Formation of dimethylketene and methacrolein by reaction of the CH radical with acetone

OH Roaming and Beyond in the Unimolecular Decay of the Methyl-Ethyl-Substituted Criegee Intermediate: Observations and Predictions

Alkene ozonolysis generates short-lived Criegee intermediates that are a significant source of hydroxyl (OH) radicals. This study demonstrates that roaming of the separating OH radicals can yield alternate hydroxycarbonyl products, thereby reducing the OH yield. Specifically, hydroxybutanone has been detected as a stable product arising from roaming in the unimolecular decay of the methyl-ethyl-substituted Criegee intermediate (MECI) under thermal flow cell conditions. The dynamical features of this novel multistage dissociation plus a roaming unimolecular decay process have also been examined with ab initio kinetics calculations. Experimentally, hydroxybutanone isomers are distinguished from the isomeric MECI by their higher ionization threshold and distinctive photoionization spectra. Moreover, the exponential rise of the hydroxybutanone kinetic time profile matches that for the unimolecular decay of MECI. A weaker methyl vinyl ketone (MVK) photoionization signal is also attributed to OH roaming. Complementary multireference electronic structure calculations have been utilized to map the unimolecular decay pathways for MECI, starting with 1,4 H atom transfer from a methyl or methylene group to the terminal oxygen, followed by roaming of the separating OH and butanonyl radicals in the long-range region of the potential. Roaming via reorientation and the addition of OH to the vinyl group of butanonyl is shown to yield hydroxybutanone, and subsequent C-O elongation and H-transfer can lead to MVK. A comprehensive theoretical kinetic analysis has been conducted to evaluate rate constants and branching yields (ca. 10-11%) for thermal unimolecular decay of MECI to conventional and roaming products under laboratory and atmospheric conditions, consistent with the estimated experimental yield (ca. 7%).

A New Pathway for Intersystem Crossing: Unexpected Products in the O(3P) + Cyclopentene Reaction

We investigated the reaction of O(³P) with cyclopentene at 4 Torr and 298 K using time-resolved multiplexed photoionization mass spectrometry, where O(³P) radicals were generated by 351 nm photolysis of NO₂ and reacted with excess cyclopentene in He under pseudo-first-order conditions. The resulting products were sampled, ionized, and detected by tunable synchrotron vacuum ultraviolet radiation and an orthogonal acceleration time-of-flight mass spectrometer. This technique enabled measurement of both mass spectra and photoionization spectra as functions of time following the initiation of the reaction. We observe propylketene (41%), acrolein + ethene (37%), 1-butene + CO (19%), and cyclopentene oxide (3%), of which the propylketene pathway was previously unidentified experimentally and theoretically. The automatically explored reactive potential energy landscape at the CCSD(T)-F12a/cc-pVTZ//ωB97X-D/6-311++G(d,p) level and the related master equation calculations predict that cyclopentene oxide is formed on the singlet potential energy surface, whereas propylketene is first formed on the triplet surface. These calculations provide evidence that significant intersystem crossing can happen in this reaction not only around the geometry of the initial triplet adduct but also around that of triplet propylketene. The formation of 1-butene + CO is initiated on the triplet surface, with bond cleavage and hydrogen transfer occurring during intersystem crossing to the singlet surface. At present, we are unable to explain the mechanistic origins of the acrolein + ethene channel, and we thus refrain from assigning singlet or triplet reactivity to this channel. Overall, at least 60% of the products result from triplet reactivity. We propose that the reactivity of cyclic alkenes with O(³P) is influenced by their greater effective degree of unsaturation compared with acyclic alkenes. This work also suggests that searches for minimum-energy crossing points that connect triplet surfaces to singlet surfaces should extend beyond the initial adducts.

