Adam M. Scheer

Direct Measurements of Conformer-Dependent Reactivity of the Criegee Intermediate CH3CHOO.

More Criegee Sightings The reaction of ozone with unsaturated hydrocarbons produces short-lived molecules termed Criegee intermediates. The simplest such molecule, H2CO2, was recently detected and monitored in the laboratory. Su et al. (p. 174; see the Perspective by Vereecken) have obtained its vibrational spectrum, which could ultimately enable direct measurements of its reactivity in the atmosphere. Taatjes et al. (p. 177; see the Perspective by Vereecken) report on the laboratory preparation and reactivity of the next heavier Criegee intermediate, which bears a methyl group in place of one of the hydrogen atoms. The reaction kinetics of an intermediate implicated in atmospheric ozone chemistry has been measured in the laboratory. [Also see Perspective by Vereecken] Although carbonyl oxides, “Criegee intermediates,” have long been implicated in tropospheric oxidation, there have been few direct measurements of their kinetics, and only for the simplest compound in the class, CH2OO. Here, we report production and reaction kinetics of the next larger Criegee intermediate, CH3CHOO. Moreover, we independently probed the two distinct CH3CHOO conformers, syn- and anti-, both of which react readily with SO2 and with NO2. We demonstrate that anti-CH3CHOO is substantially more reactive toward water and SO2 than is syn-CH3CHOO. Reaction with water may dominate tropospheric removal of Criegee intermediates and determine their atmospheric concentration. An upper limit is obtained for the reaction of syn-CH3CHOO with water, and the rate constant for reaction of anti-CH3CHOO with water is measured as 1.0 × 10−14 ± 0.4 × 10−14 centimeter3 second−1.

Rate Coefficients of C1 and C2 Criegee Intermediate Reactions with Formic and Acetic Acid Near the Collision Limit: Direct Kinetics Measurements and Atmospheric Implications

Rate coefficients are directly determined for the reactions of the Criegee intermediates (CI) CH2OO and CH3CHOO with the two simplest carboxylic acids, formic acid (HCOOH) and acetic acid (CH3COOH), employing two complementary techniques: multiplexed photoionization mass spectrometry and cavity-enhanced broadband ultraviolet absorption spectroscopy. The measured rate coefficients are in excess of 1×10−10 cm3 s−1, several orders of magnitude larger than those suggested from many previous alkene ozonolysis experiments and assumed in atmospheric modeling studies. These results suggest that the reaction with carboxylic acids is a substantially more important loss process for CIs than is presently assumed. Implementing these rate coefficients in global atmospheric models shows that reactions between CI and organic acids make a substantial contribution to removal of these acids in terrestrial equatorial areas and in other regions where high CI concentrations occur such as high northern latitudes, and implies that sources of acids in these areas are larger than previously recognized.

Radical Chemistry in the Thermal Decomposition of Anisole and Deuterated Anisoles: An Investigation of Aromatic Growth

The pyrolyses of anisole (C(6)H(5)OCH(3)), d(3)-anisole (C(6)H(5)OCD(3)), and d(8)-anisole (C(6)D(5)OCD(3)) have been studied using a hyperthermal tubular reactor and photoionization reflectron time-of-flight mass spectrometer. Gas exiting the reactor is subject to an immediate supersonic expansion after a residence time of approximately 65 mus. This allows the detection of highly reactive radical intermediates. Our results confirm that the first steps in the thermal decomposition of anisole are the loss of a methyl group to form phenoxy radical, followed by ejection of a CO to form cyclopentadienyl radical (c-C(5)H(5)); C(6)H(5)OCH(3) --> C(6)H(5)O + CH(3); C(6)H(5)O --> c-C(5)H(5) + CO. At high temperatures (T(wall) = 1200 degrees C - 1300 degrees C) the c-C(5)H(5) decomposes to propargyl radical (CH(2)CCH) and acetylene; c-C(5)H(5) --> CH(2)CCH + C(2)H(2). The formation of benzene and naphthalene is demonstrated with 1 + 1 resonance-enhanced multiphoton ionization. Propargyl radical recombination is a significant benzene formation channel. However, we show the majority of benzene is formed by a ring expansion reaction of methylcyclopentadiene (C(5)H(5)CH(3)) resulting from methyl radical addition to cyclopentadienyl radical; CH(3) + c-C(5)H(5) --> C(5)H(5)CH(3) --> C(6)H(6) + 2H. The naphthalene is generated from cyclopentadienyl radical recombination; 2c-C(5)H(5) --> C(5)H(5)-C(5)H(5) --> C(10)H(8) + 2H. The respective intermediate amu 79 and 129 species associated with these reactions are detected, confirming the stepwise nature of the decompositions. These reactions are verified by pyrolysis studies of cyclopentadiene (C(5)H(6)) and C(5)H(5)CH(3) obtained from rapid thermal dissociation of the respective dimer compounds, as well as pyrolysis studies of propargyl bromide (BrCH(2)CCH).

