Acetyl Radical Research Articles

C 2 H + H 2 C O : A new route for formaldehyde removal

The title unknown reaction is theoretically studied at various levels to probe the interaction mechanism between the ethynyl radical (HC triple bond C) and formaldehyde (H(2)C double bond O). The most feasible pathway is a barrier-free direct H-abstraction process leading to acetylene and formyl radical (C(2)H(2)+HCO) via a weakly bound complex, and then the product can take secondary dissociation to the final product C(2)H(2)+CO+H. The C-addition channel leading to propynal plus H-atom (HCCCHO+H) has the barrier of only 3.6, 2.9, and 2.1 kcal/mol at the CCSD(T)/6-311+G(3df,2p)MP2//6-311G(d,p)+ZPVE, CCSD(T)/6-311+G(3df,2p)//QCISD/6-311G(d,p)+ZPVE, and G3//MP2 levels, respectively [CCSD(T)--coupled cluster with single, double, and triple excitations; ZPVE--zero-point vibrational energy; QCISD--quadratic configuration interaction with single and double excitations; G3//MP2-Gaussian-3 based on Moller-Plesset geometry]. The O addition also leading to propynal plus H atom needs to overcome a higher barrier of 5.3, 8.7, and 3.0 kcalmol at the three corresponding levels. The title no-barrier reaction presents a new efficient route to remove the pollutant H(2)CO, and should be included in the combustion models of hydrocarbons. It may also represent the fastest radical-H(2)CO reaction among the available theoretical data. Moreover, it could play an important role in the interstellar chemistry where the zero- or minute-barrier reactions are generally favored. Discussions are also made on the possible formation of the intriguing propynal in space via the title reaction on ice surface.

Pressure dependence for the CO quantum yield in the photolysis of acetone at 248 nm: A combined experimental and theoretical study

The quantum yield of CO in the laser pulse photolysis of acetone at 248 nm and at 298 K in the pressure range 20-900 mbar (N2) has been measured directly using quantitative infrared diode laser absorption of CO. It is found that the quantum yield of CO shows a significant dependence on total pressure with Phi(CO) decreasing with pressure from around 0.45 at 20 mbar to approximately 0.25 at 900 mbar. From a combination of ab initio quantum chemical calculations on the molecular properties of the acetyl (CH3CO) radical and its unimolecular fragmentation as well as the application of statistical (RRKM) and dynamical calculations we show that CO production results from prompt secondary fragmentation (via(2a)) of the internally excited primary CH3CO* photolysis product with an excess energy of approximately 62.8 kJ mol(-1). Hence, our findings are consistent with a consecutive photochemically induced decomposition model, viz. step (1): CH3COCH3+hv--> CH3CO*+ CH3, step (2a): CH3CO*--> CH3+ CO or step (2b) CH3CO*-(+M)--> CH3CO. Formation of CO via a direct and/or concerted channel CH3COCH3+hv--> 2CH(3)+ CO (1') is considered to be unimportant.

The Unimolecular Reactions of Ionized Methylacetamides

The gas-phase decarbonylation of the N-methyl- and N,N-dimethylacetamide radical cations gives rise to the α-distonic isomers of the dimethylamine and trimethylamine molecular ions by transfer of the acetyl methyl to nitrogen in two steps: H transfer and CH2 transfer. Complete exchange af all carbon-bonded hydrogen atoms precedes loss of acetyl radicals or of ketene from the metastable amide molecular ions. The reactions are examined with deuterium labeling and with composite ab initio methods of the G3 family.

Analysis of the mechanism of photolysis of acetylpropionyl peroxide in acetone-d6 by the NMR pectroscopy data

The photolysis of acetylpropionyl peroxide solutions in acetone at different temperatures has been investigated by the NMR method with consideration for the effects of chemical polarization of nuclei and the yields of the photolysis products. Acetylpropionyl peroxide decomposes when it is found in the binding or loosening triplet electron states. In the process of photolysis, acetone not only serves as the triplet sensitizer, but also effectively homologizes with the formation of methyl and acetyl radicals; however, it practically does not participate in the photoreduction.

Quantum chemical study on reaction of acetaldehyde with hydroxyl radical

AbstractThe reaction of acetaldehyde with hydroxyl radical was studied by means of quantum chemical methods. The geometries for all the stationary points on the potential energy surfaces were optimized fully, respectively, at the G3MP2, G3. and MP2/6‐311++G(d,p) levels. Single‐point energies of all the species were calculated at the QCISD/6‐311++G(d,p) level. The mechanism of the reaction studied was confirmed. The predicted product is acetyl radical that is in agreement with the experiment.