Product Detection of the CH(X2Π) Radical Reaction with Cyclopentadiene: A Novel Route to Benzene

The reaction of the methylidyne radical (CH(X²Π)) with cyclopentadiene (c-C₅H₆) is studied in the gas phase at 4 Torr and 373 K using a multiplexed photoionization mass spectrometer. Under multiple collision conditions, the dominant product channel observed is the formation of C₆H₆ + H. Fitting the photoionization spectrum using reference spectra allows for isomeric resolution of C₆H₆ isomers, where benzene is the largest contributor with a relative branching fraction of 90 (±5)%. Several other C₆H₆ isomers are found to have smaller contributions, including fulvene with a branching fraction of 8 (±5)%. Master Equation calculations for four different entrance channels on the C₆H₇ potential energy surface are performed to explore the competition between CH cycloaddition to a C═C bond vs CH insertion into C-H bonds of cyclopentadiene. Previous studies on CH addition to unsaturated hydrocarbons show little evidence for the C-H insertion pathway. The present computed branching fractions support benzene as the sole cyclic product from CH cycloaddition, whereas fulvene is the dominant product from two of the three pathways for CH insertion into the C-H bonds of cyclopentadiene. The combination of experiment with Master Equation calculations implies that insertion must account for ∼10 (±5)% of the overall CH + cyclopentadiene mechanism.

Study of the Synchrotron Photoionization Oxidation of Alpha-Angelica Lactone (AAL) Initiated by O(3P) at 298, 550, and 700 K

In recent years, biofuels have been receiving significant attention because of their potential for decreasing carbon emissions and providing a long-term renewable solution to unsustainable fossil fuels. Currently, lactones are some of the alternatives being produced. Many lactones occur in a range of natural substances and have many advantages over bioethanol. In this study, the oxidation of alpha-angelica lactone initiated by ground-state atomic oxygen, O(³P), was studied at 298, 550, and 700 K using synchrotron radiation coupled with multiplexed photoionization mass spectrometry at the Lawrence Berkeley National Lab (LBNL). Photoionization spectra and kinetic time traces were measured to identify the primary products. Ketene, acetaldehyde, methyl vinyl ketone, methylglyoxal, dimethyl glyoxal, and 5-methyl-2,4-furandione were characterized as major reaction products, with ketene being the most abundant at all three temperatures. Possible reaction pathways for the formation of the observed primary products were computed using the CBS–QB3 composite method.

Elucidating the differences in oxidation of high-performance α- and β- diisobutylene biofuels via Synchrotron photoionization mass spectrometry

Biofuels are a promising ecologically viable and renewable alternative to petroleum fuels, with the potential to reduce net greenhouse gas emissions. However, biomass sourced fuels are often produced as blends of hydrocarbons and their oxygenates. Such blending complicates the implementation of these fuels in combustion applications. Variations in a biofuel's composition will dictate combustion properties such as auto ignition temperature, reaction delay time, and reaction pathways. A handful of novel drop-in replacement biofuels for conventional transportation fuels have recently been down selected from a list of over 10,000 potential candidates as part of the U.S. Department of Energy's (DOE) Co-Optimization of Fuels and Engines (Co-Optima) initiative. Diisobutylene (DIB) is one such high-performing hydrocarbon which can readily be produced from the dehydration and dimerization of isobutanol, produced from the fermentation of biomass-derived sugars. The two most common isomers realized, from this process, are 2,4,4-trimethyl-1-pentene (α-DIB) and 2,4,4-trimethyl-2-pentene (β-DIB). Due to a difference in olefinic bond location, the α- and β- isomer exhibit dramatically different ignition temperatures at constant pressure and equivalence ratio. This may be attributed to different fragmentation pathways enabled by allylic versus vinylic carbons. For optimal implementation of these biofuel candidates, explicit identification of the intermediates formed during the combustion of each of the isomers is needed. To investigate the combustion pathways of these molecules, tunable vacuum ultraviolet (VUV) light (in the range 8.1-11.0 eV) available at the Lawrence Berkeley National Laboratory's Advanced Light Source (ALS) has been used in conjunction with a jet stirred reactor (JSR) and time-of-flight mass spectrometry to probe intermediates formed. Relative intensity curves for intermediate mass fragments produced during this process were obtained. Several important unique intermediates were identified at the lowest observable combustion temperature with static pressure of 93,325 Pa and for 1.5 s residence time. As this relatively short residence time is just after ignition, this study is targeted at the fuels' ignition events. Ignition characteristics for both isomers were found to be strongly dependent on the kinetics of C₄ and C₇ fragment production and decomposition, with the tert-butyl radical as a key intermediate species. However, the ignition of α-DIB exhibited larger concentrations of C₄ compounds over C₇, while the reverse was true for β-DIB. These identified species will allow for enhanced engineering modeling of fuel blending and engine design.

