Article pubs.acs.org/JPCA
Product Detection of the CH Radical Reaction with Acetaldehyde Fabien Goulay,*,†,∥ Adam J. Trevitt,‡ John D. Savee,§ Jordy Bouwman,† David L. Osborn,§ Craig A. Taatjes,§ Kevin R. Wilson,⊥ and Stephen R. Leone*,†,⊥,# †
Department of Chemistry and #Department of Physics, University of California, Berkeley, California 94720, United States ‡ School of Chemistry, University of Wollongong, Wollongong, NSW 2522, Australia § Combustion Research Facility, Mail Stop 9055, Sandia National Laboratories, Livermore, California 94551, United States ⊥ Chemical Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States S Supporting Information *
ABSTRACT: The reaction of the methylidyne radical (CH) with acetaldehyde (CH3CHO) is studied at room temperature and at a pressure of 4 Torr (533.3 Pa) using a multiplexed photoionization mass spectrometer coupled to the tunable vacuum ultraviolet synchrotron radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory. The CH radicals are generated by 248 nm multiphoton photolysis of CHBr3 and react with acetaldehyde in an excess of helium and nitrogen gas flow. Five reaction exit channels are observed corresponding to elimination of methylene (CH2), elimination of a formyl radical (HCO), elimination of carbon monoxide (CO), elimination of a methyl radical (CH3), and elimination of a hydrogen atom. Analysis of the photoionization yields versus photon energy for the reaction of CH and CD radicals with acetaldehyde and CH radical with partially deuterated acetaldehyde (CD3CHO) provides fine details about the reaction mechanism. The CH2 elimination channel is found to preferentially form the acetyl radical by removal of the aldehydic hydrogen. The insertion of the CH radical into a C−H bond of the methyl group of acetaldehyde is likely to lead to a C3H5O reaction intermediate that can isomerize by β-hydrogen transfer of the aldehydic hydrogen atom and dissociate to form acrolein + H or ketene + CH3, which are observed directly. Cycloaddition of the radical onto the carbonyl group is likely to lead to the formation of the observed products, methylketene, methyleneoxirane, and acrolein. environments20−26 suffer from an incomplete understanding of the reactivity of complex organic molecules. In the gas phase, carbonyl compounds will react with radicals such as OH, CH, CN, and C2H. There are numerous studies about the reaction of the OH radical with carbonyl compounds.27−35 The preferential reaction mechanism is abstraction of an H atom by the hydroxyl radical to form water and an oxygenated radical. Theoretical investigations34−36 have shown that the reaction of OH radical with acetaldehyde is mediated by the formation of a weakly bound complex that isomerizes, and dissociates to give water and the acetyl radical, via a transition state below the initial energy of the reactants. The reaction mechanism of carbonyl compounds with carbon-centered radicals, which can lead to the formation of new oxygenated hydrocarbons, has not been studied in much detail. Hippler et al.37 investigated the reactivity of alkyl radicals with carbonyl compounds in the gas phase and found that the
1. INTRODUCTION Carbonyl compounds such as aldehydes, ketones, or carboxylic acids play a central role in the chemical evolution of numerous carbon- and oxygen-rich gas-phase environments such as the Earth’s atmosphere,1−5 combustion environments,6−9 planetary atmospheres,10 comets,11,12 and the interstellar medium.13−15 In the Earth’s troposphere, unsaturated oxygenated molecules result from anthropogenic and biogenic emissions and contribute to the oxidizing capacity of the atmosphere.16 In combustion environments, carbonyl compounds are wellestablished intermediates.7 Carbonyl chemistry also plays a major role in the conversion of biofuels and biomass molecules into usable energy.17,18 In the interstellar medium, carbonyl compounds such as formaldehyde,13 