Dissociation of the ground state vinoxy radical and its photolytic precursor chloroacetaldehyde: electronic nonadiabaticity and the suppression of the H+ketene channel.

This work is a study of the competition between the two unimolecular reaction channels available to the vinoxy radical (CH(2)CHO), C-H fission to form H+ketene, and isomerization to the acetyl radical (CH(3)CO) followed by C-C fission to form CH(3) + CO. Chloroacetaldehyde (CH(2)ClCHO) was used as a photolytic precursor to the vinoxy radical in its ground state; photodissociation of chloroacetaldehyde at 193 nm produces vinoxy radicals with internal energies spanning the G3//B3LYP calculated barriers to the two available unimolecular reaction channels. The onset of the CH(3) + CO channel, via isomerization to the acetyl radical, was found to occur at an internal energy of 41 +/- 2 kcal/mol, agreeing well with our calculated isomerization barrier of 40.8 kcal/mol. Branching to the H+ketene channel was too small to be detected; we conclude that the branching to the H+ketene channel must be at least a factor of 200 lower than what is predicted by a RRKM analysis based on our electronic structure calculations. This dramatic result may be explained in part by the presence of a conical intersection at planar geometries along the reaction coordinate leading to H+ketene, which results in electronically nonadiabatic recrossing of the transition state.

Adsorption and decomposition of acetone on Si(0 0 1)

The adsorption of acetone on the Si(0 0 1) surface was investigated using the first-principles pseudopotential method within a generalized gradient approximation to the density functional theory. Our results indicate that [(CH 3) 2CO] adsorbs at low temperature in a [2 + 2] cycloaddition reaction via CO radical to the dangling bonds of SiSi dimers. The dissociation and desorption of the methyl and acetyl radicals was also investigated, pointing out that the most probable dissociation occurs with the formation of a CH 2CH 2 structure over a SiSi dimer and with the saturation of the other dimer by hydrogen atoms in a (2 × 2) reconstruction, and the desorption of a CO molecule. Calculated vibrational modes for the adsorbed systems are presented, with a view to contribute to further experimental investigations.

Heats of Formation of the Acetyl Radical and Ion Obtained by Threshold Photoelectron Photoion Coincidence

The dissociative photoionization onsets for the production of CH 3 CO + + CH 3 _from acetone and CH 3 CO + + CH 3 CO_from butanedione have been measured by threshold photoelectron photoion coincidence (TPEPICO) in which time-of-flight (TOF) mass spectra are obtained as a function of the ion internal energy. The use of velocity focusing for threshold electrons and the subtraction of hot electron coincidences from the TPEPICO spectra allow the 0 K dissociation onset to be measured with a precision of I kJ/mol. The experimental onset for CH 3 _loss from CH 3 COCH 3 was measured to be 10.563 ′ 0.010 eV and the onset for CH 3 CO_ loss from CH 3 COCOCH 3 was found to be 10.090 ′ 0.006 eV. A 298 K heat of formation of the CH 3 CO + of 659.4 ′ 1.1 kJ/mol is obtained by combining the measured dissociation onset with the well-established heats of formation of acetone and the methyl radical. A 298 K heat of formation of the CH 3 CO_ radical of -9.8 ′ 1.8 kJ/mol is obtained by combining the measured dissociation onset with the well-known heat of formation of butanedione and the measured heat of formation of CH 3 CO + . The acetone and butanedione ionization energies were measured to be 9.708 ′ 0.004 and 9.21 ′ 0.05 eV, respectively.

Year‐round records of gas and particulate formic and acetic acids in the boundary layer at Dumont d'Urville, coastal Antarctica

