6 research outputs found

    Resonance Stabilization Effects on Ketone Autoxidation: Isomer-Specific Cyclic Ether and Ketohydroperoxide Formation in the Low-Temperature (400–625 K) Oxidation of Diethyl Ketone

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    The pulsed photolytic chlorine-initiated oxidation of diethyl ketone [DEK; (CH<sub>3</sub>CH<sub>2</sub>)<sub>2</sub>CO], 2,2,4,4-<i>d</i><sub>4</sub>-DEK [<i>d</i><sub>4</sub>-DEK; (CH<sub>3</sub>CD<sub>2</sub>)<sub>2</sub>CO], and 1,1,1,5,5,5-<i>d</i><sub>6</sub>-DEK [<i>d</i><sub>6</sub>-DEK; (CD<sub>3</sub>CH<sub>2</sub>)<sub>2</sub>CO] is studied at 8 torr and 1–2 atm and from 400–625 K. Cl atoms produced by laser photolysis react with diethyl ketone to form either primary (3-pentan-on-1-yl, R<sub>P</sub>) or secondary (3-pentan-on-2-yl, R<sub>S</sub>) radicals, which in turn react with O<sub>2</sub>. Multiplexed time-of-flight mass spectrometry, coupled to either a hydrogen discharge lamp or tunable synchrotron photoionizing radiation, is used to detect products as a function of mass, time, and photon energy. At 8 torr, the nature of the chain propagating cyclic ether + OH channel changes as a function of temperature. At 450 K, the production of OH is mainly in conjunction with formation of 2,4-dimethyloxetan-3-one, resulting from reaction of the resonance-stabilized secondary R<sub>S</sub> with O<sub>2</sub>. In contrast, at 550 K and 8 torr, 2-methyl-tetrahydrofuran-3-one, originating from oxidation of the primary radical (R<sub>P</sub>), is observed as the dominant cyclic ether product. Formation of both of these cyclic ether production channels proceeds via a resonance-stabilized hydroperoxy alkyl (QOOH) intermediate. Little or no ketohydroperoxide (KHP) is observed under the low-pressure conditions. At higher O<sub>2</sub> concentrations and higher pressures (1–2 atm), a strong KHP signal appears as the temperature is increased above 450 K. Definitive isomeric identification from measurements on the deuterated DEK isotopologues indicates the favored pathway produces a γ-KHP via resonance-stabilized alkyl, QOOH, and HOOPOOH radicals. Time-resolved measurements reveal the KHP formation becomes faster and signal more intense upon increasing temperature from 450 to 575 K before intensity drops significantly at 625 K. The KHP time profile also shows a peak followed by a gradual depletion for the extent of experiment. Several tertiary products exhibit a slow accumulation in coincidence with the observed KHP decay. These products can be associated with decomposition of KHP by β-scission pathways or via isomerization of a γ-KHP into a cyclic peroxide intermediate (Korcek mechanism). The oxidation of <i>d</i><sub>4</sub>-DEK, where kinetic isotope effects disfavor γ-KHP formation, shows greatly reduced KHP formation and associated signatures from KHP decomposition products

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

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    The pulsed photolytic chlorine-initiated oxidation of methyl-<i>tert</i>-butyl ketone (MTbuK), di-<i>tert</i>-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 HO<sub>2</sub> 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<sup>–1</sup>. The MTbuK oxidation experiments also show several minor channels, including β-scission of the initial radicals and cyclic ether formation

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

    No full text
    The pulsed photolytic chlorine-initiated oxidation of methyl-<i>tert</i>-butyl ketone (MTbuK), di-<i>tert</i>-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 HO<sub>2</sub> 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<sup>–1</sup>. The MTbuK oxidation experiments also show several minor channels, including β-scission of the initial radicals and cyclic ether formation

    Polarized Matrix Infrared Spectra of Cyclopentadienone: Observations, Calculations, and Assignment for an Important Intermediate in Combustion and Biomass Pyrolysis