acetaldehyde,15 and formic acid19 are detected in dense interstellar clouds and contribute to the chemical complexity of these environments. Very recently, reactions involving carbonyl compounds have been added to the chemical models of the atmosphere of Titan,20 the largest satellite of Saturn. Although oxygenated molecules are present in very small quantities in the atmosphere of Titan, their reactions with reactive radicals are believed to play an important function in the formation of the organic haze covering its surface.10 The accuracies of computational models developed to reproduce the chemical evolution of these gas-phase © 2012 American Chemical Society
Special Issue: A. R. Ravishankara Festschrift Received: November 24, 2011 Revised: January 6, 2012 Published: January 9, 2012 6091
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with formaldehyde42 and ketene43 at room temperature show that these reactions are fast, with reaction rates close to the collision limit, suggesting no energy barrier on the entrance channel. These kinetic experiments, however, do not provide any information about the reaction products or the reaction mechanism. Acetaldehyde is the smallest aldehyde molecule besides formaldehyde and is a good archetypal molecule to study the reactivity of the CH radical with carbonyl compounds. To our knowledge, there are no kinetic measurements for the CH + acetaldehyde reaction. The CH radical can insert into a C−H bond on either the methyl group or the aldehyde group, insert into the C−C bond, add to the double bond of the carbonyl, or abstract the aldehydic hydrogen. These multiple reaction entrance channels will lead to the formation of different C3H5O reaction intermediates that may further isomerize and dissociate to give the final products. The thermodynamically accessible exit channels are
reaction proceeds by addition of the radical onto the carbonyl group and/or by hydrogen abstraction. The methyl radical reacts by both addition and abstraction, while larger alkyl radicals preferentially react by addition to the carbonyl group. Reaction of carbonyl compounds with larger radicals such as phenyl radicals are relatively slow38 and are not likely to play a major role in gas-phase environments where the chemistry is governed by more rapid chemical reactions. The methylidyne radical (CH) is one of the most reactive carbon-containing neutral radicals, and it displays very rapid reaction kinetics with unsaturated hydrocarbons due to the barrierless attack on the π-electron systems of these molecules.39,40 Addition of a CH radical to the π system of unsaturated hydrocarbons leads to the formation of an initial cyclic intermediate, which can either directly dissociate to form cyclic products or isomerize via ring opening and further dissociate to give acyclic products.41 There are only very few studies of the reactivity of the CH radical with carbonyl compounds. Kinetic measurements of the methylidyne radical
−45.55 kJ ·mol−1
(R1a)
H2CCHO + CH2
−15.50 kJ · mol−1
(R1b)
C2H4 + HCO
−327.45 kJ ·mol−1
(R2)
H3CCH2 + CO
− 414.96 kJ ·mol−1
(R3)
H2CCO + CH3
− 324.18 kJ ·mol−1
(R4)
H3CHCCO + H
−489.93 kJ ·mol−1
(R5a)
H2CCHCHO + H
−277.43 kJ · mol−1
(R5b)
c ‐CH2CH2CO + H − 189.43 kJ · mol−1
(R5c)
c ‐H2CCOCH2 + H − 147.43 kJ· mol−1
(R5d)
−206.43 kJ ·mol−1
(R6)
CH + H3CCHO → H3CCO + CH2
H2CHCCO + H2
channel forms three C3H4O isomers that are identified, methylketene (CH3CHCO, eq R5a), methyleneoxirane (see Figure 1, eq R5d), and acrolein (H2CCH−CO, eq R5b). On the basis of these isomer distributions and on the isotope ratios of the CH and CD radicals with acetaldehyde and d3-acetaldehyde, we discuss the most likely reaction entrance channels and reaction intermediate isomerization schemes. Both insertion and cycloaddition mechanisms are necessary to interpret the experimental results. The mechanism of CH addition onto a CO bond has not been considered before.
