EPR studies of free-radical reactions of carboxyfullerenes

Carboxyfullerene is a class of water-soluble buckminsterfullerene with three malonic acids. Electron paramagnetic resonance (EPR) experiments are performed to examine the capacity of carboxyfullerenes to scavenge reactive oxygen species, methyl radicals and peroxyl radicals. Three types of reactions are reported: (1) reactions of^⋅OH and^⋅O ₂ ⁻ with carboxyfullerenes with 5,5-dimethyl-1-pyrroline N-oxide as the trapping agent, (2) reaction of^⋅CH₃ with scavenging agents, and (3) reactions of lipid alkyl/peroxyl radicals with carboxyfullerenes. The study enables one to evaluate the efficacy of carboxyfullerene as a free-radical scavenger and an antioxidatant.     

Involvement of triplet state in the photodissociation of hydrogen peroxide: experimental evidence from time-resolved EPR study

The dissociation of photoexcited hydrogen peroxide to generate a pair of hydroxyl radicals is generally believed to take place in a repulsive electronic singlet state. The results presented here, based on time-resolved EPR experiments on the spin polarisation pattern of the acetone ketyl radical (CH₃)₂C^?OH, generated on photodissociation of H₂O₂ in 2-propanol with a 248?nm laser light, strongly indicate significant involvement of a repulsive triplet state of excited hydrogen peroxide.     

Ion-molecular interactions in solutions of methanesulfonic acid in diethylamine according to IR spectroscopy data

The methanesulfonic acid (MSA)-diethylamine (DEA) binary liquid system is studied over the entire range of compositions at 30°C by using multiple frustrated total internal reflection IR spectroscopy. Solutions with acid: base equimolar ratio contain only 1 : 1 ion pairs. Upon adding the acid, a MSA molecule abstracts an anion from the 1 : 1 complex to produce a protonated DEA and an (H₃C(O₂)SO…H…OS(O₂)CH₃)⁻ anion with a strong H bond: (C₂H₅)₂(H)NH⁺ · OS(O₂)CH₃⁻ + HOS(O₂)CH₃ ↔ (C₂H₅)₂(H)NH⁺ + (H₃C(O₂)SO…H…OS(O₂)CH₃)⁻. This equilibrium is shifted to the left. The 1 : 1 complex is present in solutions even at an significant excess of the acid. To protonate the complex, it is required at least two MSA molecules. Under conditions of an excess of the base, DEA molecules do not solvate the 1 : 1 complex. The solution separates into two phases, composed of (C₂H₅)₂(H)NH⁺ · OS(O₂)CH₃⁻ complexes and pure DEA.     

Reactions and anion desorption induced by low-energy electron exposure of condensed acetonitrile

Thermal desorption spectrometry (TDS) and electron stimulated desorption (ESD) are employed to investigate mechanisms responsible for the formation of C₂H₆ in electron irradiated multilayer films of acetonitrile (CH₃CN) at 30 K. Using a high sensitivity time-of-flight mass spectrometer, we observe the ESD of anionic fragments H⁻, CH₂ ⁻, CH₃ ⁻ and CN⁻. Desorption occurs following dissociative electron attachment (DEA) via several negative ion resonances in the 6 to 14 eV energy range and correlates well with a “resonant” structure seen in the TDS yield of C₂H₆ (i.e., at mass 30 amu). It is proposed that C₂H₆ is formed by the reactions of CH₃ radicals generated following DEA to CH₃CN which also yields CN⁻. Between 2 and 5 eV, a second resonant feature is seen in the C₂H₆ signal. While DEA is observed in the gas phase at these energies, no anion desorption occurs since anionic fragments likely have insufficient kinetic energy to desorb. Since the CH₂ ⁻ ion has not been observed in gas-phase measurements, we propose that it is formed, along with HCN (that is detected in TDS) when dissociation into CH₃ ⁻ and CN is hindered by adjacent molecules.     