Unimolecular thermal decomposition of phenol and d5-phenol: Direct observation of cyclopentadiene formation via cyclohexadienone

The pyrolyses of phenol and d(5)-phenol (C(6)H(5)OH and C(6)D(5)OH) have been studied using a high temperature, microtubular (μtubular) SiC reactor. Product detection is via both photon ionization (10.487 eV) time-of-flight mass spectrometry and matrix isolation infrared spectroscopy. Gas exiting the heated reactor (375 K-1575 K) is subject to a free expansion after a residence time in the μtubular reactor of approximately 50-100 μs. The expansion from the reactor into vacuum rapidly cools the gas mixture and allows the detection of radicals and other highly reactive intermediates. We find that the initial decomposition steps at the onset of phenol pyrolysis are enol/keto tautomerization to form cyclohexadienone followed by decarbonylation to produce cyclopentadiene; C(6)H(5)OH → c-C(6)H(6) = O → c-C(5)H(6) + CO. The cyclopentadiene loses a H atom to generate the cyclopentadienyl radical which further decomposes to acetylene and propargyl radical; c-C(5)H(6) → c-C(5)H(5) + H → HC≡CH + HCCCH(2). At higher temperatures, hydrogen loss from the PhO-H group to form phenoxy radical followed by CO ejection to generate the cyclopentadienyl radical likely contributes to the product distribution; C(6)H(5)O-H → C(6)H(5)O + H → c-C(5)H(5) + CO. The direct decarbonylation reaction remains an important channel in the thermal decomposition mechanisms of the dihydroxybenzenes. Both catechol (o-HO-C(6)H(4)-OH) and hydroquinone (p-HO-C(6)H(4)-OH) are shown to undergo decarbonylation at the onset of pyrolysis to form hydroxycyclopentadiene. In the case of catechol, we observe that water loss is also an important decomposition channel at the onset of pyrolysis.

Biomass pyrolysis: Thermal decomposition mechanisms of furfural and benzaldehyde

The thermal decompositions of furfural and benzaldehyde have been studied in a heated microtubular flow reactor. The pyrolysis experiments were carried out by passing a dilute mixture of the aromatic aldehydes (roughly 0.1%-1%) entrained in a stream of buffer gas (either He or Ar) through a pulsed, heated SiC reactor that is 2-3 cm long and 1 mm in diameter. Typical pressures in the reactor are 75-150 Torr with the SiC tube wall temperature in the range of 1200-1800 K. Characteristic residence times in the reactor are 100-200 μsec after which the gas mixture emerges as a skimmed molecular beam at a pressure of approximately 10 μTorr. Products were detected using matrix infrared absorption spectroscopy, 118.2 nm (10.487 eV) photoionization mass spectroscopy and resonance enhanced multiphoton ionization. The initial steps in the thermal decomposition of furfural and benzaldehyde have been identified. Furfural undergoes unimolecular decomposition to furan + CO: C4H3O-CHO (+ M) → CO + C4H4O. Sequential decomposition of furan leads to the production of HC≡CH, CH2CO, CH3C≡CH, CO, HCCCH2, and H atoms. In contrast, benzaldehyde resists decomposition until higher temperatures when it fragments to phenyl radical plus H atoms and CO: C6H5CHO (+ M) → C6H5CO + H → C6H5 + CO + H. The H atoms trigger a chain reaction by attacking C6H5CHO: H + C6H5CHO → [C6H6CHO]* → C6H6 + CO + H. The net result is the decomposition of benzaldehyde to produce benzene and CO.