The exothermicities of these reactions are calculated using the heats of formation of H3CCO,44 CH2,45 C2H4,46 HCO,46 H 3 CCH 2 , 44 CO, 4 7 H 2 CCO, 45 CH 3 , 4 6 H 3 CHCCO, 48 H 2 CCHCHO,49 c-CH 2 CH 2 CO, 50 c-H 2 CCOCH 2 , 51 H, 46 CH,46 and CH3CHO.52 The exothermicities of the reactions R1b and R6 are calculated using the exothermicities for the dissociation of acetaldehyde53 and acrolein (H2CCH− CO),54 respectively. Figure 1 depicts the molecular structures of acetaldehyde and the expected reaction products listed above. In the present article, we investigate the reaction of the CH radical with acetaldehyde by reacting CH and CD radicals with acetaldehyde and CH radicals with partially deuterated acetaldehyde (CD3CHO, d3-acetaldehyde). The experiments are performed utilizing a slow flow reactor coupled to a photoionization mass spectrometer that uses tunable synchrotron radiation from the Advanced Light Source (ALS) synchrotron to identify product isomers according to their differing ionization energies. Five reaction exit channels are observed, abstraction/CH2 elimination (eq R1), HCO elimination (eq R2), CO elimination (eq R3), CH3 elimination (eq R4), and H elimination (eq R5). The H elimination
2. EXPERIMENTAL SECTION 2.1. Apparatus. The experiments are performed in a slow flow reactor coupled to a multiplexed photoionization mass spectrometer. A description of the apparatus is given elsewhere,55−57 and only a brief overview is presented here. The reaction takes place in a quartz reactor tube maintained at a pressure of 4 Torr (533.3 Pa) and at room temperature (total density of 1.3 × 1017 cm−3). The gas flow consists of small amounts of radical precursor and reactant molecules in a large excess of helium and nitrogen buffer gas. Prior to the experiment, a 10% gas mixture of acetaldehyde in helium is 6092
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reaction.41,58,59 The chemical reaction proceeds uniformly along the length of the reactor as the irradiated gas moves down the tube. A portion of this gas escapes from the flow tube through a 650 μm diameter pinhole in the side of the tube into a chamber at relatively low pressure, typically a few ×10−5 Torr (1.3 × 10−3 Pa). The nearly effusive beam emerging from the pinhole is skimmed by a 1.5 mm diameter skimmer before entering a differentially pumped ionization region. The gas beam is crossed by synchrotron undulator radiation that is dispersed by a 3 m monochromator at the Chemical Dynamics Beamline of the ALS at Lawrence Berkeley National Laboratory. The VUV photon current at each photon energy is measured using a photodiode (SXUV-100 International Radiation Detectors, Inc.). All masses of resulting ions are monitored near-simultaneously using an orthogonal accelerated time-offlight mass spectrometer equipped with microchannel plate detector. The experiment is repeated for 500−10 000 laser pulses, and the data are summed. Because the apparatus collects time- and photon-energy-resolved mass spectra, a threedimensional data block is available for each experiment, consisting of ion intensity as a function of mass-to-charge ratio, reaction time, and photon energy. The photoionization spectra are constructed by integrating the data versus photon energy first over the desired mass-to-charge ratio and then over a time window that corresponds to the production of the species of interest in the photolytically initiated reaction. Three independent data sets are recorded for each reaction. The photoionization spectra from each data set are normalized by the area under the curve and averaged. The error bars at a given photon energy are twice the standard deviation around the mean of the three measurements. In the following discussion, the total reaction time is considered to be the first 60 ms after the firing of the laser. Background contributions are removed by subtraction of the average signal taken in the 20 ms before the photodissociation laser pulses. Finally, these backgroundsubtracted signals are normalized for the VUV photon current at each photon energy and assembled into the photoionization spectrum. The photon energy and the energy resolution (40 meV for 600 μm exit slit width) are determined by measurement of known atomic resonances of Xe. In an independent experiment, the relative photoionization spectrum of ketene was obtained using 193 nm photodissociation of a room-temperature gas mixture consisting of 0.04% acetone in He at 4 Torr. The 193 nm excimer laser fluence inside of the reaction flow tube is ∼7.5 mJ/cm2 per pulse. Under these experimental conditions, the quantum yield for ketene production was reported to be less than or equal to 2% and is expected to be the only contribution to the m/z = 42 ion signal.60 The photoionization spectrum is recorded from 9.5 to 11.0 eV in 0.025 eV intervals. 2.2. Computational Methodology. The experimental spectra are identified, in part, on the basis of their agreement with theoretical ionization energies and Franck−Condon (FC) simulations. Adiabatic ionization energies (AIE) are calculated using the CBS-QB3 composite method61,62 implemented within the GAUSSIAN03 suite of programs.63 The corresponding B3LYP/CBSB7 normal-mode frequencies and force constants are then used in the FC spectral simulation to predict the overall shape of the photoionization spectra. The FC spectral simulation of the photoionization spectra is carried out by computation and integration of the photoelectron spectra using the PESCAL64,65 program.