On the direct formation of HO2 radicals after 248 nm irradiation of benzene C6H6 in the presence of O2

We present in this work the direct observation of HO₂ radicals after irradiation of benzene C₆H₆ at 248 nm in the presence of O₂. HO₂ radicals have been unambiguously identified using the very selective and sensitive detection of continuous wave cavity ring-down spectroscopy (cw-CRDS) coupled to a laser photolysis reactor. HO₂ radicals were detected in the first vibrational overtone of the OH stretch at 6638.20 cm^-1, using a DFB diode laser. This reaction might be important because 248 nm photolysis of H₂O₂ has often been used in the past for studying the OH^•-initiated degradation of C₆H₆, often using a large excess of C₆H₆ over H₂O₂. The possible importance of the title reaction with respect to these former laboratory studies has been quantified through comparison with HO₂ ^• signals obtained from 248 nm photolysis of H₂O₂: one obtains under our conditions (excess O₂ and total pressure of 6.6 kPa helium) from the 248 nm irradiation of identical initial concentrations [C₆H₆]=[H₂O₂] the following relative initial radical concentrations: [HO₂ ^•]=(0.28±0.05)×[OH^•]. Experiments with various O₂ concentrations have revealed that the origin of the HO₂ radicals is not the reaction of H-atoms with O₂, but must originate from the reaction of O₂ with excited C₆H₆ ^*. The quantum yield of C₆H₆ ^* formation has been deduced to ϕ=0.2±0.1. PACS  42.62.Fi; 82.20.Pm; 82.33.Tb     

A study of the temperature dependence of the rate constant for the reaction of O<Subscript>2</Subscript> with surface alkoxy radicals

A method for studying the reactions of surface alkoxy radicals with O₂ at temperatures of 230 to 300 K is described. Alkoxy radicals were generated directly in the cavity of an EPR spectrometer. Surface organic radicals, prepared from paraffin wax ((CH₃)₂(CH₂) _n , n = 16–20), were applied to Aerosil particles from a solution in heptane. The Aerosil sample was placed in the cavity of the EPR spectrometer in a cylindrical cup with a central hole for pumping out gases and exposed to H atoms. In this way, it is possible observe a steady increase in the EPR signal from the surface radicals. To measure the rate constant at tropospheric temperatures, the reaction tube was placed in a Teflon jacket, through which cool nitrogen vapor was pumped. The temperature in the reactor was varied from 230 to 300 K. The recorded EPR spectra belong to the (RO) _s radical. After obtaining a stable EPR signal from the surface radicals, treatment with H atoms was stopped, additional flow of O₂ was introduced ([O₂] = 10¹⁴–10¹⁶ cm⁻³), and the reaction of O₂ with the surface organic radicals was studied by monitoring the EPR signal decay. The temperature dependence of the rate constant for the (RO) _s + O₂ → HO₂ + ketone was obtained within T = 230–300 K. The extrapolation of the data to real tropospheric conditions ([O₂] = 10¹⁸ cm⁻³) was performed.     

A kinetic investigation on low-temperature ignition of propane with ozone addition in an RCM

To extend the temperature for propane ignition to a lower region (< 680 K), ozone (O₃) was used as an ignition promoter to investigate the low-temperature chemistry of propane. Ignition delay times for propane containing varying concentrations of O₃ (0, 100, and 1000 ppm) were measured at 25 bar, 654–882 K, and equivalence ratios of 0.5 and 1.0 in a rapid compression machine (RCM). Species profiles during propane ignition with varying O₃ concentrations were recorded using a fast sampling system combined with a gas chromatograph (GC). A kinetic model for propane ignition with O₃ was developed. O₃ shortened ignition delay times of propane significantly, and the NTC behavior was weakened. O atoms released from O₃ reacted with propane through hydrogen abstraction reactions, which led to the fast production of OH radicals. The following oxidation of fuel radicals generated additional OH radicals. Consequently, the inhibition caused by the slow chemistry of hydrogen peroxide (H₂O₂) in the NTC region was weakened in the presence of O₃. Experimental results with O₃ addition can provide extra constraints on the low-temperature chemistry of propane. Species profiles during propane ignition at 730 K with 1000 ppm O₃ addition showed the production of propanal (C₂H₅CHO), acetone (CH₃COCH₃), and acetaldehyde (CH₃CHO) was promoted significantly. Model analyses indicated that O₃ shifted the oxidation temperature of propane to a lower region, in which reactions of ROO radicals (NC₃H₇O₂ and IC₃H₇O₂) tend to generate RO radicals (NC₃H₇O and IC₃H₇O). The promotion of RO radicals led to the fast production of C₂H₅CHO, CH₃COCH₃, and CH₃CHO. The corresponding species profile highlighted the reaction relevant to ROO and RO radicals (NC₃H₇O + O₂ = C₂H₅CHO + HO₂ and 2 IC₃H₇O₂ = 2 IC₃H₇O + O₂). Rate constants of these reactions were updated, which can potentially improve the performance of the core mechanism under lower temperatures and provide references for model development of larger hydrocarbons.     