Polarized matrix infrared spectra of cyclopentadienone: observations, calculations, and assignment for an important intermediate in combustion and biomass pyrolysis.

A detailed vibrational analysis of the infrared spectra of cyclopentadienone (C5H4═O) in rare gas matrices has been carried out. Ab initio coupled-cluster anharmonic force field calculations were used to guide the assignments. Flash pyrolysis of o-phenylene sulfite (C6H4O2SO) was used to provide a molecular beam of C5H4═O entrained in a rare gas carrier. The beam was interrogated with time-of-flight photoionization mass spectrometry (PIMS), confirming the clean, intense production of C5H4═O. Matrix isolation infrared spectroscopy coupled with 355 nm polarized UV for photoorientation and linear dichroism experiments was used to determine the symmetries of the vibrations. Cyclopentadienone has 24 fundamental vibrational modes, Γvib = 9a1 ⊕ 3a2 ⊕ 4b1 ⊕ 8b2. Using vibrational perturbation theory and a deperturbation-diagonalization method, we report assignments of the following fundamental modes (cm(-1)) in a 4 K neon matrix: the a1 modes of X̃ (1)A1 C5H4═O are found to be ν1 = 3107, ν2 = (3100, 3099), ν3 = 1735, ν5 = 1333, ν7 = 952, ν8 = 843, and ν9 = 651; the inferred a2 modes are ν10 = 933, and ν11 = 722; the b1 modes are ν13 = 932, ν14 = 822, and ν15 = 629; the b2 fundamentals are ν17 = 3143, ν18 = (3078, 3076) ν19 = (1601 or 1595), ν20 = 1283, ν21 = 1138, ν22 = 1066, ν23 = 738, and ν24 = 458. The modes ν4 and ν6 were too weak to be detected, ν12 is dipole-forbidden and its position cannot be inferred from combination and overtone bands, and ν16 is below our detection range (<400 cm(-1)). Additional features were observed and compared to anharmonic calculations, assigned as two quantum transitions, and used to assign some of the weak and infrared inactive fundamental vibrations.

Unimolecular thermal decomposition of dimethoxybenzenes.

The unimolecular thermal decomposition mechanisms of o-, m-, and p-dimethoxybenzene (CH3O-C6H4-OCH3) have been studied using a high temperature, microtubular (μtubular) SiC reactor with a residence time of 100 μs. Product detection was carried out using single photon ionization (SPI, 10.487 eV) and resonance enhanced multiphoton ionization (REMPI) time-of-flight mass spectrometry and matrix infrared absorption spectroscopy from 400 K to 1600 K. The initial pyrolytic step for each isomer is methoxy bond homolysis to eliminate methyl radical. Subsequent thermolysis is unique for each isomer. In the case of o-CH3O-C6H4-OCH3, intramolecular H-transfer dominates leading to the formation of o-hydroxybenzaldehyde (o-HO-C6H4-CHO) and phenol (C6H5OH). Para-CH3O-C6H4-OCH3 immediately breaks the second methoxy bond to form p-benzoquinone, which decomposes further to cyclopentadienone (C5H4=O). Finally, the m-CH3O-C6H4-OCH3 isomer will predominantly follow a ring-reduction/CO-elimination mechanism to form C5H4=O. Electronic structure calculations and transition state theory are used to confirm mechanisms and comment on kinetics. Implications for lignin pyrolysis are discussed.