Multiple year‐round levels of acetate and formate in gas and aerosol phases were investigated at Dumont d'Urville (DDU, a coastal Antarctic site) by using mist chamber and aerosol filter sampling. Formate and acetate aerosol levels range from <0.5 ppt in winter to 3 ppt in summer. With corresponding gas phase levels of more than a hundred of pptv, formic and acetic acids are mainly (99%) present in the gas phase, representing the 2 major acidic gases before inorganic species (HCl, HNO3 and SO2) there. Mixing ratios of formic acid are minimal from May to August (70 pptv) and increase regularly toward November–February months when levels reach ∼200 pptv. Mixing ratios of acetic acid exhibit a more well‐marked seasonal cycle with values remaining close to 70 pptv from April to October and strongly increase during November–February months (mean value of 400 pptv). These seasonal changes suggest that the 2 carboxylic acids mainly originate from biogenic emissions of the Antarctic ocean whose variations follow the annual cycle of sea ice extent and solar radiation via photochemical production of alkenes from dissolved organic carbon released by phytoplankton. In summer, acetic acid levels show daily variations with maxima at noon and minima at night whereas formic acid levels peaks later in the afternoon. These dial variations in summer suggest that carboxylic acids are rapidly produced during the day and lost at night due to dry deposition on wet surface. It is suggested that the reactions of peroxy acetyl radical produced from propene with HO2 and CH3O2 in these poor NOx environments represent in summer the dominant chemical mechanisms producing acetic acid whereas ozone‐alkene reactions remain of minor importance at that season. Neither ozone‐alkene reactions nor aqueous phase HCHO oxidation can explain the summer levels of formic acid. In winter the long range transport of alkenes emitted at more temperate oceanic regions and reactions with ozone could account for the observed level of formic acid and possibly of acetic acid.

Theoretical study on potential energy surface of the C 2HF 2O and C 2F 3O radicals

The potential energy surface of the C 2HF 2O and C 2F 3O radicals were computed at the B3LYP density functional and the CCSD(T) ab initio levels of theory. For C 2HF 2O radical, the resonance dominating methyl radical (HFC –CFO) is found to the most stable isomer, followed by the difluoro acetyl ([F 2HCCO] ) isomer by 9.9 kcal/mol. However, the most stable C 2F 3O radical isomer is trifluoro acetyl radical ([F 3CCO] ), followed by the methyl isomer (F 2C –CFO) by 1.8 kcal/mol. Both acetyl radicals are relatively stable towards dissociation of being endothermic with low barriers, while other transformations (cyclization, hydrogen, and fluorine arrangements) are endothermic and have very high barriers.

Pressure and temperature‐dependent quantum yields for the photodissociation of acetone between 279 and 327.5 nm

The photodissociation of acetone has been studied over the wavelength (λ) range 279–327.5 nm as a function of temperature (T) and pressure (p) using a spectroscopic method to monitor the acetyl (CH3CO) radical fragment. Above 310 nm the quantum yield (QY) is substantially smaller than previous measurements, and decreases with T. The QYs for production of CH3CO + CH3 and CH3 + CH3 + CO have been parameterised as a function of λ, p and T and used to calculate the altitude dependence of the photolysis frequency. In the upper troposphere (UT) the acetone photolysis lifetime is a factor of 2.5–10 longer, dependent upon latitude and season, than if the previously recommended QYs are used.

Branching Fractions for H2O Forming Channels of the Reaction of OH Radicals with Acetaldehyde

Branching fractions for two hydrogen abstraction channels of the OH + CH3CHO reaction producing H2O + CH3CO (a) and H2O + CH2CHO (b) were determined using chemical ionization mass spectrometry. Reaction took place in a high-pressure turbulent flow reactor at 200 Torr of carrier gas N2. The branching fraction for the abstraction from the methyl group, (5.1+2.4/-1.7) %, was determined at room temperature by direct detection of the vinoxy radicals (CH2CHO) using proton-transfer ionization. The total H2O yield of (97.7 ± 4.7) % was obtained at T = 298 and 248 K by comparison with OH + C6H12 reaction, confirming that in this temperature range reaction proceeds by H-atom abstraction. Secondary reactions of the acetyl and vinoxy radicals in the presence of O2 and NO2 were studied by monitoring the stable products CH2O, (HCO)2, and CH3OH using proton-transfer ionization. It was found that formaldehyde is the major coproduct of OH in the CH3CO + O2 reaction.