Transformations of hydrocarbon radicals moving along a stainless steel tube

Transport of exhausted thermonuclear fuel in the ITER divertor and pumping duct was modeled on a specially designed dc glow discharge setup using mass spectrometry, optical and electron microscopy, and electron probe microanalysis. Transport and deposition of hydrocarbon radicals transferred in an H₂/C _x H _yx mixture through a hollow stainless steel anode at a total mixture pressure of 8–212 Pa and a methane content to 15 mol % were considered. It was shown that deposition of radicals and ions (CH₃, C₂H₃, C₂H₅) with kinetic energies of 0.03–3 eV on the anode inner surface at 600 K was suppressed to a large extent. In the temperature range of 600–800 K, deposition of ions and radicals with kinetic energy of ~3 eV was partially restored with the formation of soft a-C:H films, while thermalized radicals were not condensed.     

Radical formation in lithium trifluoromethane-sulfonate for ESR dosimetry

Lithium trifluoromethane-sulfonate (Li-TFMS:CF₃SO₃Li) irradiated by γ-rays showed an electron spin resonance (ESR) powder spectrum having the rhombicg-factor ofg _xx = 2.0259 ± 0.0005,g _yy = 2.0112 ± 0.0005 andg _zz = 2.0025 ± 0.0005 and a triplet hyperfine coupling constant ofA _xx/gβ= 0.8 ± 0.15 mT.A _yy andA _zz are not obtained because of the broadened spectrum. The energy levels,g-factor,A _xx/gβ and optical absorption spectrum of several conceivable radicals such as C^⋅F₂SO₃Li, CF₃-S-O^⋅ and CF₃-S-O-O^⋅ have been calculated by softwares MOPAC-V2 and Gaussian-98 based on ROHF (Restricted Hatree-Fock for open shell molecule). The most probable radical was ascribed to CF₃-SO^⋅ from both calculated and experimental results. The response to γ-ray dose and the thermal stability have been studied in addition to the effect of UV illumination for possible use of the signal intensity in ESR dosimetry. The obtained number of free radicals per 100 eV (G-value) was 1.23 ± 0.40.     

Photophysical Properties of New Terbium (III) Organophosphonates

This paper reports on the synthesis, characterization and photophysical properties of the Tb³⁺ organophosphonates, TbH(O₃PR)₂, methylphosphonate (R=CH₃), ethylphosphonate (R=C₂H₅), propylphosphonate (R=C₃H₇), and phenylphosphonate (R=C₆H₅). The layered Tb³⁺ organophosphonates were characterized by X-ray diffraction, IR spectroscopy, TG and elemental analysis. The interlayer distances of the Tb³⁺ organophosphonates evaluated by the X-ray diffractogram were 9.50 Å for TbH(O₃PCH₃)₂, 12.18 Å for TbH(O₃PC₂H₅)₂, 14.84 Å for TbH(O₃PC₃H₇)₂ and 15.20 Å for TbH(O₃PC₆H₅)₂. The Tb³⁺ luminescence data revealed highly green emissive materials when they were excited at 368 nm, where the characteristic ⁵D₄ → ⁷F _J (J=6, 5, 4 and 3) transitions of Tb³⁺ were observed at 488, 543, 585 and 619 nm, respectively. The lifetime of the Tb^{3+ 5}D₄ → ⁷F₅ transition (λ_exc=368 nm and λ_em=543 nm) for the Tb³⁺ organophosphonates was evaluated from the decay curves, which values were of 2.88, 2.22, 2.14 and 2.59 ms, respectively for TbH(O₃PCH₃)₂, TbH(O₃PC₂H₅)₂, TbH(O₃PC₃H₇)₂ and TbH(O₃PC₆H₅)₂. TG analysis revealed that these materials are thermally highly stable, with no water molecule in their composition, which makes them potential luminophores.     