Pyrolysis of Cyclopentadienone: Mechanistic Insights from a Direct Measurement of Product Branching Ratios

The thermal decomposition of cyclopentadienone (C5H4═O) has been studied in a flash pyrolysis continuous flow microreactor. Passing dilute samples of o-phenylene sulfite (C6H4O2SO) in He through the microreactor at elevated temperatures yields a relatively clean source of C5H4═O. The pyrolysis of C5H4═O was investigated over the temperature range 1000-2000 K. Below 1600 K, we have identified two decomposition channels: (1) C5H4═O (+ M) → CO + HC≡C-CH═CH2 and (2) C5H4═O (+ M) → CO + HC≡CH + HC≡CH. There is no evidence of radical or H atom chain reactions. To establish the thermochemistry for the pyrolysis of cyclopentadienone, ab initio electronic structure calculations (AE-CCSD(T)/aug-cc-pCVQZ//AE-CCSD(T)/cc-pVQZ and anharmonic FC-CCSD(T)/ANO1 ZPEs) were used to find ΔfH0(C5H4═O) to be 16 ± 1 kcal mol(-1) and ΔfH0(CH2═CH-C≡CH) to be 71 ± 1 kcal mol(-1). The calculations predict the reaction enthalpies ΔrxnH0(1) to be 28 ± 1 kcal mol(-1) (ΔrxnH298(1) is 30 ± 1 kcal mol(-1)) and ΔrxnH0(2) to be 66 ± 1 kcal mol(-1) (ΔrxnH298(2) is 69 ± 1 kcal mol(-1)). Following pyrolysis of C5H4═O, photoionization mass spectrometry was used to measure the relative concentrations of HCC-CHCH2 and HCCH. Reaction 1 dominates at low pyrolysis temperatures (1000-1400 K). At temperatures above 1400 K, reaction 2 becomes the dominant channel. We have used the product branching ratios over the temperature range 1000-1600 K to extract the ratios of unimolecular rate coefficients for reactions 1 and 2 . If Arrhenius expressions are used, the difference of activation energies for reactions 1 and 2 , E2 - E1, is found to be 16 ± 1 kcal mol(-1) and the ratio of the pre-exponential factors, A2/A1, is 7.0 ± 0.3.

Tailoring next-generation biofuels and their combustion in next-generation engines.

Increasing energy costs, the dependence on foreign oil supplies, and environmental concerns have emphasized the need to produce sustainable renewable fuels and chemicals. The strategy for producing next-generation biofuels must include efficient processes for biomass conversion to liquid fuels and the fuels must be compatible with current and future engines. Unfortunately, biofuel development generally takes place without any consideration of combustion characteristics, and combustion scientists typically measure biofuels properties without any feedback to the production design. We seek to optimize the fuel/engine system by bringing combustion performance, specifically for advanced next-generation engines, into the development of novel biosynthetic fuel pathways. Here we report an innovative coupling of combustion chemistry, from fundamentals to engine measurements, to the optimization of fuel production using metabolic engineering. We have established the necessary connections among the fundamental chemistry, engine science, and synthetic biology for fuel production, building a powerful framework for co-development of engines and biofuels.

Facile Rearrangement of 3-Oxoalkyl Radicals is Evident in Low-Temperature Gas-Phase Oxidation of Ketones

The pulsed photolytic chlorine-initiated oxidation of methyl-tert-butyl ketone (MTbuK), di-tert-butyl ketone (DTbuK), and a series of partially deuterated diethyl ketones (DEK) is studied in the gas phase at 8 Torr and 550-650 K. Products are monitored as a function of reaction time, mass, and photoionization energy using multiplexed photoionization mass spectrometry with tunable synchrotron ionizing radiation. The results establish that the primary 3-oxoalkyl radicals of those ketones, formed by abstraction of a hydrogen atom from the carbon atom in γ-position relative to the carbonyl oxygen, undergo a rapid rearrangement resulting in an effective 1,2-acyl group migration, similar to that in a Dowd-Beckwith ring expansion. Without this rearrangement, peroxy radicals derived from MTbuK and DTbuK cannot undergo HO2 elimination to yield a closed-shell unsaturated hydrocarbon coproduct. However, not only are these coproducts observed, but they represent the dominant oxidation channels of these ketones under the conditions of this study. For MTbuK and DTbuK, the rearrangement yields a more stable tertiary radical, which provides the thermodynamic driving force for this reaction. Even in the absence of such a driving force in the oxidation of partially deuterated DEK, the 1,2-acyl group migration is observed. Quantum chemical (CBS-QB3) calculations show the barrier for gas-phase rearrangement to be on the order of 10 kcal mol(-1). The MTbuK oxidation experiments also show several minor channels, including β-scission of the initial radicals and cyclic ether formation.