A historical record of formate and acetate from a high‐elevation Alpine glacier: Implications for their natural versus anthropogenic budgets at the European scale

High‐resolution records of formate and acetate from a Col du Dôme (CDD, 4250 m elevation, French Alps) ice core were used to investigate the impact of man‐made activities on the midtropospheric levels of these species over Europe since 1925. Formate whose summer levels remained unchanged (80 ± 20 ng g−1) over the 1925–1995 time period is the major monocarboxylate present in summer CDD snow layers. In contrast, the acetate summer level being close to 15 ng g−1 prior to 1950 increased by a factor of 3 from 1950 to 1975 and declined during the 1980s. The difference in past changes of these monocarboxylates is due to recent acidification of the atmosphere which has lowered the scavenging efficiency in a larger extent for formate than for acetate. Formate levels corrected from effect of past acidity changes show a long‐term trend similar to the acetate one. These past changes are likely related to vehicle emissions (direct emissions and secondary production from alkenes) which strongly increased from 1950 to 1980. Because of improvement of engines and more stringent emission standards the CDD summer levels of monocarboxylates declined in the 1980s, and the 1990 levels are only slightly higher than their preindustrial levels. Therefore, except at the end of the 1970s when the anthropogenic input was as high as the natural one, natural sources appear to dominate the budget of the two monocarboxylates in the European midtroposphere. It is shown that natural sources include direct emissions by vegetation, oxidation of isoprene and monoterpenes, and possibly in‐cloud oxidation of formaldehyde into formic acid and gas phase oxidation of peroxyl acetyl radical into acetic acid.

A DFT study on the radical, monomer and dimer of α-keto pyruvic acid: equilibrium structures and vibrational analysis of stable conformers

The equilibrium structures and vibrational frequencies of the radical, monomer and dimer of α-keto pyruvic acid have been investigated by using hybrid density functional theory. The calculated geometries and the corresponding vibrational spectra of the three stable monomer conformers are found to be in good accord with earlier results. The present work provides the first detailed structure and vibrational analysis of the two stable dimer conformers, one of which is consistent with the X-ray diffraction and spectroscopic data, and also shows that there is only one stable form for the radical, with the others likely dissociated to the acetyl radical.

Modeling the chemistry of plasma polymerization using mass spectrometry.

The goal of the project is a solvent free painting shop. The environmental technologies laboratory is developing processes of plasma etching and polymerization. Polymerized thin films are first-order corrosion protection and primer for painting. Using pure acetylene we get very nice thin films which were not bonded very well. By using air as bulk gas it is possible to polymerize, in an acetylene plasma, well bonded thin films which are stable first-order corrosion protections and good primers. UV/Vis spectroscopy shows nitrogen oxide radicals in the emission spectra of pure nitrogen and air. But nitrogen oxide is fully suppressed in the presence of acetylene. IR spectroscopy shows only C=O, CH(2) and CH(3) groups but no nitrogen species. With the aid of UV/Vis spectra and the chemistry of ozone formation it is possible to define reactive traps and steps, molecule depletion and processes of proton scavenging and proton loss. Using a numerical model it is possible to evaluate these processes and to calculate theoretical mass spectra. Adjustment of theoretical mass spectra to real measurements leads to specific channels of polymerization which are driven by radicals especially the acetyl radical. The estimated theoretical mass spectra show the specific channels of these chemical processes. It is possible to quantify these channels. This quantification represents the mass flow through this chemical system. With respect to these chemical processes it is possible to have an idea of pollutant production processes.

Ultrafast dissociation dynamics of ketones at 195 nm

The photodissociation dynamics of the 3s Rydberg state of three ketones (CH 3CO–R, R=C 2H 5, C 3H 7, and iso-C 4H 9) and the ensuing dissociation of the nascent acetyl radical following 195 nm excitation were investigated by ultrafast photoionization spectroscopy. The 3s state the lifetimes of these ketones are similar (2.5–2.9 ps), though lifetimes of the acetyl radical range from 8.6 ps for CH 3CO–C 2H 5, 15 ps for CH 3CO–C 3H 7, to 23 ps for CH 3CO–( iso-C 4H 9), which suggests that for larger R more vibrational degrees of freedom compete for the excess energy so that less energy is partitioned into the internal energy of the acetyl radical.

An ab-initio study of some homolytic substitution reactions of acyl radicals at silicon, germanium and tint.

Ab initio calculations using the 6-311G**, cc-pVDZ, and (valence) double-zeta pseudopotential (DZP) basis sets, with (MP2, QCISD, CCSD(T)) and without (UHF) the inclusion of electron correlation, and density functional (BHandHLYP, B3LYP) calculations predict that homolytic substitution reactions of acetyl radicals at the silicon atoms in disilane can proceed via both backside and frontside attack mechanisms. At the highest level of theory (CCSD(T)/cc-pVDZ//MP2/cc-pVDZ), energy barriers (deltaE double dagger) of 77.2 and 81.9 kJ mol(-1) are calculated for the backside and frontside reactions respectively. Similar results are obtained for reactions involving germanium and tin with energy barriers (deltaE double dagger) of 53.7-84.2, and 55.0-89.7 kJ mol(-1) for the backside and frontside mechanisms, respectively. These data suggest that both homolytic substitution mechanisms are feasible for homolytic substitution reactions of acetyl radicals at silicon, germanium, and tin. BHandHLYP calculations provide geometries and energy barriers for backside and frontside transition states in good agreement with those obtained by traditional ab initio techniques.