Dissociative photoionization of l-menthone: An experimental and theoretical study

The dissociative photoionization mechanism of l-menthone has been investigated with photoionization mass spectrometry using synchrotron radiation. The adiabatic ionization energy (IE) of l-menthone and the appearance energies (AE) of its major fragment ions C₉H₁₅O⁺, C₉H₁₇⁺, C₈H₁₆⁺, C₇H₁₁O⁺, C₆H₁₀O⁺, C₆H₉O⁺, C₅H₈O⁺, C₅H₁₀⁺, C₄H₆O⁺, C₅H₉⁺, C₄H₈⁺, C₄H₇⁺, C₃H₇⁺, C₃H₆⁺, C₂H₂O⁺, and CH₃⁺ are determined with their photoionization efficiency (PIE) spectra in the photon energy region of ∼8−15.5 eV. Breakdown diagrams identifying the major products are presented. Dissociative photoionization channels for formation of these fragment ions are proposed based on comparison of determined experimental appearance energies and energies predicted with the DFT calculations. According to our results, the experimental dissociation energies are in fair agreement with the theoretical values of the possible photodissociation channels of C₁₀H₁₈O.     

1,4-二氧六环的真空紫外同步辐射光解离电离研究

The photoionization and photodissociation of 1,4-dioxane have been investigated with a reflectron time-of-flight photoionization mass spectrometry and a tunable vacuum ultraviolet synchrotron radiation in the energy region of 8.0-15.5 eV. Parent ion and fragment ions at m/z 88, 87, 58, 57, 45, 44, 43, 41, 31, 30, 29, 28 and 15 are detected under supersonic conditions. The ionization energy of DX as well as the appearance energies of its fragment ions C₄H₇O₂⁺, C₃H₆O⁺, C₃H₅O⁺, C₂H₅O⁺, C₂H₄O⁺, C₂H₃O⁺, C₃H₅⁺, CH₃O⁺, C₂H₆⁺, C₂H₅⁺/CHO⁺, C₂H₄⁺ and CH₃⁺ was determined from their photoionization efficiency curves. The optimized structures for the neutrals, cations, transition states and intermediates related to photodissociation of DX are characterized at the B3LYP/6-31+G(d,p) level and their energies are obtained by G3B3 method. Possible dissociative channels of the DX are proposed based on comparison of experimental AE values and theoretical predicted ones. Intramolecular hydrogen migrations are found to be the dominant processes in most of the fragmentation pathways of 1,4-dioxane.     

Experimental and kinetic modeling study of C2H4 oxidation at high pressure

A detailed chemical kinetic model for oxidation of C₂H₄ in the intermediate temperature range and high pressure has been developed and validated experimentally. New ab initio calculations and RRKM analysis of the important C₂H₃ + O₂ reaction was used to obtain rate coefficients over a wide range of conditions (0.003-100 bar, 200-3000 K). The results indicate that at 60 bar and medium temperatures vinyl peroxide, rather than CH₂O and HCO, is the dominant product. The experiments, involving C₂H₄/O₂ mixtures diluted in N₂, were carried out in a high pressure flow reactor at 600-900 K and 60 bar, varying the reaction stoichiometry from very lean to fuel-rich conditions. Model predictions are generally satisfactory. The governing reaction mechanisms are outlined based on calculations with the kinetic model. Under the investigated conditions the oxidation pathways for C₂H₄ are more complex than those prevailing at higher temperatures and lower pressures. The major differences are the importance of the hydroxyethyl (CH₂CH₂OH) and 2-hydroperoxyethyl (CH₂CH₂OOH) radicals, formed from addition of OH and HO₂ to C₂H₄, and vinyl peroxide, formed from C₂H₃ + O₂. Hydroxyethyl is oxidized through the peroxide HOCH₂CH₂OO (lean conditions) or through ethenol (low O₂ concentration), while 2-hydroperoxyethyl is converted through oxirane.     

Influence of heme periphery upon electronic structúre and Mössbauer spectra parameters of hemoglobin

Quantum-chemical analysis of electronic structure peculiarities and mossbauer spectra parameters was performed for penta-coordinated complex of ferro-protoporphyrin with imidazole (Fe(+2)PPIm). Peripheral substitutes (−CH₃, −C₂H₃, −C₂H₄COOH) introduced in the porphyrin macrocycle simulated real chemical structure of protoporphyrin (PP) in heme group of Hb. Calculations displayed that doubly occupied molecular orbitals (MO) of the peripheral substitutes (−CH=CH₂ and −CH₂−CH₂−COOH) always appeared near the occupat ion border. The orientation of vinyl fragment have the essential influence upon Fe₅ dorbital populations and quadrupole splitting_AE_Q for⁵B₁ and⁵B₂ terms. The values of isomer shift were insensitive to that modification of fragment orientation.     