Figure 1. Molecular structures of acetaldehyde (C2H4O), C2H3O isomers, ketene, C3H4O isomers, and C3H3O.
prepared in a 3.79 L stainless steel cylinder at a total pressure of 2000 Torr (2.668 × 105 Pa). Liquid bromoform is placed in a glass vessel and kept at a temperature of 8 °C. A flow of He bubbling through the liquid maintains a constant total pressure in the vessel of 725 Torr (96.4 × 103 Pa). The total gas flow (100 sccm) is obtained by mixing 1 sccm (cubic centimeter per minute at 273 K and 1.013 × 105 Pa) of the acetaldehyde/ helium mixture and 2 sccm of the bromoform/helium mixture with 15 sccm of nitrogen and 82 sccm of helium. All of the gas flow rates are maintained constant using individual mass flow controllers. The densities of bromoform and acetaldehyde in the gas flow are 7.2 × 1012 and 1.3 × 1014 cm−3, respectively. The samples of deuterated bromoform and partially deuterated acetaldehyde (CD3CHO) are prepared in the same manner, and the densities in the final flow are the same. The purities of gases and reactants are as follows: He, 99.999%; bromoform, >99%; d-bromoform, 99.5 atom % D, with 1% carbon tetrabromide; acetaldehyde, 99.6%; and d3-acetaldehyde, 98 atom % D. By recording a mass spectrum of the partially deuterated acetaldehyde, we determine that the sample contains approximately 5% of CD2HCHO, mass 46. A uniform initial concentration of CH radicals is produced coaxially in the flow by 248 nm multiphoton photolysis of bromoform using an unfocused beam of an excimer laser with a 4 Hz repetition rate. The typical photolysis fluence inside of the reaction flow tube is ∼25 mJ/cm2 in a 20 ns pulse. The density of nitrogen in the flow (1.93 × 1016 cm−3) is sufficient to quench any vibrationally excited CH radicals formed during the photolysis of bromoform on a time scale faster than the 6093
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3. RESULTS The absorption cross section of acetaldehyde at 248 nm is ∼1 × 10−20 cm−2.66 The two main photodissociation channels are CH4 + CO and HCO + CH3.67−69 Formation of the acetyl radical by H loss and ketene (C2H2O) by H2 loss is energetically feasible but has not been observed experimentally at this wavelength. Figure 2 displays the mass spectrum
laser pulses. The data are corrected by subtracting the mass spectrum obtained under the same laser and flow conditions except without bromoform (see Figure 2). Masses corresponding to four reaction exit channels, eqs R1, R2 and/or R3, R4, and R5, are identified. No signal is observed at m/z = 55, corresponding to the H2 elimination channel (eq R6). Figure 4
Figure 2. Mass spectrum obtained by photodissociation of acetaldehyde at 248 nm, integrated for 10000 laser pulses at 10.1 eV photon energy.