A Sol-Gel Type of Electrolyte for a Dye-Sensitized Solar Cell: Attenuated Total Reflectance (ATR) Vibrational Spectra Studies

Gel electrolytes based on triethoxy silane with a covalently linked mid-poly (propylene glycol) chain (ICS-PPG) and an incorporated KI + I2 redox couple were prepared for use in a dye-sensitized photoelectrochemical cell (DSPEC). Infrared ATR spectra measurements of ICS-PPG and ICS-PPG/KI + I2 catalyzed with HCl or AcOH were performed and the changes to the vibrational bands of the solvent (EtOH), the urea groups and the SiOEt groups were used to correlate the hydrolyzing effect of HCl and AcOH, strengthening the interactions between urea groups during gelation and the evaporation of EtOH. The measurements of the I/U characteristics of the DSPEC with ICS-PPG/KI + I2/HCl and ICS-PPG/KI + I2/AcOH showed that the cell with the latter type of electrolyte exhibited a higher efficiency, which was related to the influence of indirect hydrolysis in the case of the AcOH catalyst. ATR spectra analysis confirmed the hydrolyzing effect of AcOH on ICS-PPG, which involved the anionic substitution of the acetyl radical.

Reaction of H + ketene to formyl methyl and acetyl radicals and reverse dissociations

AbstractThermochemical properties for reactants, intermediates, products, and transition states important in the ketene (CH2CO) + H reaction system and unimolecular reactions of the stabilized formyl methyl (C·H2CHO) and the acetyl radicals (CH3C·O) were analyzed with density functional and ab initio calculations. Enthalpies of formation (ΔHf°298) were determined using isodesmic reaction analysis at the CBS‐QCI/APNO and the CBSQ levels. Entropies (S°298) and heat capacities (Cp°(T)) were determined using geometric parameters and vibrational frequencies obtained at the HF/6‐311G(d,p) level of theory. Internal rotor contributions were included in the S and Cp(T) values. A hydrogen atom can add to the CH2‐group of the ketene to form the acetyl radical, CH3C·O (Ea = 2.49 in CBS‐QCI/APNO, units: kcal/mol). The acetyl radical can undergo β‐scission back to reactants, CH2CO + H (Ea = 45.97), isomerize via hydrogen shift (Ea = 46.35) to form the slight higher energy, formyl methyl radical, C·H2CHO, or decompose to CH3 + CO (Ea = 17.33). The hydrogen atom also can add to the carbonyl group to form C·H2CHO (Ea = 6.72). This formyl methyl radical can undergo β scission back to reactants, CH2CO + H (Ea = 43.85), or isomerize via hydrogen shift (Ea = 40.00) to form the acetyl radical isomer, CH3C·O, which can decompose to CH3 + CO. Rate constants are estimated as function of pressure and temperature, using quantum Rice–Ramsperger–Kassel analysis for k(E) and the master equation for falloff. Important reaction products are CH3 + CO via decomposition at both high and low temperatures. A transition state for direct abstraction of hydrogen atom on CH2CO by H to form, ketenyl radical plus H2 is identified with a barrier of 12.27, at the CBS‐QCI/APNO level. ΔHf°298 values are estimated for the following compounds at the CBS‐QCI/APNO level: CH3C·O (−3.27), C·H2CHO (3.08), CH2CO (−11.89), HC·CO (41.98) (kcal/mol). © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 35: 20–44, 2003

OH formation from CH 3CO+O 2: a convenient experimental marker for the acetyl radical

OH formation has been observed directly from the reaction between CH 3CO and O 2. This reaction proceeds via the formation of energised CH 3CO(O 2), which can either dissociate to OH or be collisionally stabilized to CH 3CO(O 2). The kinetics of OH formation were measured as a function of pressure at 295 and 213 K, giving rate coefficients in good agreement with the literature. The OH yield was also determined as a function of pressure enabling the reaction to be further quantified.