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    A detailed vibrational analysis of the infrared spectra of cyclopentadienone (C<sub>5</sub>H<sub>4</sub>O) in rare gas matrices has been carried out. <i>Ab initio</i> coupled-cluster anharmonic force field calculations were used to guide the assignments. Flash pyrolysis of <i>o</i>-phenylene sulfite (C<sub>6</sub>H<sub>4</sub>O<sub>2</sub>SO) was used to provide a molecular beam of C<sub>5</sub>H<sub>4</sub>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 C<sub>5</sub>H<sub>4</sub>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, Γ<sub>vib</sub> = 9a<sub>1</sub> ⊕ 3a<sub>2</sub> ⊕ 4b<sub>1</sub> ⊕ 8b<sub>2</sub>. Using vibrational perturbation theory and a deperturbation–diagonalization method, we report assignments of the following fundamental modes (cm<sup>–1</sup>) in a 4 K neon matrix: the a<sub>1</sub> modes of X̃ <sup>1</sup>A<sub>1</sub> C<sub>5</sub>H<sub>4</sub>O are found to be ν<sub>1</sub> = 3107, ν<sub>2</sub> = (3100, 3099), ν<sub>3</sub> = 1735, ν<sub>5</sub> = 1333, ν<sub>7</sub> = 952, ν<sub>8</sub> = 843, and ν<sub>9</sub> = 651; the inferred a<sub>2</sub> modes are ν<sub>10</sub> = 933, and ν<sub>11</sub> = 722; the b<sub>1</sub> modes are ν<sub>13</sub> = 932, ν<sub>14</sub> = 822, and ν<sub>15</sub> = 629; the b<sub>2</sub> fundamentals are ν<sub>17</sub> = 3143, ν<sub>18</sub> = (3078, 3076) ν<sub>19</sub> = (1601 or 1595), ν<sub>20</sub> = 1283, ν<sub>21</sub> = 1138, ν<sub>22</sub> = 1066, ν<sub>23</sub> = 738, and ν<sub>24</sub> = 458. The modes ν<sub>4</sub> and ν<sub>6</sub> were too weak to be detected, ν<sub>12</sub> is dipole-forbidden and its position cannot be inferred from combination and overtone bands, and ν<sub>16</sub> is below our detection range (<400 cm<sup>–1</sup>). 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

    Photoionization Mass Spectrometric Measurements of Initial Reaction Pathways in Low-Temperature Oxidation of 2,5-Dimethylhexane

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    Product formation from R + O<sub>2</sub> reactions relevant to low-temperature autoignition chemistry was studied for 2,5-dimethylhexane, a symmetrically branched octane isomer, at 550 and 650 K using Cl-atom initiated oxidation and multiplexed photoionization mass spectrometry (MPIMS). Interpretation of time- and photon-energy-resolved mass spectra led to three specific results important to characterizing the initial oxidation steps: (1) quantified isomer-resolved branching ratios for HO<sub>2</sub> + alkene channels; (2) 2,2,5,5-tetramethyltetrahydrofuran is formed in substantial yield from addition of O<sub>2</sub> to tertiary 2,5-dimethylhex-2-yl followed by isomerization of the resulting ROO adduct to tertiary hydroperoxyalkyl (QOOH) and exhibits a positive dependence on temperature over the range covered leading to a higher flux relative to aggregate cyclic ether yield. The higher relative flux is explained by a 1,5-hydrogen atom shift reaction that converts the initial primary alkyl radical (2,5-dimethylhex-1-yl) to the tertiary alkyl radical 2,5-dimethylhex-2-yl, providing an additional source of tertiary alkyl radicals. Quantum-chemical and master-equation calculations of the unimolecular decomposition of the primary alkyl radical reveal that isomerization to the tertiary alkyl radical is the most favorable pathway, and is favored over O<sub>2</sub>-addition at 650 K under the conditions herein. The isomerization pathway to tertiary alkyl radicals therefore contributes an additional mechanism to 2,2,5,5-tetramethyltetrahydrofuran formation; (3) carbonyl species (acetone, propanal, and methylpropanal) consistent with β-scission of QOOH radicals were formed in significant yield, indicating unimolecular QOOH decomposition into carbonyl + alkene + OH
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