obtained at 10.1 eV for the photodissociation of acetaldehyde at 248 nm, without bromoform in the flow and averaged for 10 000 laser pulses. The main photoproducts are observed at m/z = 15 and 42 and are likely to be the methyl radical and ketene. At the photon energy of 10.1 eV, methane (CH4, AIE = 12.61 eV)70 and carbon monoxide (CO, AIE = 14.0 eV)71 are not ionized and consequently do not contribute to the mass spectra. The ion signals of the formyl radical at m/z = 29 and acetyl radical at m/z = 43 are close to or below the experimental detection limit at this photon energy. The detection of m/z = 42 suggests that H2 elimination from acetaldehyde at 248 nm to form ketene is a favorable photodissociation channel. At a fluence of 25 mJ/cm2, only 3 × 10−4 of the acetaldehyde absorbs the 248 nm light, resulting in a total density of the acetaldehyde photoproducts in the reaction flow of ∼8 × 1010 cm−3. Hereafter, when analyzing the photoionization spectra of the products of the CH reaction with acetaldehyde, we neglect any contribution from CH3CHO photoproducts. Figure 3 displays the mass spectrum obtained at 10.1 eV for the reaction of CH with acetaldehyde and averaged for 10 000
Figure 4. Time traces of the ion signal at m/z = (a,b) 43, (c) 29, (d) 15, (e) 42, and (f) 56 for the reaction of the CH + CH3CHO integrated from 8.7 to 10.6 eV for (a,c,d), from 8.7 to 10.2 eV for (b), and at 10.1 eV for (e,f).
displays the kinetic traces for the main products of the reaction, integrated from 8.7 to 10.6 eV (a,c,d), from 8.7 to 10.2 eV (b), and at 10.1 eV for (e,f). The signal rise times for all of the products are within the temporal resolution of the experiment, which suggests that the reaction of CH with acetaldehyde is fast, similar to its reactions with other closed-shell molecules.72 After the initial rise, the temporal profiles of the ion signals at m/z = 42 and 56 in panels (e) and (f) are relatively constant over the entire reaction time. The slight increase in ion signal after the initial rise may be due to either a nonhomogeneous initial CH density due to absorption of a small amount of the laser light by the bromoform or slight misalignment of the photolysis laser. The products at m/z = 42 and 56 are likely to be closed-shell molecules produced by CH3 elimination (eq R4) and H elimination (eq R5), respectively. The ion signals at m/z = 43 (C2H3O), 29 (HCO or C2H5), and 15 (CH3) decay rapidly, which is indicative of reactive radicals that undergo self-reaction or react with other species in the flow or with the tube walls. The CH2 elimination channel (eq R1) results in the formation of either the acetyl radical (H3CCO) or the vinoxy radical (H2CCHO) at m/z = 43. The methylene radical (CH2) at m/z = 14 has an ionization energy of 10.35 eV,73 which is higher than the photon energy used to record the mass spectrum displayed in Figure 3. Using high-resolution mass spectra, we identify the signal at m/z = 29 as being both HCO from eq R2 and C2H5 from eq R3; the CO (m/z = 28, AIE = 14 eV)74 coproduct (eq R3) is not ionized at this photon energy. We detect a small amount of ethylene (AIE = 10.5 eV)75 from eq R2. Additional signals are detected at m/z = 57 and 58 (see Figure 3). The signal at m/z = 58 is laser-independent and
Figure 3. Mass spectrum of CH + CH3CHO reaction products obtained at 10.1 eV photon energy. The ion contribution from acetaldehyde photodissociation recorded at the same photon energy has been subtracted. 6094
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Table 1. Adiabatic Ionization Energies (AIE) for the Expected Isomers Produced by Reaction of the CH Radical with Acetaldehyde CHx radicals AIE (eV) mass 29 species AIE (eV) C2H2O isomers (mass AIE (eV) C2H3O isomers (mass AIE (eV) C3H4O isomers (mass AIE (eV) C3H5O isomers (mass AIE (eV) a
42) 43) 56) 57)
CH99 10.64 HCO74 8.12 ketene83 9.62 acetyl radical78 7.0 methylketene83 8.95b propionyl78 6.6
CH273 10.396 C2H5101 8.12
vinoxy radical79 10.16−10.85 cyclcopropanone84 9.1
CH3100 9.83
methyleneoxiranea 9.29
acrolein85 10.1
2-propyn-1-ol86 10.5
Present work. bVertical value.
displays an ion onset corresponding to acetone, most likely an impurity emanating from the bromoform bubbler and not totally removed from the mass spectrum by the prelaser background subtraction. The signal at m/z = 57 has an appearance energy between 9.7 and 9.8 eV and displays a fast rise (
