. The formation of IIa could be attributed to the good solubilities of 2_M and 2_D in these solvents and the high packing efficiencies of these amorphous solids. IIa was also obtained when crystals of 2_D (IIb) were ground together. The solid-state formation of IIa would not only involve the locational exchange of 2_M and 2_D, but would also involve chemical exchanges.     

Generation of OH radicals in redox reactions of cobalt corrin complexes

Corrin complexes of cobalt reduce molecular oxygen to H₂O₂ in the presence of a reducing agent (ascorbic acid or ubiquinol Q₉H₂). The complex of divalent cobalt [Co^II(cor)]_OMe ⁺ClO₄ ^– breaks down H₂O₂ according to a radical mechanism, with the formation of OH radicals.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2679–2683, December, 1989.     

Direct kinetic study of the reaction of OH radicals with methyl-ethyl-ketone

The low-pressure discharge flow technique with resonance fluorescence monitoring of OH has been applied to study the kinetics of the overall reaction:

Kinetic measurements of methyl and ethyl nitrate reactions with OH radicals

Reaction rate coefficients of methyl and ethyl nitrates with OH radicals were determined by the relative rate method in 1 atmosphere of oxygen. Reactions were initiated by the photochemical formation of OH radicals utilizing the reaction: H₂O+O(¹D)→2OH. O(¹D) was obtained through a stationary photolysis of excess ozone in an experimental system under black light irradiation. Measurements were carried out for various combinations with different reference materials. Rate coefficients obtained were (0.30±0.032 (2σ)×10⁻¹³ cm³molecule⁻¹s⁻¹ (Temp.: 304–310 K) for methyl nitrate and (2.0±0.70)× 10⁻¹³ cm³molecule⁻¹s⁻¹ (298–310 K) for ethyl nitrate. For methyl nitrate, this data indicates the preference of a smaller rate coefficient between the two values reported in the literature [1,2], which have shown large discrepancies of more than one order of magnitude. For ethyl nitrate, only one measurement has been reported [2]. However, the present result suggests that the reported value was overestimated by a factor of more than two. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet: 29: 933–941, 1997.