The research on the synthesis and structure of some new ferrocene derivatives

The reaction of C₅H₄RLi with FeCl₂ gave nine new compounds of Fe(C₅H₄R)₂ [R=C(CH₃)₂C₆H₄CH₃-p(-m,-o), C₆H₁₀C₆H₅, C(Me)₂C₆C₄OCH_3-o, C₆H₁₀C₆H₄CH₃-p(,-m,-o), C₆H₁₀C₆H₄OCH₃-p]. The compositions of compounds were determined through elementary analysis. The structural determination was made by IR and H²NMR. Mossbauer spectia were taken at room temperature. The IS and QS values are 0.41–0.45mm/s and 2.3–2.5mm/s., respectively. The solid state structure of the complex has been determined by a single crystal x-ray diffraction study, crystal data for Fe[C₅H₄C(CH₃)₂C₆H₅]₂: a=17.988(2)A, b=17. 411(2)A, c=7.496(1)A, α=β=90°, r=112.23°, Z=4, monoclinic form, space group C2/c. Our conclusions are: in π-acceptor ligand, the nucleophilic substituents decrease and the electrophilic substituts increase the metal to ligand electron cloud shift, which results in a decrease or an increase in the strength of the coordinate bonds and in the stabilization of the complexes by their steric effect.     

Theoretical studies of ethylene addition to transition metal compounds with carbene and oxo groups LnM(CH2)(O)

Quantum chemical calculations using density functional theory at the B3LYP level in combination with relativistic effective core potentials for the metals and TZ2P valence basis sets have been carried out for elucidating the reaction pathways of ethylene addition to MeReO₂(CH₂) ( C1 ). The results are compared with our previous studies of ethylene addition to OsO₂(CH₂)₂ ( A1 ) and OsO₃(CH₂) ( B1 ). Significant differences have been found between the ethylene additions to the osmium compounds A1 and B1 and the rhenium compound C1 . Seven pathways for the reaction C1 +C₂H₄ were studied, but only the [2+2]_Re,C addition yielding rhenacyclobutane C5 is an exothermic process with a high activation barrier of 48.9 kcal mol^?1. The lowest activation energy (27.7 kcal mol^?1) is calculated for the [2+2]_Re,C addition, which leads to the isomeric form C5 ′. Two further concerted reactions [3+2]_O,C, [3+2]_O,O, and [2+2]_Re,O and the addition/hydrogen migration of ethylene to one oxo ligand are endothermic processes which have rather high activation barriers (>35 kcal mol^?1). Four isomerization processes of C1 have very large activation energies of >65 kcal mol^?1. The ethylene addition to the osmium compounds A1 and B1 are much more exothermic and have lower activation barriers than the C₂H₄ addition to C1 . Copyright © 2007 John Wiley & Sons, Ltd.     

Mössbauer and molecular orbital (MO) study of localized and delocalized mixed-valence organo-FeIIFeI bisandwiches

A Mössbauer study of two mixed-valence Fe^IIFe^I compounds C₅H₅Fe(C₅H₄)₂ FeC₆(CH₃)₆ (1) and ¦C₆(CH₃)₆ Fe(C₅H₄)₂ Fe C₆(CH₃)₆¦⁺ (2 ⁺) was carried out from 4.2K to room temperature. Zero-field spectra show two types of iron atoms for1 and one type for2 ⁺. Hence1 is a localized mixed-valence complex and2 ⁺ a delocalized mixed-valence complexe. High magnetic field spectra for 2⁺ give a negative sign for the EFG and show the valence electron is delocalized on the two centers. IEHT-MO calculations confirm the results and allow to explain the temperature independence of the quadrupole splitting (QS) of2 ⁺.     