Experimental Evidence of Dioxole Unimolecular Decay Pathway for Isoprene-Derived Criegee Intermediates

Ozonolysis of isoprene, one of the most abundant volatile organic compounds emitted into the Earth's atmosphere, generates two four-carbon unsaturated Criegee intermediates, methyl vinyl ketone oxide (MVK-oxide) and methacrolein oxide (MACR-oxide). The extended conjugation between the vinyl substituent and carbonyl oxide groups of these Criegee intermediates facilitates rapid electrocyclic ring closures that form five-membered cyclic peroxides, known as dioxoles. This study reports the first experimental evidence of this novel decay pathway, which is predicted to be the dominant atmospheric sink for specific conformational forms of MVK-oxide (anti) and MACR-oxide (syn) with the vinyl substituent adjacent to the terminal O atom. The resulting dioxoles are predicted to undergo rapid unimolecular decay to oxygenated hydrocarbon radical products, including acetyl, vinoxy, formyl, and 2-methylvinoxy radicals. In the presence of O₂, these radicals rapidly react to form peroxy radicals (ROO), which quickly decay via carbon-centered radical intermediates (QOOH) to stable carbonyl products that were identified in this work. The carbonyl products were detected under thermal conditions (298 K, 10 Torr He) using multiplexed photoionization mass spectrometry (MPIMS). The main products (and associated relative abundances) originating from unimolecular decay of anti-MVK-oxide and subsequent reaction with O₂ are formaldehyde (88 ± 5%), ketene (9 ± 1%), and glyoxal (3 ± 1%). Those identified from the unimolecular decay of syn-MACR-oxide and subsequent reaction with O₂ are acetaldehyde (37 ± 7%), vinyl alcohol (9 ± 1%), methylketene (2 ± 1%), and acrolein (52 ± 5%). In addition to the stable carbonyl products, the secondary peroxy chemistry also generates OH or HO₂ radical coproducts.

Product Detection of the CH Radical Reactions with Ammonia and Methyl-Substituted Amines

Reactions of the methylidyne (CH) radical with ammonia (NH₃), methylamine (CH₃NH₂), dimethylamine ((CH₃)₂NH), and trimethylamine ((CH₃)₃N) have been investigated under multiple collision conditions at 373 K and 4 Torr. The reaction products are detected by using soft photoionization coupled to orthogonal acceleration time-of-flight mass spectrometry at the Advanced Light Source (ALS) synchrotron. Kinetic traces are employed to discriminate between CH reaction products and products from secondary or slower reactions. Branching ratios for isomers produced at a given mass and formed by a single reaction are obtained by fitting the observed photoionization spectra to linear combinations of pure compound spectra. The reaction of the CH radical with ammonia is found to form mainly imine, HN═CH₂, in line with an addition-elimination mechanism. The singly methyl-substituted imine is detected for the CH reactions with methylamine, dimethylamine, and trimethylamine. Dimethylimine isomers are formed by the reaction of CH with dimethylamine, while trimethylimine is formed by the CH reaction with trimethylamine. Overall, the temporal profiles of the products are not consistent with the formation of aminocarbene products in the reaction flow tube. In the case of the reactions with methylamine and dimethylamine, product formation is assigned to an addition-elimination mechanism similar to that proposed for the CH reaction with ammonia. However, this mechanism cannot explain the products detected by the reaction with trimethylamine. A C-H insertion pathway may become more probable as the number of methyl groups increases.