Polysilyl radicals: EPR study of the formation and decomposition of star polysilanes

Electron paramagnetic resonance (EPR) spectroscopy was fruitfully used for studying the formation and the reactions of the star polysilane radical (Me₃SiMe₂Si)₃Si (1).1, which was successfully generated both thermally and photochemically from a variety of precursors, was found to be significantly more stable kinetically than the (Me₃Si)₃Si radical. Thus, (Me₃SiMe₂Si)₃Si^⋅ has a half-life time of ca. 6 min at 20°C, while (Me₃Si)₃Si^⋅ can be observed only at −25°C. Density-functional quantum-mechanical calculations show that1 and (Me₃Si)₃Si^⋅ have the same thermodynamic stability. The high kinetic stability of1 is attributed to its backfold “umbrella”-type conformation where the β-silyl groups point “inwards” towards the radical center. This conformation protects the radical center of1 from dimerization and other reactions. The EPR spectrum of1 and in particular the Si α-hyperfine coupling constant of 5.99 mT shows that1 is less pyramidal than (Me₃Si)₃Si^⋅ but is more pyramidal than (i-Pr₃Si)₃Si^⋅, with an estimated SiSiSi bond angle around the radical center of 118∘. Photolysis and thermolysis of [(Me₃SiMe₂Si)₃Si]₂ also involves the intermediacy of1. Photolysis of [(Me₃SiMe₂Si)₃Si]₂ leads to (Me₃SiMe₂Si)₄Si, while thermolysis produced the less strained isomer of 1, (Me₃SiMe₂Si)₃SiSi-Me₂Si(Me₃SiMe₂Si)₂SiMe₃. In this study we provide the first direct evidence that silyl radicals are involved as intermediates in the reactions of silanes with di(tert-butyl)mercury.     

When hydrogen is slower than methane to ignite

Hydrogen (H₂) is known to be the fastest fuel to ignite among all practical combustion fuels. In this study, for the first time, longer ignition delay times (IDTs) for the H₂ and H₂ blended CH₄ mixtures were measured compared to those for pure CH₄. This work investigates the ignition characteristics of H₂, CH₄, and 50% CH₄/50% H₂ mixtures using a rapid compression machine at pressures ranging from 20 to 50 bar and at equivalence ratios (φ) from 0.5 to 2.0 in air in the temperature range 858–1080 K. The experimental IDTs are simulated using a newly updated kinetic mechanism, NUIGMech1.3, and good agreement is observed. At lower temperatures the IDTs of H₂, CH₄, and the 50% CH₄/50% H₂ mixtures are similar to one another, and the IDTs of the 50% CH₄/50% H₂ mixtures are longer than those for pure CH₄ at temperatures below 930 K. At temperatures below 890–925 K, depending on the operating pressure and equivalence ratio, the hydrogen mixtures are the slowest to ignite, with IDTs being 2.5 times longer than those recorded for CH₄ at a pressure of 40 bar at 890 K for φ = 1.0, and at 875 K for φ = 2.0. At low temperatures alkyl (Ṙ = ĊH₃ and Ḣ) radicals add to O₂ producing RȮ₂ radicals, which then react with HȮ₂ radicals forming ROOH (H₂O₂ and CH₃OOH) and O₂. For H₂, the self-recombination of HȮ₂ radicals leads to chain propagation which inhibits reactivity, whereas for CH₄, the reaction between RȮ₂ (CH₃OȮ) and HȮ₂ leads to chain branching, increasing reactivity. Furthermore, CH₃OOH decomposes more easily to produce CH₃Ȯ and ȮH radicals than does H₂O₂ to produce two ȮH radicals. Thus, mixtures containing higher H₂ concentrations are slower to ignite compared to those with higher CH₄ concentrations at low temperatures.     

Exceptionally high decarboxylation rate of a primary aliphatic acyloxy radical determined by radical product yield analysis and quantitative 1H‐CIDNP spectroscopy

Symmetrical (RCO₂CO₂R; R = XCH₂CH₂) and asymmetrical (RCO₂CO₂R′; R = C₉H₁₉CH₂CH₂, R′ = CH₃ or m‐ClC₆H₄) primary diacyl peroxides were thermally decomposed under different conditions to analyze the decarboxylation rates of the thermally generated acyloxy radicals. Quantitative models of the geminate product yields, and qualitative and quantitative ¹H‐CIDNP spectroscopy were used to obtain the decarboxylation rate estimates. Results reported here suggest that, unlike short chain acyloxy radicals such as propanoyloxyl, long chain acyloxy radicals possess the highest decarboxylation rates of all known acyloxy radicals, estimated at (0.5–1.5) × 10¹² s^?1 between 80 and 140 °C. Given the nature of the dissociative state of acyloxy radicals, such rates appear to be the result of destabilization of the former by the steric bulk of the long chain substituents. Additionally, the rate of this order of magnitude suggests a nearly concerted decarboxylation of primary diacyl peroxides. Copyright © 2011 John Wiley & Sons, Ltd.