Insights into gas-phase reaction mechanisms of small carbon radicals using isomer-resolved product detection

Gas-phase radical reactions of CN and CH with small hydrocarbons are overviewed with emphasis on isomer-resolved product detection.

Pyrolysis Pathways of the Furanic Ether 2-Methoxyfuran

Substituted furans, including furanic ethers, derived from nonedible biomass have been proposed as second-generation biofuels. In order to use these molecules as fuels, it is important to understand how they break apart thermally. In this work, a series of experiments were conducted to study the unimolecular and low-pressure bimolecular decomposition mechanisms of the smallest furanic ether, 2-methoxyfuran. Electronic structure (CBS-QB3) calculations indicate this substituted furan has an unusually weak O-CH3 bond, approximately 190 kJ mol(-1) (45 kcal mol(-1)); thus, the primary decomposition pathway is through bond scission resulting in CH3 and 2-furanyloxy (O-C4H3O) radicals. Final products from the ring opening of the furanyloxy radical include 2 CO, HC≡CH, and H. The decomposition of methoxyfuran is studied over a range of concentrations (0.0025-0.1%) in helium or argon in a heated silicon carbide (SiC) microtubular flow reactor (0.66-1 mm i.d., 2.5-3.5 cm long) with reactor wall temperatures from 300 to 1300 K. Inlet pressures to the reactor are 150-1500 Torr, and the gas mixture emerges as a skimmed molecular beam at a pressure of approximately 10 μTorr. Products formed at early pyrolysis times (100 μs) are detected by 118.2 nm (10.487 eV) photoionization mass spectrometry (PIMS), tunable synchrotron VUV PIMS, and matrix infrared absorption spectroscopy. Secondary products resulting from H or CH3 addition to the parent and reaction with 2-furanyloxy were also observed and include CH2═CH-CHO, CH3-CH═CH-CHO, CH3-CO-CH═CH2, and furanones; under the conditions in the reactor, we estimate these reactions contribute to at most 1-3% of total methoxyfuran decomposition. This work also includes observation and characterization of an allylic lactone radical, 2-furanyloxy (O-C4H3O), with the assignment of several intense vibrational bands in an Ar matrix, an estimate of the ionization threshold, and photoionization efficiency. A pressure-dependent kinetic mechanism is also developed to model the decomposition behavior of methoxyfuran and provide pathways for the minor bimolecular reaction channels that are observed experimentally.

A quantum Monte Carlo study of the reactions of CH with acrolein

To assist understanding of combustion processes, we have investigated reactions of methylidyne (CH) with acrolein (CH2CHCHO) using the quantum Monte Carlo (QMC) and other computational methods. We present a theoretical study of the major reactions reported in a recent experiment on the subject system. Both DFT and MP2 computations are carried out, and the former approach is used to form the independent-particle part of the QMC trial wave function used in the diffusion Monte Carlo (DMC) variant of the QMC method. In agreement with experiment, we find that the dominant product channel leads to formation of C4H4O systems + H with leading products of furan + H and 1,3-butadienal + H. Equilibrium geometries, atomization energies, reaction barriers, transition states, and heats of reaction are computed using the DFT, MP2, and DMC approaches and compared to experiment. We find that DMC results are in close agreement with experiment. The kinetics of the subject reactions are determined by solving master equations with the MultiWell software suite.

Flow tube studies of the C(3P) reactions with ethylene and propylene

Product detection studies of C(³P) atom reactions with ethylene, C₂H₄(X¹A_g) and propylene, C₃H₆(X¹A′) are carried out in a flow tube reactor at 332 K and 4 Torr (553.3 Pa) under multiple collision conditions.